Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
  • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
  • H01L21/768—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics
  • H01L21/76897—Formation of self-aligned vias or contact plugs, i.e. involving a lithographically uncritical step
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/28—Manufacture of electrodes on semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/268
  • H01L21/283—Deposition of conductive or insulating materials for electrodes conducting electric current
  • H01L21/285—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation
  • H01L21/28506—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers
  • H01L21/28512—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers on semiconductor bodies comprising elements of Group IV of the Periodic Table
  • H01L21/28525—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers on semiconductor bodies comprising elements of Group IV of the Periodic Table the conductive layers comprising semiconducting material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
  • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
  • H01L21/76—Making of isolation regions between components
  • H01L21/762—Dielectric regions, e.g. EPIC dielectric isolation, LOCOS; Trench refilling techniques, SOI technology, use of channel stoppers
  • H01L21/76202—Dielectric regions, e.g. EPIC dielectric isolation, LOCOS; Trench refilling techniques, SOI technology, use of channel stoppers using a local oxidation of silicon, e.g. LOCOS, SWAMI, SILO
  • H01L21/76213—Dielectric regions, e.g. EPIC dielectric isolation, LOCOS; Trench refilling techniques, SOI technology, use of channel stoppers using a local oxidation of silicon, e.g. LOCOS, SWAMI, SILO introducing electrical inactive or active impurities in the local oxidation region, e.g. to alter LOCOS oxide growth characteristics or for additional isolation purpose
  • H01L21/76216—Dielectric regions, e.g. EPIC dielectric isolation, LOCOS; Trench refilling techniques, SOI technology, use of channel stoppers using a local oxidation of silicon, e.g. LOCOS, SWAMI, SILO introducing electrical inactive or active impurities in the local oxidation region, e.g. to alter LOCOS oxide growth characteristics or for additional isolation purpose introducing electrical active impurities in the local oxidation region for the sole purpose of creating channel stoppers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
  • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
  • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
  • H01L29/0684—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by the shape, relative sizes or dispositions of the semiconductor regions or junctions between the regions
  • H01L29/0692—Surface layout
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
  • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
  • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
  • H01L29/08—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions with semiconductor regions connected to an electrode carrying current to be rectified, amplified or switched and such electrode being part of a semiconductor device which comprises three or more electrodes
  • H01L29/0821—Collector regions of bipolar transistors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
  • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
  • H01L29/66007—Multistep manufacturing processes
  • H01L29/66075—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials
  • H01L29/66227—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
  • H01L29/66234—Bipolar junction transistors [BJT]
  • H01L29/66272—Silicon vertical transistors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
  • H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers
  • H01L27/04—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body
  • H01L27/08—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including only semiconductor components of a single kind
  • H01L27/082—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including only semiconductor components of a single kind including bipolar components only
  • H01L27/0823—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including only semiconductor components of a single kind including bipolar components only including vertical bipolar transistors only

Definitions

• the present invention relates to processes for the fabrication of very small integrated bipolar transistors, and more particularly to a self-aligned process for fabricating a bipolar transistor with polysilicon contacts.
• Device horizontal geometry depends largely on the photolithographic techniques and tools available.
• the resolution provided by a particular photolithographic process determines the minimum feature size that can ordinarily be made in each masking step. Additionally, at very small feature sizes, alignment between mask steps becomes very critical.
• improvements continue to be made conventional photolithography provides reliable resolution down to just under 2 urn. Similarly, obtaining alignment tolerances much below 0.5 um. is very difficult with conventional equipment. As a result, obtaining an economic yield of operative devices becomes very difficult both as device size is reduced and as number of devices per chip is increased.
• MOSFET metal-oxide-semi ⁇ conductor
• MOS metal-oxide-semiconductor
• bipolar devices can maintain gate delays of about one half nanosecond or better when capacitively loaded.
• Bipolar technologies have generally lacked the advantages of MOS technologies. Bipolar devices generally require substantially more power than MOS devices. They also typically require substantially more substrate area per device. Conventional bipolar technologies using emitter-coupled logic (ECL) typically produce transistor areas of over 250 um. 2 and dissipate over 10 mW per gate (5 mW per equivalent gate) when operated at speeds providing system propagation delays of 300-500 ps. Common packaging technologies limit total chip power dissipation to around 10 . These factors result in a substantially lower packing density than MOS devices, limiting bipolar densities to less than 10,000 transistors per chip or about 3000 gates (compared to 10-20,000+ for CMOS), or a substantial tradeoff in speed. Also, because of their complexity, prior bipolar technology yields are low compared to MOS technologies by a factor of two or more (10-15%).
