CA1199715A - Masking techniques in chemical vapor deposition - Google Patents

Abstract

ABSTRACT OF THE INVENTION

Various mask configurations and techniques for their employment in a chemical vapor deposition system are disclosed. These masks can be utilized in the fabrication of semiconductor devices. The masks have at least one aperture therein and may be either removed after device processing or formed as an integral part of the semiconductor device being fabricated. In either case, semiconductor devises can be formed with one or more layers characterized by desired spatial variations in their thickness and/or contour.
The integral masking techniques provide for incorporated self alignment which simplifies device processing. The fabrication of semiconductor injection lasers are disclosed as examples of applications of the masking techniques.

Description

~L~,6~37~L~

MASKING TECHNIOUES IN CHEMICAL VAPOR DEPOSITION
Background of the Invention This invention relates to the fabrication of semiconductor devices via chemical vapor deposition and, in particular, the fabrication of such devices in S metalorganic chemical vapor depositions (MO-CVD) with nonplanar layer characteristics by means o~ mas~ing techniques employed during growth.
It has been established in research and development of semiconductor injection lasers having an active layer and/or cladding layers which are nonplanar and have spatial variation in their thickness exhibit improved properties, such as, low threshold current, linear light output versus current characteristics and stable fundamental transverse mode control. Such nonplanar variations are discussed in U.S. Patent 4,335,461 entitled "Injection Lasers With lateral Spatial Thickness Variations (LSTV) In The Active Layer" and assigned to the assignee herein~
To date, such nonplanar lasers have been successfully grown by liquid phase epitaxy (LPE~.
Within the past several years, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MO-CVD) have become important processes in the fa~ri_~lioIl of- ~;incle crystal semiconductor integrated devices, including injection lasers. M~E is a growth process carried out under ultra high vacuum conditions, by the evaporation of the crystal constituents and dopants and beam deposited on substrates. MO-CVD is a gaseous crystal growth technique in which compounds, such as, (CH3)3 Ga, are caused to react with other gases, such as, AsH3, and appropriate dopants, in the vapor phase to produce single crystalline or polycrystalline deposits. These two procedures have, to a large extent, replaced the conventional LPE crystal growth techniques, owing to their improved control over (1) layer thickness, (2) crystal composition, (3) layer smoothness, (4) 1~ '; 4 'I';'i lS

abruptness of interfaces, and (5) uniform doping profiles.
~ EP processes permit nonplanar variations in layex contours and thicknesses as desired. For example, LPE
growth of channeled substrate lasers produced curved contours and thickness variations in deposited layers on the substrate. However, MBE and MO-CVD processes characteristically do not produce the same type of growth variations. Depending upon deposit rate, flow rate, substrate temperature, etc., the deposited layers or films tend to "match" the contour and shape of the depositing surface. It would be desirable to start with a substrate surface with a curved contour having a curved contour or taper adequate to produce the tapered variations during growth, as taught in the previously mentioned U.S. patent. ~owever, it is not readily easy to fabricate the desired curvature in a substrate prior to growth. It would be simpler to develop the desired contour during growth, as done in the past, and obtain better accuracy and control in the desired contour and thickness variations that MBE and MO-CVD processes would provide.
One way of accomplishing these spatial variations in MBE is by employing a mask having an aperture. The mask is positioned between the elemental sources and the substrate. Only elemental materials propagating through the mas~ aperture will deposit on th~ surface of the substrate.
But what about masking in MO-CVD processes? One 3U would conclude that an apertured mask in MO-CVD will be of little help. MO-CVD involves the flow of gases through a reactor that engage a supported substrate where pyrolyzation of vapor mixtures of elemental compounds in these gases occurs. Turbulence is present in the flow of these gases in the region of the subs~rate. One would, therefore, postulate that because of the turbulent nature of the gas Elow in this region, it would be inept for one to conclude that apertured masking may be a viable way of producing 3 ~ ~

desired layer spatial variations during MO-CVD growth processes. With an apertured mask positioned over the substrate upon which deposition is to occur, the turbulent motion of gases about and in the mask aperture would surely lead to uneven and nonunifor~
spatial variations in tapered contours and layer or film thicknesses.
Summary of khe Invention According to an aspect o~ this invention, masking techniques can be successfully employed ln chemical vapor deposition, such as, MO-CVD. Nonplanar shaped lay~rs with spatial variations in both uniform contours, taper and thickness deposited on semiconductor structures can be produced in MO-CVD
deposition system by introducing a mask in the heated deposition zone of the system during the pyrolyzation of vapor mixtures of elemental compounds including the semiconductor materials to be deposited and wherein the mask has at least one aperture. The mask may have more than one aperture and the configuration of the mask aperture may be of any size or shape, e.g~, curved, round, parallelogram, trapezoid, triangle, ellipse, square, etc.
The mask may be a removable mask, positioned over the structure, e.g., semiconductor injection laser upon which deposition is to occur, either in spaced relation to the depositing surface or in engagement with that surface. The mask may be an integral mask comprising a deposited layer of the structure or formed in the structure, such as, formed in a semiconductor substrate. Whether of the removable or of the integral type, variations in the mask aperture dimensions and the spacing relative to the structure surface upon which deposition is to occur can provide accurate control of the deslred spatial variations in deposited layers or films~
Integral masks have the advantage over removable masks of being fabricated of thinner mask dimensions (in the low ~ m range). Micro-semiconductor structures are possible having micro spatial variations. However, 3a , the dimensions of composite removable mask structure can approach the small dimensions of the integral mask.
Spacing of removable masks from khe depositing surface may be accomplished b~ supportin~ the mask in spaced relation from the depositing sur~ace. In the case of integral masking, a well or channel may be formed in the structure through the aperture of the mask. This is advantageous in the fabrication of semiconductor injection lasers because not only can the one or more layers le.g. the active layer) of the completed device have desired spatial variations but also current confinement definition and alignment are automatically achieved duriny growth in a congruent manner, which was not previously possible in any other process.
The masking techniques disclosed may be used in the deposition of amphorous, polycrystalline or single crystalline materials and layers.
According to an aspect of this invention, a semiconductor device may be fabricated by a chemical vapor deposition to have one or more layers of predetermined lateral spatial thickness variation. The lateral spatial thickness variation may be formed by means of pyrolyzation of vapor mixtures of semiconductor materials comprising the layer or layers through an aperture of a mask employed during the chemical vapor deposition thereof~
A particular example of such a semiconductor device is a semiconductor injection laser device which is fabricated by metal organic chemical vapor deposition (MO-CVD) with the aid of an apertured mask wherein the active layer and possibly other layers comprising the laser device are deposited in the MO-CV~
reactor through a mask aperture onto a laser substrate so that the layers so deposited are characterized by a la~eral spatial thickness variation or tapered contour wherein the thickest region of the variation is central of the mask aperture.

3b The mask structure may be removable and removed after deposltion of such layers or may be an integral part of the device structure, i.e., an integral layer with an aperture formed therein.
An aspect of the inven-tion is as follows:
A method employed during the fabrication of a semicon-ductor device to determine the center point of a plurality of semiconductor layers deposited on a semiconductor substrate, the deposited top layer thereof characterized by lateral spatial variation in thickness with at least one position therealong having a minima or maxima cross sectional thickness, said position beiny said center point due to the method used in the deposition of said layers comprising the steps of projecting confined radiation onto the surface of said top layer producing a pattern of interference color fringes, determining said center point by examining said interference fringes produced by said projection and distinguishing said one position from other areas of said top layer adjacent thereto by the intensity or color variation created at said position due to said interference fringes.
Other objects and attainments together with a fuller understanding of the lnvention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.
Brief Description of the Drawings Figure 1 is a schematic representation of a MO-CVD
reactor system suitable for practising the method according to this invention;

Figure 2 is a perspcctiYe view of Ihe susccptor of the system shown in Figure 1 with a gencric illuslralion of the mask as applied to a semiconductor structure upon which deposi~on is to occur;

S Flgure 3 is a side elevation of the view shown in Figure 2. In this Figurc and subsequent Figures, the mask according ~o this invention is shown in cross hatched lines for purposes of clarity;

Figure 4 is a removable mask according to ~his invention, and having a cavity so that the 10 mask aperture is spaced firom the surface of the structure upon which deposition is to occur, Figure 5 is ano~er rernovable spaced mask similar to that shown in Figure 4 but supported in alternate rnanner;
~s Figure 6 is still another removable, spaced mask similar to that shown in Fïgures 4 and 5 but supported in an alternate manner, ~gure 7 is an inlegral rnask according to this invention, and integral wiLh a 20 semiconductor s~ructure comprising a semiconductor substrate;

Flgure 8 is anolher il]ustration of an integra1 mask according to this inYention integral with a semiconductor structure comprising a semiconductor layer or film deposiled on a semiconductor substrat~;
2s F;gure 9 is another illustration of an inte~ral mask which is formed as part o~
semiconductor struet~re comprising a semiconductor substrate;

Figure lO is a graphic illustration of thc conlour and shape that may be grown through 30 dle apenure of a mask according to ~he method oî ~his inYen~ion;

Fïgurl~s 11 and 18 are diagrammatic illustrations relating lo inlegral mask configurations.
For puJposes of simplicity, the growth is shown, in most c~ses, as a singlc ]aycr bu~
reprcsent~tive, ]loweve~, of or~e or more dcposi~ed ]ayer.s of di~crenl elementa]
35 compounds Figure 11 is a side elevation of a channelcd scmiconduc~or stnlcture comprising a subsuate with an inlcgral mask;

Figure 12 is a side elevation of a channeled scmiconductor structure similar to Figure 11 S but having a mask lip or overhang;

Figure 13 is a side elevation of a channcled scmiconductor siructure similar to Figure 12 but having a differently shaped struclure channel;

o Figure 14 is a side eleva~ion of a channeled semiconductor structure simil~r to Figure 12 but having a mesa formed in the channel;

Figure lS is a side elevation of a channeled semiconductor structure similar to Figure 12 but having ano~her channel formed in the main char.nel of the structure;

Figure 16 is a side eleva~ion of a channcled semiconductor structure similar to Figure 12 excep~ that the semiconductor siructure comprises an interrncdiate dcposited layer belw~en a semiconductor substrate and a dcposited mask;

20 Figure 17 is a side elevation of a channeled semiconductor structure similar to Figure 16 excep~ the deposited interrnediate layer is of thicXer cross-section;

Figure 18 is a side elevation o~ a semiconductor structure similar to Figure 17 but wiehollt a channel formed in the substrat~;

Figurcs 19 through 31 are diagrammatic i]lustrations relating to rcmovable masl~configurations. For purposes of sirnp1i/ity, tlle growth is shown, in most cases, as a single layer b~e rcprcscnlativc, ho Ycver, of one or morè deposited layers of different elcmental compounds:

Figure 19 is a side elevation of a semiconductor stnlcture with a mask having a cavily similar to mask shown in Figure 4;

Fi~ure 20 is a side elevation of a scmiconduclor s~ruclure with a f~at surfaee mask similar 3S to mask shown in Figure 3:

7~

I igure 21 is a side elevation of a semiconduclor struclure having a mask similar to the mask of ~'igure 20 except having an vutwardly beveled mask aperlure;

Figure 22 is a side elevation of a semiconductor structure havin~ a mask sirnilar to ~he 5 mask of Figure 20 excepl having an inwardly beveled mask aperture Figure 23 is a side elevation of a semiconductor structure similar to Figure 22 bu~ having a mask with an inwardly beYeled mask aperiure and a mask cavity simi1ar to the mask caYity shown in mask of Figure 19;

Figure 24 is a side elevalion of a semiconductor structure similar to Figure 23 except provided with a deeper mask cavity;

Figure 25 is a side elevation of a semiconductor structure havin~g a mask aperture simi]ar 5 to the mask aperture in Figure 21 and provided with a mask cavity:

Figure 26 is a side elevation o~ a semiconducaor structure wi~ a composite mas~
structure;

20 Figure 27 is a side elevation of a channeled semiconductor structure having a flat masl~
structure like the mask shown in Figure 20;

Figure 28 is a side elevation oF a channeled semiconductor structure with a cen~ral mesa disposed in the structure channel and a mask structure like the mask shown in Figure 19:
Figure 29 is a side elevadon of a channeled semiconductor struclure similar to the mask shown in Figure 28 except having an upwardly disposed masX cavity;

Figure 30 is a side elevation of a channeled semiconduc~or structure with a pair30 poinied shaped mesas centrally disposed in the stn~clure ch~nnel and a mask structure like the mask shown in Figure 20;

Figure 31 is a side e~eva~ion of a channeled V-shaped semiconductor struclure employing a mask stmcture like the m~sk disclvsed in Fi~ure 28:
3~
]-igure 32 is a diagrammatic ilius~ration of a double helerostnJctllre injeclion l~scr with a s nonplanar ac~ivc rcgion having dcsircd spatial varlialions, grown in a channclcd substra~c with a rcmovablc mask cmploycd during dcposition:

Fi,gure 33 is a diagrammatic illuslration of anolhcr doublc hctcrostmcturc injcction lascr 5 with a nonplanar active rcgion having dcsircd spatial variations, grown in a channclcd substralc with a rcmovablc mask cmploycd dcposition;

Figure 34 is a scanning elcctron microscope photomicrograph of a side clcvation a scmiconductor structure after complction of an initial etching stcp, the structure o cornprising a scmiconductor subsLrate and two contiguous, dcpositcd scmiconductor laycrs;

Figure 35 is a photomicrograph of thc s~ructurc shown in Figure 34 aflcr complction of a sccond e~ching stcp, the struclurc now bcing thc same as that shown in Figure 8;
Figure 36 is a photomicrograph of a side elevation of a scmiconductor structurc similar to that shown in Figure 35 cxcept the structure inc]udcs scvcral intcnncdiate dcposilcd scmiconductor tayers;

20 hgure 37 is a photomicrograph of a side elevation of a doublc hc~crostructurc injcction lascr grown by M3-CVD cmploying an intcgral singlc ~rystalline mask during growth;
i-igurc 33 is a photomicrograph of the ~amc ]ascr shown in Figure 37 but of grcater magnificaLion 2s Figurc 39 is a photomicrograph of a sidc clcva~ion of still anothcr doub]e hclcrostructure injcction laser grown by MO-CVD cmploying an integral polycrystalline mask during growth;

30 Figurc 40 is a diagrammatic illustJation of a partia1 side clcvation of an injcction lascr grown by MO-CVD cmploying an inlcgral mask and il]ustrating currcnt confincmcnt and alignrncnt tcchniqucs in complcLing thc fabrication of thc ]ascr; and Figur 41 is a diagramma~ic illustration of a parlial sidc clcvation of an injcclion ]ascr 35 g,rown by MO-CVI~ cmploying a rclnnvablc mask and iliustrating currcnt cnnfincmcnt and alignmcnt Icchniqucs lo~ard complc~ing thc ~abrication of Ihc lascr.