• Bipolar transistors produced by conventional LSI technologies typically have a cutoff frequency f of around 3 GHz at 2mA and exhibit 50fF of junction capacitance.
• the designer may choose between an epitaxial or a triple diffusion technology.
• the epitaxial process predominates in commercial use. Examples of epitaxial processes are shown in U.S. Pat. No. 4,381,953 to Ho et al., U.S. Pat. No. 4,483,726 to Isaac et al., U.S. Pat. No. 4,433,471 to Ko et al., U.S. Pat. No. 4,495,010 to Kranzer et al. and in "A Bipolar Process That's Repelling CMOS," Electronics, December 23, 1985, pages 45-47. Examples of triple diffusion technology are described in U.S. Pat. No.
• the designer also has a number of choices of how 5 to define various device features, such as active transistor area.
• Photolithographic techniques are widely used but, as mentioned above, are limited to the resolution of photolithography.
• In the fabrication of discrete bipolar devices it is also known to employ 0 controlled undercut techniques to form device features smaller than those provided by photolithography. This technique is described by F. Morandi et al., in "Controlled Undercut Microwave Devices," SGS-Fairchild, S.p-.A., Milan, Italy, International Electron Devices 5 Meeting, 23-25 October 1968, Washington, D.C. (abstract published by IEEE 1968), page 108.
• the self-alignment techniques strive to form the horizontal features in both dimensions and for several process steps using a single photolithographic mask.
• Deposition of polysilicon on the substrate surface followed by out-diffusion of the dopant impurities into the substrate or epitaxial layer has also been used in these self-aligned techniques.
• the above-cited patent to Oh et al. uses this technique to form base regions self-aligned with base contacts provided by the polysilicon.
• Konaka et al., cited above discloses use of a highly-doped polysilicon emitter contact to form a diffused emitter.
• One object of the invention is to build an integrated circuit fabrication process which produces high density, fast, low power, bipolar integrated circuits.
• a second object of the invention is to bring to bipolar technology the MOS technology advantages of high yield, high density and low power while obtaining the speed advantages of bipolar devices.
• a third object of the invention is to enable fabrication of VLSI bipolar circuits.
• Another object is to increase the speed of VLSI circuits.
• a further object of the invention is to reduce the power requirements of high-speed bipolar transistors.
• Yet another object of the invention is to improve yield in the production of bipolar integrated circuits, particularly in LSI and VLSI levels of complexity.
• An additional object is to reduce both collector resistance and collector-base capacitance without having to trade off increased base resistance and collector-substrate capacitance.
• Photolithographic resolution problems are solved by limiting the use of photolithography to the definition of features and feature sizes that are readily within the resolution capabilities of generally available photolithographic techniques. These techniques are supplemented, where necessary to define smaller horizontal feature sizes, by a combination of horizontally additive and 8 subtractive masking techniques. These techniques are selected and controlled so as tp define, with greater accuracy, feature sizes smaller than photolithographic techniques alone can readily produce. Alignment concerns are alleviated, with increased packing density, by adopting a masking strategy which renders most alignment steps noncritical within the tolerances that are readily provided by conventional alignment equipment. In general, this is done by separating the steps which define device features in each horizontal dimension.
• a long, narrow collector region is formed in one collector masking step, the width of the collector region ultimately defining emitter length. Then, in a subsequent contact masking step, device features requiring a predetermined spacing and ' size are defined along the length of the collector region. The device features in this masking step are made long enough that they can readily transect the collector region even if the mask is misaligned at the tolerance limits of the alignment equipment. The collector region itself is made long enough to permit a similar degree of misalignment of the mask along its length. Self alignment of the collector region, the active transistor, and th collector, base and emitter contacts, is preferably used.
• the self-alignment scheme is designed to take advantage of the noncritical spacing of the preceding steps and uses steps which, themselves, are relatively simple and virtually fool ⁇ proof.
• a first pattern-defining mask is used to define the collector region, with self-aligned surrounding channel stop and surface isolation, and forming the collector in the substrate, preferably by implantation and diffusion.
• a second pattern-defining mask is used to define two spaced-apart openings which transect the collector region. Low resistivity regions are formed in the surface of the substrate within these openings, after which the openings are selectively closed, e.g., by thermal oxidation.
• Removal of the second mask exposes collector and base contact regions at opposite ends of the collector and a central active device or emitter region, in which a vertical bipolar transistor, preferably NPN, is formed.
• a single polysilicon layer can be used to form base, collector and emitter contacts, without building oxide box structures or using reactive ion etching.
• the process is carried out using a triple diffusion, rather than epitaxial, technique.
• the low resistivity region between the collector contact and active device region makes a buried layer unnecessary.
• a further improvement of the preferred process and resultant transistor enables many of the tradeoffs that designers must ordinarily make in designing integrated bipolar devices to be decoupled.
• this improvement uses what applicant refers to as a "keyhole" structure for the collector.
• the keyhole collector is defined by the first pattern-defining mask, with a collector region that i ⁇ wider in the collector contact region and adjoining low resistivity region extending to the emitter region, and is narrower in the base contact and emitter regions and intervening low resistivity region. This structure decouples the collector resistance Rex and collector-base capacitance Ccb.
• FIG. 1 is a top plan view of a silicon substrate 11 showing a collector region in which an NPN bipolar transistor is to be formed in accordance with the invention.
• FIG. 2 is a cross-sectional view taken along lines 2-2 in FIG. 1 showing initial deposition, collector photolithography, etching and field implantation steps.
• FIG. 3 is a view similar to FIG. 2, showing a nitride etch and undercut step.
• FIG. 4 is a view similar to FIG. 3, showing the steps of stripping the low temperature oxide from the nitride layer and local oxidation of the field regions.
• FIG. 5 is a view similar to FIG. 4, showing the steps of stripping the nitride layer and collector photolithography, ion implantation and drive-in.
• FIG. 6 is a view similar to FIG. 5, showing the steps of low pressure vapor depositing successive layers of nitride and low temperature oxide, contact photolithography, and successive etching of the oxide and nitride layers.
• FIG. 7 is a top plan view of the collector region as shown in FIG. 6.
• FIG. 8 is a view similar to FIG. 6, showing P+ photolithography and ion implantation steps to form a conductor in the substrate between the base contact and emitter regions.
• FIG. 9 is a view similar to FIG. 8, showing N+ photolithography and ion implantation steps to form a conductor in the substrate between the emitter and collector contact regions.
• FIG. 10 is a view similar to FIG. 9, showing the step of undercutting the nitride layer.
• FIG. 11 is a view similar to FIG. 10, showing the steps of stripping the low temperature oxide layer, base oxidation of the exposed silicon substrate, stripping of the nitride layer, and base photolithography and P implantation of the emitter and 12 base contact regions of the substrate.
• FIG. 12 is a view similar to FIG. 11, showing the steps of deposition of a polysilicon layer, photolithography and N+ implantation of the polysilicon in the collector contact and emitter regions.
• FIG. 13 is a view similar to FIG. 12, showing the steps of depositing of a third pair of successive layers of nitride and oxide, photolithography and etching of the oxide layer to selectively expose the nitride layer over the base oxide and field oxide regions.
• FIG. 14 is a view similar to FIG. 13, showing the steps of etching the nitride layer, stripping the oxide layer and local oxidation of the exposed polysilicon.
• FIG. 14a is a cross-sectional view of a resistor fabricated in accordance with the process of the invention, at the stage of fabrication of the transistor shown in FIG. 14, showing the photolithography, etching and local oxidation steps as used to define the width of the resistor.
• FIG. 15 is a view similar to FIG. 14, showing the . steps of stripping the nitride layer, photolithography and P implantation of the polysilicon in the base contact region.
• FIG. 16 is a view similar to FIG. 15, showing the further steps of low pressure chemical vapor deposition of a fourth pair of layers of nitride and oxide and photolithography and etching to selectively expose the polysilicon contact layers in the collector and base contact regions and the emitter region.