Detailed Description of the Preferred Embodiments In Figure 1 there is shown a conventional MO-CVD
reactor system 10 for practicing this invention and for the fabrication of semiconductor devices, such as, injection lasers. The employment of the mask configurations and making techniques to be discussed are not limited to MO-CVD. These masks may be readily employed in other chemical vapor deposition systems and in molecular beam epitaxy (MBE). In the case of MBE, however~ the desired degree of spatial variations may not be as ea~ily achieved.
System 10 will be described in conjunction with elemental compounds used in fabrication of GaAs/GaAlAs injection lasers. However, employing the masking techniques to be disclosed, any other depositable materials may be used.
Prior art discussion of MO-CVD systems is found in an article of Russell D. Dupuis and P. Daniel Dapkus entitled "Preparation & Properties of Gal_xAlxAs-GaAs Heterostructure Lasers Grown by Metalorganic Chemical Vapor Deposition", IEEE Journal of Quantum Electronics, Vol. QE-15, No. 3, pp.128-135, March, 1979.
System 10 comprises sources 12, 14 and 16, respectively, trimethylgallium (TMGa), trimethylaluminum (TMAl~, and arsenic hydride (ASH3).
Sources 12 and 14 are bubbler sources with purified hydrogen provided fxom source 15. The hydrogen is bubbled through these sources at a controlled rate via the mass flow controllers 17. Physical vapor phase mixtures of these compounds are pyrolyzed in hydrogen generally between 600 to 850C to form thin solid films according to the net reaction:
H?
~l-x)[(CH3)3Ga] + X[(CH3)3Al] -~ ASH
Ga(l x)AlxAs + 3CH~
~he metalorganics TMGa, TMAl and DEZn are liquids near room temperature with relatively high vapor pressures. Hydrogen gas from source 15 is used as a carrier to ~ransport these source vapors into vertical Dr,;l~' .IL- ~J 7 _ILs.
8a reactor 18. Susceptor 20 is supported within the reactor on a rotatable rod 22. The semiconductor structure 24, upon which deposition is to occur, is positioned on the top of susceptor 20. The terms "semiconductor structure" as used herein means a semiconductor substrate or one or more previously deposited semiconductor layers on a semiconductor substrate.

7:~LS
9, 'rhc Rl~' hcating coil 28 providcs hcat to thc dcpositon zonc 30, surrounding susccptor 20 and structure 24, ~o wilhin thc abovc mcntioncd tcmpcraturc rangc Lo pyroly~c thc vapor phasc mixturcs of the sourcc compounds. The alloy composition of thc dcpositcd film is contro11cd by thc rclatiYe pat~ial prcssurcs of thc Ga and ~1 mckllorganic source 5 compounds.

For p-typc zinc doping, a source 32 of dicthylzinc (DEZn) is cmploycd and for n-type Se doping, a sourcc 34 of hydrogcn sclcnidc (H2Sc) is cmployel 0 Tlte flows of mctalorganics and hydridcs arc prcciscly controllcd to dcsired molccular proportions for introduction into the rcactor 18 by mcans Or the mass tlow controllcrs 17.
The growth ratcs are typically From 1,000-~0,000 A pcr mil)ute. Thc thickness of layers and the extcnl Or doping can be prcciscly controllcd by the appropriatc timcd scqucncing of the path flow values 19. Exhaust flow valves 23 arc uscd in purging ~he systcm 10.
Bricfly, the process for forming layers on a s~ructure 24 compriscs thc steps of (1) evacuating the rcaclor 18; (2) flushing thc rcactor 18 with hydrogcn; (3) hcatillg the dcposition zone 30 to thc desircd dcpos;tion tempcraturc within thc range of 600C to 850C; (4) cquilibrating the flow gas from thc compound sourccs by connccting the 20 appropriate sources to exhaust while also bubbling hydrogcn through sclcctcd mctalorganic sources 12, 14 or 32 at a contro11ed rale to cquilibrate the vapor flows at dcsired ratios; (53 introducing the sclcctcd rcactants into the rcactor 18 for a givcn pcriod of timc to form a thin film or laycr of dcsircd thickncss on thc exposcd surface of the structure 24; and (6) thcrcaftcr c~hausting all rcactant gascs from thc rcactor 18 and 25 cooling thc structure while purging the rcactor wiîh hydrogen.

The rcacLant gases entcr the reactor 18 via main flow valvc 25 and sprcad ~hroughout the physical volume of rcactor. Vnlikc MBE the envirorlmcnt compriscs a vapor phase mixturc of reactant matcrials that will pyrogcnically rcact in zone 30. Illcrc bcing the 30 physical movcrncnt of gascs in and about the arca of thc susccplor 20 and thescmiconductor structurc 24, thcre are also somc divcrgcnt gas flow crcalcd in this rcgion.
Un;forrn and unobstructcd growths arc, thus, possiblc on thc stnlcturc surface.

Rcccntly thcre have bccn dcvclopmcnts in thc scmiconduct()r fic]d o~ dcsigning and 35 rabricaling scmicunduclor dcviccs v~ith s~rip or boundcd compositcs or mc~a typc huricd structurcs. 'I'hcsc slructurcs arc furmcd via additionlll and intcnncdialc pr~ccssin, slcps which usually invoke selective etching. An example of such a device is an injection laser disclosed in U.S.
Patent 4,371,966 entitled "Heterostructure Lasers With Comhination Active Strip And Passive Waveguide Strip"
and assigned to the assignee herein. Masking techniques would be desirable to form these mesa type patterns or layers directly by deposition to eliminate intermediate steps of removal of the growth structure from the process and apply ~elective etch techniques to form the desired strip or mesa type structures.
Crude forms of masking have been employed in LPE
for growing desired patterns directly on substrates through mask patterns but with limited success.
Masking techniques have been also employed in MBE with a good degree of success because growth takes place in an ultra high vacuum chamber and the beams of elemental constituents are, for the most part, unidirectional.
In MO-CVD, however, the reactant gases entering the xeactor are multi-directionalO Attempts to employ apertured masks in a potentially turbulent environment is highly suspect of not producing uniform and desirably contoured deposited patterns via mask apertures. I have discovered, to the contrary, that apertured masks may be employed in MO-CVD to form mesa type patterns through mask apertures having desired spatial variations in pattern contour and thickness.
These spatial variations are accomplished by several factors: (1) mask size, (23 dimensional size of the mask aperture; (3) the thickness of the mask and mask aperture, and (4) spacing relative to the surface below the mask aperture upon which deposition is to occur.
Mask structuxes may be either of the removable or integral type. If o~ ~he integral type, their presence provides for "automatic" fulfilment of alignment for location and fabrication of current confinement means for semiconductor devices, such as, injectlon lasers.
From my development of mask parameters and structures as well as masking techniques in MO-CVD, I
have found that the non~directional aspect of the 7~
lOa reactant gases may, indeed, not be as paramount as one might believe. Although it is not altogether clear why masking during growth is successful in MO-CVD, it appears reasonable that one reason for success is that when the gas components, such as H2 and CH3, dissociate from the liberated elements or compounds deposited, they are comparatively of much lighter mass and because of the thermal dissociation, have attained high kinetic energy. Because of these two factors, they move at much higher velocities than other molecular components and are able to move expeditiously away from the mask aperture and the deposition zone.