• FIG. 16a is a cross-sectional view of the resistor taken along lines 16a-16a in FIG. 13a, showing the photolithography and etching steps at a stage of fabrication corresponding to FIG. 16 as used to define the length and contact regions of the resistor. '
• FIG. 16b is a cross-sectional view taken along 'the 13 same plane as FIG. 16, showing fabrication of a substrate contact in accordance with the process of the invention.
• FIG. 17 is a view similar to FIG. 16, showing metallization of the contacts.
• FIG. 18 is a top plan view to scale which is a composite of the views of FIGS. 1 and 7, the first (collector) and second (contact) pattern-defining masks shown in solid lines, and the polysilicon patterning mask (see FIG. 13), shown in dashed lines.
• FIG. 19 is an electrical schematic model of a bipolar transistor, illustrating the parameters affected by the keyhole transistor structure.
• FIG. 20 is a view similar to FIG. 18, showing the a composite of the first and second pattern-defining masks and polysilicon patterning mask as used to form a keyhole transistor in accordance with the improved invention.
• FIG. 21 is a top plan view of the keyhole transistor structure produced by the collector and contact feature masks of FIG. 20, shown at a stage corresponding to FIG. 11.
• FIGS. 1-18 relate to the fabrication of a bipolar transistor, in accordance with the method of the present invention, on the silicon substrate in a predetermined, rectangular collector region 20.
• a plurality of rectangular regions 20 of uniform length and width are positioned at a uniform spacing.
• region 20 is shown as 3.4 um. wide and 12 um. long.
• a suitable spacing is provided by an 8 um.
• pad oxide (Si0 2 ) layer is thermally grown on the surface of the substrate by dry oxidation, as known in the art.
• the bulk of the substrate is identified by reference numeral 21, with a reference surface 22, upon which the process steps are carried out.
• the pad oxide layer is not shown, but is understood to form surface 22 of the substrate.
• the process commences with low pressure chemical vapor deposition (LPCVD) of a layer 24 of nitride, followed by a layer 26 of low temperature oxide (Si0 2 ). Each of these layers is deposited to a thickness of approximately 1,000 angstroms.
• LPCVD low pressure chemical vapor deposition
• Si0 2 low temperature oxide
• a collector photolithography etching step In this step, the region in which the bipolar transistor device is to be formed is covered with a layer of photoresist 30 which is rectangular in shape and has long, narrow dimensions, e.g., 12 um. x 3.4 um.
• the oxide layer 26 is etched away around the photoresist to expose the surface of nitride layer 24.
• HF-etch is used in all oxide etching steps.
• a field implantation step in which the field regions surrounding the regions covered by photoresist layer 30 are ion implanted with boron to a depth of about 4,000 angstroms and a P+ concentration to form implanted channel stops 32.
• the next step is a first nitride etching step, to remove the nitride layer from the exposed field regions of the substrate.
• This etching step is carefully controlled to provide an undercut or recess 33, beneath the margins of oxide 15 layer 26, for a distance 35 of about 5,000 angstroms.
• the photoresist layer 30 is removed.
• This and subsequent nitride etching and stripping steps are carried out in a refluxer with a boiling (158°C), concentrated phosphoric acid bath.
• the etch rate is about 40 angstroms per minute.
• the etch duration is controlled to determine the amount of undercut.
• the amount of undercut is preferably controlled to within plus or minus 20%. This undercutting spaces or insets the edge of the nitride layer relative to the inner boundary of implanted channel stops 32.
• the wafer is next subjected to an etching step to strip off the remainder of oxide layer 26, leaving the upper surface of nitride layer 24 exposed. Then, the wafer is subjected to an oxidation step at 950°C for five hours locally to oxidize the exposed field regions, and thereby form field oxide layer 34. This step pushes a "bird's beak" oxide formation 36 beneath the margins of the nitride layer. The distance 37 of the protrusion of the bird's beak formation beneath the nitride is controlled, by controlling the thickness 38 to which the field oxide is grown, to precisely define the final dimensions of a narrow, reduced collector region.
• the oxide layer is preferably grown to a thickness 38 of about 8,000 angstroms, and the bird's beak correspondingly intrudes a distance of about 5,000 angstroms beneath nitride layer 24.
• the initial collector region width of 3.4 um. is reduced by an average of 1.0 um. by the nitride undercutting and then by an average 1.0 um. , providing an average final width of 1.4 um.
• the length of the collector region is reduced to about 10 um. This produces a reduced collector region 20a, as shown in FIG. 7.
• the channel stop ions diffuse to form a broadened channel stop 40 16 of a P doping concentration. In a later step, shown in FIG.
• a collector region implant 43 is diffused downward and laterally toward the channel stop. With the foregoing initial spacing, the resulting diffusion 44 forms a wide, shallow-gradient P-N junction with channel stop 40. This wide junction exhibits less capacitance than appears in epitaxial processes.
• the foregoing steps employ a variation of the known LOCOS methods (see Philips Research Report 26, pages 162-63).
• the prior LOCOS methods were developed in MOS technologies to define the dimensions of MOS source and drain regions. They are not believed to have been used before in fabricating bipolar devices, particularly to critically define a small, nominally 1.4 um. emitter length and a spacing of the edges of the active device region from the junction between the diffused field channel stop 40 and collector region 44.
• the undercutting and local oxidation steps are statistically independent and variations are characterized by a Gaussian distribution. Controlling each to within, e.g., 20% tolerances (3 sigma), the combined precision of definition of the emitter- length will be satisfactorily maintained at 1.4 um. +/-.28 um. , i.e., between about 1.1 um. and 1.7 um.
• channel stop 40 remains spaced sufficiently from reduced collector region 20a over the foregoing range of variation.
• a second nitride etch step is used to strip nitride layer 24, exposing substrate surface 22.
• a second photolithography step in which the field oxide layers are covered with a layer 42 of photoresist while leaving the silicon surface exposed in the reduced collector region.
• This step is followed by a collector region implant step, in which phosphorus ions are implanted to a depth of about 4,000 angstroms and an N+ 17 concentration to form implanted collector region 43.
• Photoresist layer 42 is then stripped and the wafer is annealed at a temperature of 1100°C for two hours, to diffuse or drive in the collector implant ions to a depth of about 1.4 um. This forms a much broader and deeper diffused collector region 44 with an N doping concentration.
• a 1,500 angstrom layer 46 of nitride (Si-N.) followed by a 1,000 angstrom layer 48 of oxide (SiO-) are low pressure chemical vapor deposited on the entire surface of the wafer, including over the field oxide and exposed substrate surfaces 22.
• the oxide layer 48 is etched within openings 54, 56, the resist is stripped, and nitride layer 46 is etched through these openings to selectively expose portions of substrate surface 22 within the openings.
• openings 54, 56 are rectangular and parallel, with a spacing 58 and a width 60 of, e.g., 2 um. and a greater length 62, e.g., 4 um.
• the regions thus defined in this step are identified as the emitter region 64, the collector contact region 66, and the base contact region 68.
• Their longer dimension 62 is oriented approximately normal to the longer dimension of the collector region 20a.
• the spacing 58 between openings 54, 56 defines the emitter width which, for high density VLSI applications, is preferably 2 um. or less.
• the 2 um. width and spacing of the openings are selected to be easily within the resolution capabilities of conventional photolithographic techniques. As these techniques improve, these dimensions can be reduced to scale the present method 18 to smaller device geometries.
• openings 54, 56 in relation to the length of collector region 20, and the fixed spacing 58 in a single mask of such openings makes alignment in this step substantially noncritical.
• these openings are centered both widthwise and lengthwise on reduced collector region 20a, but need not be.
• the openings can be displaced from a centered position, either lengthwise along region 20a, or widthwise of the region. So long as openings 54, 56 transect region 20a with some minimal length, e.g., 1 um., of region 20a at each end to form collector and base contacts, an operative device can be fabricated.
• widthwise and lengthwise alignment tolerances of +/-lum. are permitted while conventional alignment equipment can readily meet +/-0.5 um. tolerances.
• a fourth photolithography step is performed to provide an implant mask, photoresist layer 70 defining an opening 72 which encompasses opening 54. Boron ions are implanted through opening 54 to form a P+ low resistivity region or conductor 74 from an active transistor, to be formed in emitter region 64, to the base contact region 68. Implantation is conducted at relatively low energies so that conductor 74 is relatively shallow, e.g., 1,000 angstroms as implanted. At that energy level, the ions do not penetrate the nitride and oxide layers 46, 48. Consequently, the alignment of implant mask for this step is also substantially noncritical.
• photoresist layer 70 is removed.