~199~5 Thc simplcst mask structure is s3)own in Figurcs 2 and 3. Mask 26 compriscs a tlat composi~c having at lcas~ onc apcrlure 27. Mask 26 may be inlegral with slructure 24, such as, an layer or film, or may be a rcmovablc stnJcLure. The mask 26 may be made of any number of ma~erials, such as, silicon dioxidc, gallium aluminum arscnide, gallium arscnide, silicon niLrite, aluminum oxide, etc.

Special considcralion can be given in mask dcsign in ordcr to minimize contamination and to rcstrict the flow of reactant gases around and from regions undcr the mask. This is particularly true for removable masks. ln Figure 4 the rcmovable mask 3~ has an outer 0 perimetrical lip 33. Mask 32 a1so has an aperture 34. Positioning of the mask 32 on suuclure 24 provides for the aper~re 34 to be spaced from the surface 24' on which dcposition is to occur. The configuration of this particular mask stn cture, as comparcd to mask 26 in Figure 2, is îhat growsh will be pcrrnittcd to exlcnd over surface 24' bcyond the confincs or dimensions of the aperture 34.
'' The reactor 18 can bc modificd to include an asscmbly within the reaclor lo provide for the inscrtion and rcmova] of masks during thc dcposition processes.

An examp]e of the employment of a mask 32 is as follows. Mask 32 was made of si1icon with <110> orientation. The mask was about 3 mi]s thick (dimcnsion A in Figure 4~ and the width of the apert~re was S mils wide and 25 mi]s long. The spacing B was about 6~m. The growth on the surface of the structure 24 was of Gaussian shapc about lO mi1s wide, 30 mils long and 4.4 ,um high. The contour oî the growth ~Yas similar to the contour pattcrn 66 shown in Figure lQ.
~s In Figure S, mask 36 is similar to mask 32 in Figure 4 exccpt it is provided wilh a pcrimetrical lip 37 îor supporting the mask in spaced rclation from surface 24'( of scmiconductor structure 24. Lip 37 is designed to engage the sur~ace of susccptor 22 ]caving no spacing for reactant gases to escape under thc mask lip 37. Mask 36 is also shown with two apcrtures 35' and 3S".

In Figure 6. mask 3~ is also provided ~o be main~ined in spaccd rc]ation ~rom the surface 24' of a scmiconductor structure 24. The spaccd rcla~ion, howcvcr, is accl)mplishcd by thc pcrimclrica] edgc or lip 29 proYidcd on thc scmiconductor structure 24. Mask 38 is p1anar has two apcrtures 3~' and 38".

Mask 32, 36 and 38 in Figurcs 4-6 arc all dcsigncd to bc rcmovablc masks, that is, thcy arc cmploycd during thc growth proccss and may subscqucn~ly bc rcrnorcd prior to the complction of proccssing in rcactor 18.

s The mask structures shown in Figurcs 7, 8 and 9 arc intcgral masks. In Figure 7, the scmiconductor structurc 24 is providcd with a channc1 40. A dcpositcd mask 42 isprovidcd with an apcrturc 44 aligncd wi~h thc centcr of channel 40. ~hc mask laycr 42 may comprisc polycr,vstallinc matcrial, a amorphous matcrial or a singlc crystal matcrial.
For example, structure 24 may bc a substrate of ga]lium arscnidc (GaAs). Mask 42 may lo comp~ise a dcpositcd laycr of SiO2, Si3N4 or A121:)3. A important aspect of masl~ 42 is the cantilevercd lips 46 extending ovcr thc channel 40 of thc serniconductor sLructure Z4.

- The sclf-aligned mask 42 is made by the vapor dcposition of SiQ2 on thc substrate 24.
Ncxt, an elongated aperture 44 is ctchcd through the SiO2 mask laycr cmploying a SiO;
LS ctch. This is followcd by a sclcctive etch for GaAs to for n the channcl 40 in substrate 24 through the apcrture 44. This two step ctching proccss leaves Ihc mask cantilevcrcd lips 46 ovcr both sidcs of the channcl 4Q With this type of mask structurc, growtb of non-planar laycred structurcs can easily bc pcrforrncd by MO-CVD. ln the case of thecxarnp7e of the previous paraOraph, the growth through the SiO2 maskcd apcrturc 44 on the GaAs substrate will be cr~stallinc while the gro~vth on the surfacc of the mask will be polycrystalline. Discussion conccrning growth will be explaincd in greater dctail in subscquent figures.

In Figurc 8, semiconductor strucnlrc 24 compriscs a substratc 48 on which is a dcposited layer 50. Layer 50 rnay comprise~ for cxamplc, Gal xAIxAs. Mask layer 52 is dcposilcd on laycr SO and will subscqucnt!y bc thc mask structurc for the scmiconductor s~cture 24. Substrate 48, fior example, may bc ~lOU> oricnt~tion, n-dopcd Ga~s. Layer 50 may be Gao~A106As. Mas1~ layer S2 may be undopcd GaAs. Layer 52, as wcll as other intcgra3 mask laycrs to be hercinafler discusscd, may proton or ion implantcd or oxyge~
or Gc dopcd ~o rcndcr thcn clcclrically insu1aling. Such a 1aycr may ~o~n part oF the currcnt confine slructurc of scmiconductor dcrice comprising a pluraliI~ of scmiconductor laycrs subscqucntly dcposi~cd through apcrture 56 in ch;mncl 57.

Laycr S2 may 3]SO bc n-dopcd CiaAs whilc laycr 50 may bc p dopcd Gal xA3xAs to rorm a rcvcrsc junction and ~orm part of lhc currcnt conïlncmcnt mcans for a scmiconductor dcvicc dcpositcd in channcl 57.

The preparation of this mask structure for subsequent growth is accomplished as follows, reference being made also to the microphotographs of Figures 34 and 35.
Figure 35 is an actual photomicrograph of the structure illustrated in Figure 8 except for substrate orientation. With selective masking, an elongated aperture 56 is etched through the gallium arsenide layer 52. Figure 34 shows the result of this single etching step wherein the etchant has also extended a little into the intermediate layer 50 of Gal xAlxAs.
This first etching step is followed by a second etching step comprising an etchant for Gal xAlxAs such as, HCL
or HF etchant. The mask aperture 56 now performs the function of a mask for performing this second etching step. The etching process, over a selected period of time, will produce a channel 54 in layer 50 and extending beneath the elongated edges of the aperture 56 forming the extended cantilever lips 58. The structure resulting from this second etching step is shown both in Figures 8 and 35.
The ~dges of the lips 58 can have different angled contours depending on the crystal orientation of structure 24. For example, in Figure 8, the upwardly open bevelled edges are obtained by a ~100~
orientation of the substrate 48 with the etch d channel psrpendicular to the (011) cleavage plane. On the other hand, V-shaped edges are obtained by a ~100>
orientation of the substrate 48 with the etched channel perpendicular to the (011) cleavage plane, as illustrated in Figures 34 and 35.
The photomicrograph ~hown in Figure 36 is similar to that shown in Figure 35 except that the structure 24 comprises two addi.tional deposited layers. Structure 24 may, for example, comprise a ~100~ orientation substrate 48, an undoped layer 50 of GaO 4Alo 6As, a p-type layer Sl of GaO 4Alo 6As, an n-type layer 53 of Ga~ 4Alo 6As and the single crystal mask 5~ of undoped GaAs with the channel etched perpendicular to the (011) cleavage plane~ Layers 51 and 53 will form a reverse 7~5 junction forming part of the current confinement for a semiconductor device formed in channel 57.
The mask structure need not be formed as an integral layer or a film on the semlconductor structure 24. As shown in Figure 9, the mask structure may be actually part of the semiconductor structure 24, per se. Using a GaAs etch, a mask opening is formed in the body of the substrate forming a dovetail channel 60 defining an aperture 62 and forming the elongated lips 6~. The channel 60 etched perpendicular to the (011~
cleavage plane is sufficiently deep so as to function as a mask structure to obtain contoured growth on the surface of the channel. The profile of the side walls 65 of the channel can be varied depending upon the etchant as is known in the art. See, for example, the channel profile in Fi~ure 13.
In all of these removable and integral mask structures in Figures 4-9, MO-CVD growths may be performed through the apertures of the masks by the deposition of materials or compounds from the reactant gases onto the surfaces of the channels formed beneath the mask apertures. The extent of the growth, that is, the height, thickness and curvature of the growth is controlled by the size and shape of the mask aperture, the thickness of the mask and the amount of the channel volume beneath the mask.
A rule of thumb is that the width of the mask aperture should be greater than thickness of the mask.
This ratio is particularly important in order that a major portion of the reactant gases make initial contact and deposit on the channel bottom beneath the mask aperture before making substantial contact with the surfaces of the mask aperture edges or channel extremities. In this sense, the mask thickness should be comparatively thin, but this dimension also depends on the mask aperture width.
The thickness, for example, of a removable or integral type mask may typically vary between 2 to 5 mils with an aperture width between 4 to 8 mils. A