• the mask and ion implant procedure illustrated and described in FIG. 8 is repeated to 19 implant phosphorus ions through opening 56 to selectively implant within a larger opening 76 in photoresist 78 a shallow, e.g., 1,000 angstroms as implanted, N+ low resistivity region or conductor 80 between the emitter region 64 and collector contact region 66.
• photoresist layer 78 is removed.
• the wafer is subjected to an etching step, like that described in connection with FIG. 3, to undercut nitride layer 46 to form recesses 82 under oxide layer 48.
• This step is controlled to space the edge of nitride layer 46 a predetermined distance 84 (e.g., 2,500 angstroms) from the implant boundaries of conductors 74, 80.
• This step reduces the as-masked dimension of emitter region 64 from 2 um. to about 1.5 um.
• oxide layer 48 is stripped.
• the wafer is subjected to a thermal oxidation step (900°C, 1/4 hour) to form thin (e.g., 1,000 angstrom) base oxide films 86, 88 on the exposed substrate surface in openings 54, 56, respectively.
• These films overlap implanted conductors 74, 80, respectively, by an amount determined by the distance 84 of undercut 82.
• This step produces some additional reduction in width of the emitter region, e.g., to about 1.0 um.
• the implants forming conductors 74, 80 also slightly diffuse downward and laterally.
• nitride layer 46 is stripped. Then, a fifth photolithography step is performed to provide a layer 90 of photoresist covering collector contact region 66 and extending over a portion of base oxide film 88, while leaving the emitter region 64 and base contact region 68 exposed. This is followed by a base implant step in which regions 64, 68 are implanted with boron ions to form a base implant 92 'in emitter region 20
• Implantation is conducted at energy levels comparable to those used in performing the P+ implant in FIG. 8. The doping concentrations, however, are 5 somewhat lower, producing a P concentration. Some of the implanted ions penetrate the base oxide layers 86, 88. This somewhat increases the doping concentration in P+ region 74. It somewhat reduces the N-type doping concentration of region 80, although such region
• implants 92 and 93 are connected beneath the margins of base oxide layers 86 to P+ region 74.
• the doping profile is further illustrated and discussed in
• photoresist 90 is removed and the pad oxide (not shown ) is stripped.
• a thin (e.g., 1,000 angstrom) layer 94 of LPCVD, undoped polysilicon is applied to the entire surface of the
• photoresist layer 96 having an opening 98 over emitter region 64 and an opening 100 over collector contact region 66. These openings are sized, spaced, oriented and aligned in the same way as openings 54, 56 (FIGS. 6 and 7). Arsenic ions are implanted through these openings
• resist layer 96 is stripped.
• a nitride layer 102 and an oxide layer 104 are deposited, by low pressure chemical vapor deposition, over the entire substrate.
• This is followed by a seventh photolithography step, to define a photoresist layer 106 selectively covering the emitter region 64 and the collector and base contact regions 66, 68, while leaving openings 108 over the field oxide and openings 110, 112.
• openings 110, 112 are noncritically aligned over the base oxide regions 86, 88, respectively.
• Oxide layer 104 is etched through these openings.
• resist layer 106 is stripped and nitride layer 102 is etched. Then, oxide layer 104 is stripped. This procedure leaves the upper surface of polysilicon layer 94 exposed atop the field oxide 34 and over base oxide regions 86, 88. The polysilicon layer 94 over the emitter region 64 and collector and base contact regions 66, 68 is shielded by the remaining portions of nitride layer 102.
• the wafer is subjected to an oxidation step at 950°C. for one hour to locally oxidize the exposed portions of polysilicon layer 94.
• the polysilicon is converted to polyoxide 114, which insulates the remaining regions of polysilicon 94 over the emitter region 64 and collector and base contact regions 66, 68 from one another.
• the oxidation step also drives ions into the substrate from the doped regions of the polysilicon in collector contact regions 66 and emitter region 64.
• the out-diffusion of arsenic ions from the polysilicon in the emitter region forms the emitter 116 of the transistor. It also forms the collector contact conductor 118. This diffusion is launched only from the interface of the polysilicon layer with the reference surface.
• Diffusion provides a concentration gradient that is greatest at the reference surface. Consequently, formation of a parasitic, off-vertical, edge transistor surrounding the vertical transistor is minimized. Therefore, the performance characteristics (i.e., breakdown voltage, speed) of the vertical transistor are not degraded by presence of a parasitic transistor.
• nitride layer 102 is stripped.
• the polysilicon layer 94, exposed in this opening is then shallowly implanted with boron ions to render the base contact conductive.
• photoresist layer 120 is stripped.
• a thin (250 angstrom) and nitride layer 124 and a 2,000 angstrom oxide layer 126 are deposited by low pressure chemical vapor deposition.
• This deposition step is followed by a ninth photolithography step, in which a layer of photoresist (not shown) is applied and patterned to form an opening 128 over the entire collector region.
• this step serves to define other devices (see FIGS. 16a and 16b) formed on the substrate in preparation for metallization.
• the oxide layer 126 is etched through opening 128, the photoresist is removed, and the nitride layer 124 is etched to expose the upper surfaces of the polyoxide layer 114 and polysilicon layer 94 in the emitter region 64 and collector and base contact regions 66, 68.
• metallization is preferably accomplished by an additive process such as ion plating.
• the metallization lines are sized and spaced at a 4 um. pitch, but can be scaled along with preceding steps in the process as 23 photolithographic techniques permit.
• the particular metallization process and structure form no part of the present invention and so are only briefly described.
• the preferred metallization method and structure is conventional (see, for example. Summers, D., "A Process for Two-Layer Gold IC Metallization," “Solid State Technology,” December 1983, pages 137-141, and references cited therein) .
• this metallization process calls for depositing a thin film of palladium, reacting the palladium with the exposed surface of the polysilicon to form a suicide, and stripping the unreacted palladium.
• a thin layer of a barrier metal such as titanium tungsten (TiW) is deposited, followed by deposition of a plating layer of palladium.
• TiW titanium tungsten
• This is followed by a metal photolithography step and ion plating a thick layer of gold onto the palladium to form emitter contact 130, collector contact 132 and the base contact 134. Additional insulative and conductive layers may be applied to form multilayer interconnects, as will be understood by those skilled in the art.
• a resistor 140 is formed atop field oxide 34 at a location spaced conveniently (e.g., 4 um. pitch) from other structures.
• the polysilicon layer 94 deposited in the steps shown in FIG. 12, is patterned in the manner as shown in FIG. 13. Then, as shown in FIG. 14a, photoresist 106 and oxide layer 104 are removed, leaving an elongate strip of nitride layer 102 shielding a narrow portion of the polysilicon layer 94.
• the width of this strip is conveniently set at 2 um., and initially defines the width of resistor 140.
• the exposed portions of the polysilicon are oxidized. This step isolates resistor 140 and defines a reduced, final width of the resistor, about 1.7 um. 24
• the nitride layer over the resistor is removed.
• the resistor polysilicon is then implanted with P-type boron ions simultaneously with implanting the base contact region 68.
• nitride and oxide layers 124, 126 are deposited and patterned as shown in FIG. 16a.
• This step defines the length 142 of the resistor, which can vary as required by the circuit design. A typical resistor length is 6.5 um.
• This step also provides contact openings at each end of resistor 140 for contact by metallization lines in the manner shown in FIG. 17. The contact openings are preferably sized and oriented in the manner shown in FIG. 7 so that alignment is noncritical.
• FIG. 16b shows how a substrate contact 150 is formed in the disclosed method.
• a contact is formed in the same manner as a collector region, described above, with the differences next described.
• the region where the substrate contact is to be formed is left covered by photoresist 40, so that it does not receive an N-type collector implant. It is covered by nitride and oxide layers 46, 48 and remains covered throughout the steps shown in FIGS. 6-10.
• the region is opened for P-type implant 93a.
• the doping profile of implants 93, 92a has a peak, as-implanted concentration at a predetermined depth illustrated in FIG. 16b by dashed line 95. This depth is preferably about 1,000' angstroms and is determined by controlling implant energy. This depth is essentially constant in both silicon and silicon dioxide.
• the implant can extend beneath the bird's beak 36 but, because of the orientation of the surfaces of the bird's beak structure, the P-doped region terminates at the 25 silicon-silicon dioxide interface as shown.
• nitride, oxide and photoresist layers 102, 104, 106 to pattern the polysilicon contact and to isolate it by local oxidation of the surrounding polysilicon.
• the substrate contact region is then opened in the steps of FIG. 15 for P-type implantation of the polysilicon layer.