7~
14a specific example would be a mask 3.5 mils thick at the - mask lips with an aperture width of 4 to 5 mils, follows the above mentioned rule. Thinner mask dimensions are more easily achieved with integral masks. The thickness and aperture width of integral masks may typically be 1 to 5 ~ m and 2 to 30 ~ m, respectively.
By the use of these mask structures, a three dimensional controlled, contoured growth is possible in MO-CVD. The preferred mask design for contoured shapes and configurations is to have a region (e.g. channel 57) under the mask in which the reactant gases can spread laterally, depositing compounds in the chanffel volume in a tapered or contoured manner. The channel volume and mask aperture width selection permit control over both aspects of spatial variation of the growth -1) the curved contour and extent of the growth and 2) the thickness and height of the growth, albelt a single layer or a plurality of layers. The wider the aperture, the lower the taper rapidity of the growth, i e. a more level growth profile, with channel volume presumed constant. With the same mask and channel parameters, the spatial variations of the growth may be reproduced in a continuous and substantially identic~l manner.
A three-dimensional type profile or pattern 66 of a contoured growth with a Gaussian shaped cross section is illustrated in Figure lQ. These mesa like patterns may be employed in the fabrication of semiconductor devices requiring three dimensional a7 ~ ~

contours, such as~ the formation o~aclive regions in injcction lascrs h~ving dcsircd spa~ial variations in lapcrcd contour and thickncss.

Thc prcccding discussion has bcen in connec~ion with thc fabrication of difrcrcnt typcs of S mask s~ruclures. ll-e descrip~ion of ~he rcmaining fi~urcs involvcs thc use of various rcmovable and fixed mask structures in thc dcposition of one or more layers of scmiconductor compounds in MO-CVD.

ll~e purpose of Flgures 11 through 31 is to illustrate the differcn~ type of growths lo possible with various ~ypes of inlegral and rcmovable mask configurations. Figurcs 11 through 18 illustrate integral type mask structurcs. In these Figurcs mask ovcrgrowth is shown since the mask rcmains as an intcgral part of the fabricatcd dcvice. Flgurcs 19 through 31 illustrate rcmovable mask type structures. ln all these Figures, thc masl;
structures are shown cross-hatche~ for purposes of clarity. ~n the fig~rcs rclating to rcmovablc mask structures, ovcrgrowth on thc mask is not illustratcd since Lhc masks are removed during or after complction ol growth.

Tl-e growth in Figures 11 through 31 is pcrforrncd in the MO-CVD systcm 10 of Flgure 1 and in most cases is shown as a single 1ayer for purposes Or simplicity. This represCnLation, however, is a]so intcnded ~o rcprcsent the bu]k of a plura1ity of dcposited layers, such as, illustrated in Figure 31. Dctai1cd multilayer structures are discusscd in Figures 32, 33, 37, 38 and 39.

In Fgures 11 through 15, the strl3cturcs shown each comprise an oricnlcd crysta]line 2s scmiconductor (such as, dopcd or undopcd Ga~s) subslratc 70, an oxide (SiO2) or nitride (Si3N4) mask 72, a polycrystalline growth 74 over thc mask surface and a single crystal growth 76 dcpositcd through the aperture 78 of the mask 72. The growth 76 forrns a spatial ~ariation in tapcred contour or rapidity and in thickness, as illustratcd at 80. The growth extcnds in a uniform contourcd shapc in the substratc channe]s 71, 73 and 75 away from the ccntral axis of the apcrture 78 tov~ard thc channcl cxtrcmitics. /l~lso the growth extcnds around the cdgcs of thc mask lips 82 and tapcrs on the undcrsurraces of thc mask lips toward thc channcl exiremities.

In Figurc II, a sclcctivc ctch is pcr~ormcd to Form thc c]ongatcd apcrturc 78 in ]aycr 74.
Ch3nncl 71 is ~onmcd, as by sclcctiYc ctch, in~o thc substratc 70 fonning ch;lnnci 71. Two diffcrcnt ctchants m~y bc nccdcd for clching thc matcri~]s of thc mask nnd of Ihe subslratc.

7~i ln Figurcs I2 and 13 the subs~rate channcls 73 and 75 arc formcd by an ctchant that is not cffcctive on thc mask matcrial as prcviously cxplaincd rclativc to Flgure 7. A two step etching treatrncnt forms ~he channcls 73 75 and mask lips 82. The diffcrcnce in the cross-scctional shape of channcls n and 7~ is duc to crystal oricnta~ion of structure 24 as s known in the arL

In Figure 14 the thickness varia~ion is more pronounced and the tapcr rapidity is greater as comparcd to prcvious structurcs duc lo thc prescnce of the mcsa 84 forrncd in channel 73. Mesa 84 is easily formed by slripe masking the ccntral portion of channcl 73 10 and procceding further with the sc~ond ctching s~ep.

In Flgure IS a second channel 86 is forrned in substrate channel 73 employing conventiona1 selectiYe mask tcchniques. Composi~e layers 76.1 76.2 and 76.3 dcmonstrate the different shapcd contours that can be formed when scquentially dcpositing diffcrent elcmental compounds through mas& apcrlure 78. Layer 76.1 is contoured concave due to the prescnce of channcl 86. However during the growth of 1ayer 76.2 this will cventually become p]anar due to thc prcsence of the apcrturcd mask 72. Continued growth of ]ayer 76.2 will bccome convex contourcd so that layer 76.2 will have an eye shapcd contour.
The final layer 76.3 has an eveD more convex contoured as the growth rcaches theaperture 78.

In Flgures IS through 18 the structurcs shown each comprise an <IOD> oriented crystalline scmiconductor (such as dopcd or undoped Gal~s) substrate 70 a single cryst~
laycr 88 (such as for example Ga~ xAIxAs where 0.3~ x<I) a singlc crysta]line mask 90 (such as for example Gal yf~IyAs whcrc Ky<0.3) a single crystal growth 92 ovcr surface of the mask 90 and a single crystal growth 94 dcpositcd through the aperture 96 of ~he rnask 90. The growth 94 forrns a spatial variation in tapercd contour and in thickness as i~lustratcd by the contour 98.

ln the slructures of Figurcs 16 and I7 thrcc diffcrcn~ ctching stcps arc pcrforrncd prior to growth. First therc is sclective ctching of the mask 90 to form lhc mask apcrture 96.
The complction of Ihis stcp is illustratcd in Figure 34 as prcviously discusscd. The sccond stcp is thc elching of thc channcl 93 through 13ycr 88 and into thc substralc 70.
lllc third stcp is thc ctching of laycr 88 through thc mask apcrtusc 96 to f~rm ch lnoel 9I in laycr 88 producing thc cantilc~cr lips 95.1he structurc shown in ~i~ure 18 diffcrs from thosc of Figurcs 16 ~nd 17 in that thc sccond c~ching stcp is not pcrforrncd i.e.

9t7~L~

thcrc is no channcl 93. lhc struclurc of Figurc 18 is the samc as that shown in the photomicrograph of Figurc 35 cxccpt for initial substratc oricntation.

To bc no~cd from Figurcs 16 through 18 is the diffcrcnccs in thc dcgrcc of tapcr rapidity 98 and growth thickncss of growth 94 duc to diffcrcnccs in the width of thc apcrturc 96, the thickness of thc mask 90, thc thickness of ]aycr 88, the volumc and width of channel 91 and thc prescncc or abscncc of thc substratc channcl 93.

While the mask structures of Figures 11-18 are charactcrizcd as intcgral, the ovcrgrowth 0 72 and 94 and cvcn the masks 72 and 90 may be rcmoved, as by wet or dry ~pla~rna) ctching, bcfore comp]etion of furthcr fabrication processes.

In Figures 19 through 3L the structures shown each cornprise an oricntcd crystalline scmiconduc~or substrate 70, such as, GaAs, a rcmovable mask structure having an aperturc 105, and a rcsultant growth 100 forrncd on the surface of the substrateemploying MO-CVD system 10. These rcmovabl- mask structures may be fabricated from Si GaAs, SiC, Graphite, SiO2, Si3N4, '~'23 as well as many other types of matcria]s. Each of the mask structures shown in ~ese figures has a dif}~rcnt attribute. In some cases there is a modification to the substrate depositing surface. These dif~crcnt configurations illustrate how variations in the mask paramelers and geomctry aree~lp~oyed to control and produce desircd spatial variations i11ustrated by the contour 102 of each of the growths 100.

in Figure 19 thc mask 97 is provided with an apcrturc 105 and an undcrgro~ve or 2s channcl forming the lips 107 and chamber 95 when the mask is positioncd on the surface of the structure 24. Masl~ 97, as positioncd on subslrate 70, lis simi]ar to the intcgrai mask configuration shown in Figure 18. Thus, thc spatial variation of resultant growths 98 and 1ûO in thcse Figurcs arc quite sirnilar.

In Figure 20, masl ~9 has a flat configuration. Masl s }01 and 103 of Figurcs 21 and 22 are also flat mask configurations with mask ]û} having an ou~wardly bevclcd mas~apcrturc 105.1 and mask 1a3 having an inwardly bcvclcd mask apcrture 105.2. To bc notcd is thc diffcrcncc in thc contour of thc growths 102 duc to thc diffcrcncc in thcse mask apcnurcs. ïllc rapidity of spatial variation rclative to thc contourcd curYature and thickncss of the growth 100 is quitc pronounccd in ~igurcs 20 and 21 as cornparcd to the samc spaIial ariation for thc groulh ~00 in Figurc 22.

~.$~t~

In Figurc 23 mask 104 is of similar configuration lo mask 97 of Flgurc 19 but has a mask apcrturc 105.2 îike ~hat shown in Fi~ure 22. Thc spatial variation rclative to the tapcrcd contour is similar ~o that Or growth 100 in Figure 22 but is of rcduced taper rapidity. The tapcr rapidity can bc incrcascd for growth 100 as cvidcnccd by Figure 24 5 by incrcasing thc apcrturc width and thc cxtcnt or volumc of chamber 95 .

In Figure 25 mask 106 has an upwardly bcvclcd apcrture 105.1 in combination with a mask cavity to form chambcr 95. Mask 106 is also providcd with lips 107 that include rccess 109. Reccss 109 provides for a thinncr mask thickness at the apcrture 105.1 which 0 will provide a largcr growth 100. To be notcd is the extcndcd nature of the growth 100 as comparcd to growth 100 in Figure 21 but ha~ ing a larger tapcr rapidity as compared with the growths of Flgurcs 23 and 24.

In Figure 26 there is shown a composite mask 108 consisting of t~vo componcnts.
Component 108.1 may comprise fior example graphite Si or GaAs. The thinner componcnt 108.2 may comprise SiC SiO2 Si3N4 or Al203. A prcfcrrcd cornbination o~
malcria]s for componcnts 108.1 and 108.2 is graphite for componcnt 108.1 with SiC for componcnt 108.2 bccause these matcrials can bc made to match rclativc to strain and thcrrnal cxpansiorL The advantage of this composite mask is that the largcr component 108.1 is a rigid support for the much thinncr mask component 108.2. The composite mask 108 is designcd for fabricating much smaller dimcnsional growths wherc the ma~k aperture 105 may be Icss than 6 um wide and the channcl about 3 um dcep.

Composite masl~ 108 is fabricatcd by first ctching the channcl pattcrn 108.4 in the bottom 2s of component 108.1 of oricntcd crystallinc silicon. Next a film of SiO2 (such as 0.1 to 2 um~ is dcpositcd on the etchcd surfacc forming componcnt 108.2 Thir(i an SiO2 sclective etch is pcrforrncd to form the mask aperture 105. Last the oppositc sur~ace of componcn~ 108.1 is e~ched to forrn the rcsess 108.5. thc eLchant uscd does not etsh component 108.2. Composi~c masl~ 108 is a simple struclure to produce and has its grca~cst ulility in fabricating micro growths 1~.

Figurcs 27 through 31 disclosc rcmovablc masks employcd ith channclcd slructurcs 24.
In cach of thcsc figurcs growth 1~ occurs in thc channcl 111 of the subs~ratc 70 below thc apcrturc 105 of the mask s[ructurc producing diflcrcnL dcsircd spatial ~ariation in ~he contours 102.

7~5i In Figurc 27, thc substratc 70 is providcd with a channcl 111. Mas~; 110 has a tlat configuration. ïlic channcl 111 pcrrnits the growth to sprcad lalcra11y bcnca~h thc mask during dcposition.

llle configurations of Figurcs 28 and 29 are similar to Figure 27 cxccp~ for the masX lip.
Substrale 70 is also providcd with a channcl 111, as in thc casc of Figure 27, but the channcl furthcr includcs thc mcsa 113. The mask lips 107 are formcd by sclcc~ivc ctching a channc7 into a surface of thc maslt struc~ure 112, 114. This cn1argcs the vo1umc of the charnbcr formcd bclow the mask whcn the mask has becn positioncd on the substrate 70 0 thereby pcrmitting enhancemcn~ of lhe latcral extcnt of the dcposition. 2n Figure 29, the mesa 113 is narrower in width than mesa 113 in Figure 2B.

In Figure 30, the sLructure 24 and rnask 110 are gcomctrically thc same as that shown in Figure 27, except that channe1 111 is provided with a pair of mesas 115 having atriangular cross-section. This configuration wi11 providc for h;gh and a~rupt spatia2 variations in the contour 102.

In Flgure 31, substrate 70 has a V-shaped channcl 117. Mask 112 of Flgure 28 is emp]oycd lo provide the channe2 or spacing 119. Growth ]00 compriscs three layers 100.1,100.2 and 100.3. Additiona2 layers may be dcpositcd on ]ayer 100.3. Bccause of the mask apcrture ]05, the cx~ent of channe1 119 and the presencc of channel 117, a M~
CYD fabricatcd device may ~e provic2cd ~vith a complctcly buried s~rip in the form of layer 100.2. For cxarnple, strip lOQ2 may bc dopcd or undopcd GaAs and function as the s~rip active ]aycr or region in a strip heterostructure injccLion ]aser.
~5 Rcference is now made to Figures 32 and 33 which disc]ose scmiconductor hetcrostructure injection lasers ~abricated in MO-CVD cmploying the removab]e mask techniqucs just discussea ~n Figures 32 and 34, hctcrostructurc injcction lascrs 120A and 12013 ct)mprisc substrate 122 of n-~aAs and cptiaxia] growth 126. Growth 126 compriscs n-Gal xAIxAs cladding laycr 126.1, undopcd GaAs active laycr 126.2 and p-Gal xAlxAs cladding ]aycr 126.3.
Additional Ga~s/GaAlAs laycrs may bc providcd in thc structurc as contact and atlditional cladding laycrs. as is wcl] known in the ar~
Grt)wlh 126 is rormcd in channcl 124. A rcmovablc mask, such as, mask llO or 112, may , .

be employed and is positioned on the surface o~
substrate 122 during growth in reactor 18. After completion of the deposition of growth 126, the structure is removed from reactor 18 and the mask is removed. Selecti~e proton or ion implant 128 is performed to form the insulating barrier, indicated by the dotted line in each of the Figures, leaving the semiconductive channel 136 for current confinement to the active radiation emitting region 126.20 of layer 126.2. Such a current confinement technique is known in the art. It should be not~d that the implant penetrates through the active layer 126.2 but is sufficiently far enough from the lasing region 126.20 so as not to interfere with the operation of laser 120A, 120B.
The metalized layer 130 is, then, deposited on the top surface of the device and metal contacts 132 and 134 appropriately fixed. In the case of the metalization 130 in Figure 33, there is a break 138 in the metalization due to applying the metalization vapor ~rom an angular position as indicated by arrow 139.
These two laser structures demonstrate how removable masking in MO-CVD permits controlled optimi~ation of the spatial variation 127 of the nonplanar active region 126.20 with desired taper rapidity and active region thickness in accordance with the teachings of U.S. Patent No. 4,355,461.
Figures 37, 38 and 39 are photomicrographs of heterostructure injection lasers fabricated in MO-CVD
using an integral mask. The laser structure of Figure 37 differs from that of Figure 39 by the material used for the mask. In Figure 37~ the mask is single crystalllne material (Ga~s), whereas in Figure 39, the mask is an amorphous material (SiO2). As a result, the growth on the amorphous mask will be polycrystalline while the growth on the single crystalline mask will be single crystalline, which is evident from an examination of these Figures~

7~i An added advantage of the integral mask laser structures is that the presence of the mask aperture, which provides for in place, "automatic" alignment over the desired lasing region of the device. This greatly simplifies subsequent current confinement procedures and subsequently applied metalization. There is no necessity of an intermediate step to determine the center point oE the deposited growth beneath the mask aperture.
In Figures 37 and 38, heterostructure injection laser device 140 is fabricated as follows: One starts with the structure shown in Figure 35, the fabrication of which has been previously explained relative to Figure 8. This structure comprises substate 48 of C100 oriented n-GaAs with the etched channel perpendicular to the (011) cleavage plane, layer 50 of undoped GaO 4Alo 6As and mask 52 having an aperture 56 and etched chamber 57 formed undex mask lips 58. Layer 50 and mask 52 may be fabricated of single crystalline materials having electrically insulating properties, such as, oxygen or Ge doped GaAs and GaAlAs.
The Figure 35 structure is next placed on the susceptor 20 in reactor 18 and layers 1~2-150 are deposited forming the laser structure 141 in chamber 57 beneath the mask aperture 56. These layers comprise base layer 142 of n-GaAs, cladding layer 144 of n-GaO 7Alo 3As, active layer 146 of undoped GaAs (active region 146.1 being part of laser structure 141 while the remaining portion 146.2 of this layer being deposited on the mask 52, as in the case of the other sequentially deposited layers), cladding layer 148 o p-GaO 7Alo 3As and contact layer 150 of p-GaAs.
Conventional polishing, metalizations for contacts, cleaning and bonding is then performed. A Cr-Au metallization is shown at 152 in Figures 37 and 38.
Because of the size of chamber 57, the width of mask aperture 56 and the control of the deposition rate, the spatial variation of active region 146.1 may 7~

be controlled in accordance with the teachings of U.S.
Patent 4,335,461.
In Figure 39, the heterostructure injection laser device 160 is fabricated as follows. A 0.15 ~m thick SiO2 layer 164 is deposited by electron beam evaporation on a clean ~100~ oriented Si doped GaAs substrate 162~ An 8 ~m aperture 166 is then formed in the SiO2 layer 164 by conventional photolithographic techniques and plasma etching. Next, about a 3 ~ m deep channel 168 is etched into the GaAs substrate by 5~
solution of H2SO4:H202 and H2O. The underetching below the SiO2 mask 164 during this etching step creates the chamber 170 defined by the cantilevered lips 173 of mask 164 formed over the channel 164. The extent of each lip 173 is about 1.5 ~m.
Since the aperture 166 in the mask 164 is narrower than the extent of chamber 170, the growth rate during deposition at the center of the channel 168 is faster than the growth rate in the channel beneath the mask lips 172.
During growth in system 10~ the following single crystal layers are sequentially deposited through -the mask aperture 166: cladding layer 172 of n-GaO 7Alo 3As, active layer or region 174 of p or n doped or undoped GaAs, cladding layer of p-GaO 7Alo 3As and contact layer of p GaAs~
During growth, polycrystalline material, comprising the compounds of layers 172-178, is deposited on surface of mask 164 forming a polycrystalline electrically insulating layer 180.
The presence of the mask aperture 164 causes the materials to be deposited with a curved tapered contour in channel 168. Also, as the polycrystalline material forming layer 180 is deposited on mask 164l the aperture 166 narrows in width and thereby acts to increase the thickness of active layer 174 in the center of channel 168 as compared to lateral regions of the same layer (al~hough this is difficult to discern from the micrographs because the thickness variations 22a are very small~. Thus, the ratio of the diminishing aperture width to the depth of each of the grown layers determines the flnal thickness variation that will occur laterally along each layer. This taper and thickness spatial variation provides lateral waveguidance, as taught in U.S. Patent 4,335,461.
The measured light output versus current characteristics at 300 K under pulsed operation (100 nsec pulse 1 kH2 repetition rate~ of various fabricated laser devices 160 produced linear optical power output up to 130 mA and a power output per facet in excess of 15 mW. In some cases, some of the fabricated devices 160 had a current threshold ranging from 32 to 42 mA.
Figures 40 and 41 illustrate how more refined stripe alignment and current confinement may be provided in the previously described laser devices 120, 140 and 160.
In Figure 4n, MO-CVD fabricated laser device 190 includes laser structure 191, generally identical to structure 141 in Figure 37 at 141, deposited on substrate 48. Deposit of single cxystalline materials, forming layer 192, on the sur~ace of mask 52 has occurred during epitaxial growth of laser structure 191. Upon completion of this growth, but prior to deposit of metalization 152 and contacts 132 and 136, an electrically insulating layer 194 is deposited over the entire exposed surface of the device 190, forming deposited layers 194.1 and 194.2O Conventional metalization techniques can now be applied. The mask lips 58 with deposits will serve as a shadow mask and metalization vapors ~ill not penetrate into the open regions of chamber 57.

":

Onc rni~ suspcct a scrious drawback if insulating matcrials uscd in lhis growth proccss in systcr~ 10 mighl causc contamination, e. g., Lhc subscqucnl dcposition of oxidcs or nitritcs a~cr the dcposi~ion of III-V or Il-VI clcmcnts or compoun~is. ~lowcvcr, Si3N~
laycrs ~94 havc bccn succcssfuly ~rown aftcr thc dcposition of thc 1ascr structurc 191.
S Si3N~3 is onc of thc casicst insulating compounds to grow in thc MO-CV]? systcm 10 at ~his poir~1 since thcir gas mixtures (5% SiH4 in H2 and NH3) arc bclicvcd to bc the Ieast ellcctcd lby thc background impurties alrcady prcscnt in thc the rcac~or 18. It is bclicvcd, ha~ this can bc cx~cnded ~o othcr insulating 1ayers, such as, Al;~03, SiO2 and SiC.

~0 Next, by employing an optical microscope, projcclion mask aligncr or clcctronlilhograplly, dc~cnnination can be rcadily rnade of the ccnlcr of laser s~ruc~ure 191 becausc ~f the intcrfcrcnce fringes crcatcd by the microscopc light rcflccting from Lhe surface of insula~ing layer portion 194.1. Thcse fringcs rcsult rrorn the varia~ion in the ihickness of layer 194, the color patlern at ccnter point bcing quite distinguishable. For e~ample" ;n the case of layer portion 294.1 compriscd of Si3N4, the interrcrcnce pattern could be a decp rich blue co]or at the center point and vary to lightcr blucs and other ligl~ter co]ors away from the center poinL This method of alignment perrnits theforrnatioin of the stripe l9B in the laycr portion 194.1 by photolithographic and plasma etch Icchniques. After the formation of stripe 198, the mctalization 152 may be vapor 20 dcposited~ Thus, a very confined curren~ channel can be fabricatcd to confine the current f~ow 20t~ to a small region of the ac~ive layer 293 of structure 191 thcrcby lowcring ~he a~rrent ~hreshold of the device 190.

The sam~ process ean be employcd to dctcrrnine thc centcr point of an applicd photo

2~ rcsist lau~r. For example, top layer 194.1 may be a spun photo rcsist laycr. However, because of the manner of i~s application to structure 191 having spatial variation, i.e., curved a~ntour, the photo rcsist laycr will be thinncr a~ the top of this struc~ure as compare~ to adjacent regions. lllis process for deterrnining the centcr point may be uscd whcthcr ~is position is of minimum or maximum cross-sec~ional thickness, anel rcgardless 30 of thc pærticular matcrial uscd for the top layer.

llle bca~ of confined radiation may bc polychromatic, monochroma~ic or vcry narrow bandwid~ or Or single wavclcng~ xamp]cs arc mcrcury vapor lamps or a lascr bcarn.
6)f spcci3~ intcrcst is a ]ascr bcam tlJnable to a wavclcngth within a parIicular b~ndwidth 35 or a dischargc lamp having ccrtain spcc~ral lines which can sclcc~ivcly be fillcred.

l~y projccling thc confincd radiation onlo Ihc surfacc Or thc lop laycr 194.1, a paltcrn of intcrrcrcncc fringcs is produccd, from which a dctcrrnination of thc ccntcr point of thc ]aycr can bc madc distinguishing thc ccnlcr point position by dcfinitivc rcsolution of intcnsity or color varia~ions crca~cd at this position duc to thc produccd fringcs.
Figure 41 illustratcs thc smploymcnt of this alignment and currcnt confincrncnt tcchniquc rclativc to rcrnovablc mask cmbodimcnts. Thc ]ascr structurc 214 having active laycr or rcgion 216 is grown, cmploying, for example, a mask 97 shown in Figure 19 or a mask 104 in Figure 24, on subs~ra~e 70. With ~he rcmovablc mask still in place~ the 10 c]cctsically insulating layes 218 (such as, Si3N4) is grown.

llle mask is then rcmovcd from the reactor 18 and an additional elcctrically insulatin~
layer 220 is dcposi~cd on the dcvice. Sincc the masl~ has bccn removed. layer 122 will cover the enLire surfare of the sLructure.
L~
The device 210 is then removcd from thc rcactor 18 and using thc alignmcnt tcchnique just dcscribcd and convcntional photolithographic and plasma ctch tcchniques, thc s~ripe 222 can be forrncd at the exact ccnter point of lascr stlucturc 214.

20 While the invention has been described in conjunction with spccific cmbodimcnts, it is evident that many alternatives, rnodifications and variations will be apparcnt to tllose sk;l1ed in the a~t in light of the forcgoing dcscription. Accordingiy, it is intendcd to cmbrace all such altcrnativcs. modifications, and variations as fall within thc spirit and scopc of the appendcd claims. An altcrnaLiYc cxample of a mask structure is the provisio~
2s of a cavity or channel cxtcnding from one end to the othcr in a surface Or a flat mask struc~ure. This nonplanar surface of the mask is laid face down on Lhe substratc so that an end facc of thc cavity is exposcd, as positioncd on the subsLrate. This cnd face of the caYi~y rorrns an apcrlure to the extcnt that rcactant gases can pcnctrate into space J'o~ned by thc cnd face. A tapercd stmcture can be formcd on thc substrate surfacc, such as~ ror 3D cxample, a tapcrcd optical coupler.

, .,

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method employed during the fabrication of a semiconductor device to determine the center point of a plurality of semiconductor layers deposited on a semiconductor substrate, the deposited top layer thereof characterized by lateral spatial variation in thickness with at least one position therealong having a minima or maxima cross sectional thickness, said position being said center point due to the method used in the deposition of said layers comprising the steps of projecting confined radiation onto the surface of said top layer producing a pattern of interference color fringes, determining said center point by examining said interference fringes produced by said projection and distinguishing said one position from other areas of said top layer adjacent thereto by the intensity or color variation created at said position due to said interference fringes.

2. The method of claim 1 including the step of thereafter forming a photo resist pattern at said center point.

3. The method of claim 1 including the steps of projecting a beam of radiation comprising a narrow band of radiation wavelengths onto the surface of said top layer producing a pattern of interference fringes, scanning said beam across the surface of said top layer, tuning the wavelength of said beam of radiation to enhance said interference fringes and the definitiveness of intensity fringe variations.

4. The method of claim 1 wherein said radiation is a beam of collimated laser radiation.

5. The method of claim 1 wherein said top layer comprises an electrically insulating material.

6. The method of claim 3 wherein said material is selected from the group consisting of Si3N4, A12O3, SiC and SiO2.