• nitride and oxide layers 124, 126 are deposited, masked and patterned to selectively expose substrate contact 150. This step provides a contact opening for contact to the substrate by a metallization line in the manner shown in FIG. 17.
• LSI and VLSI bipolar circuits have been successfully manufactured in production quantities using the foregoing process.
• One such circuit is 16 x 16 bit fixed point multiplier, produced in both ECL and TTL versions. This circuit is fabricated on a 180 mil x 180 mil die with an adder array totalling 11.6K transistors in an area of 121 mils x 135 mils. Control circuitry, on-chip latches and ECL 10KH input/output buffers bring the total transistor count to 13.8K in a 166 mil x 178 mil area. Typical power dissipation is 1.8 W in the adder array alone; input/output buffers, latches and control circuitry raise the total typical power to 2.9 W (2.2 W in TTL) .
• the typical multiply propagation time for the worst-case path from input latch to output latch is 12 ns.
• two-level series gating is used with a single -5.2 V supply.
• One subcircuit used in the multiplier is a 44-transistor, carry-save adder.
• the total gate size including fifteen polysilicon
• Propagation delays are typically 300-600 ps per gate, depending on gate complexity and loading.
• transistor level At the transistor level,
• TO ' - are on the order of 5-10 fF.
• the foregoing process exhibited very high yields, about 50% averaged over 10 production runs of 10 wafers each. Higher yields on some runs indicate that yields could increase to over 60% with
• FIG. 18 shows a
• the patterned oxide layer is 27 used to pattern the underlying nitride layer 102. Unet ⁇ hed portions of layer 102 protect the . base and collector contact regions at opposite ends of the collector and the central emitter or active device region during local oxidation of the polysilicon layer. This step oxidizes the exposed areas of polysilicon surrounding the device and covering the low resistivity regions as shown in FIG. 14.
• the collector feature, identified by reference numeral 150A is rectangular, with an initial length of 12 micrometers and width of 3.4 micrometers.
• the contact-defining features, identified by numerals 154A, 156A, are rectangular.
• the latter features are spaced, sized and ideally positioned so as to divide the length of the collector feature into contact regions 64, 66, 68 (see FIG. 7) of substantially equal length and width.
• the polysilicon patterning mask features identified by numerals 164A, 166A, 168A to correspond to the contact regions, are rectangular. They are sized, spaced and ideally aligned edge-to-edge with the contact-defining features.
• FIG. 20 shows a general model of the bipolar transistor.
• the model shows an ideal bipolar transistor 170 having a parasitic series resistance Rbx in its base, a parasitic series resistance Rex in its collector, a parasitic shunt capacitance Ccb between its collector and base, and a parasitic shunt capacitance Ccs between its collector and the substrate. Shown in dashed lines are another transistor having an emitter connected to the base of transistor 170 and a load resistance R ⁇ in series with the collector of transistor 170.
• base resistance Rbx is essentially determined by the length, cross-sectional area and resistivity of the P- type low resistivity region 74.
• Collector resistance is similarly determined by the length, area and 28 resistivity of region 80.
• Capacitance Ccb is determined by the area of interface of the Nrtype collector diffusion 44 with the P-type base 92, base contact diffusion 93 and low resistivity region 74, and the relative doping concentrations about this interface. Capacitance Ccs is similarly determined by the area of the interface between the collector diffusion 44 and the P-type substrate 21 and channel stops 40 and their relative dopings. As mentioned above, spacing the collector diffusion 44 apart from the channel stops 40 helps hold down this capacitance. In transistor design, it is desirable to control base resistance as it, among other factors discussed below, limits device speed. It is also desirable to control collector resistance as it limits device saturation characteristics. In the rectangular collector design of FIG. 18, increasing collector width to decrease Rex increases the entire width of the transistor, including the base and adjoining low resistivity region. This increases the area of interface between the P-type base and N-type collector diffusions and, in turn, increases base-collector capacitance Ccb.
• Gate switching time is proportional to the time constant
• Ccb (Rbx + Rex + R ⁇ )
• the first factor indicates the dominant effect of base- collector capacitance Ccb.
• Control of load resistance R is limited because it involves tradeoffs of power and noise margin as well as speed.
• Collector resistance Rex should be reduced because of its effects on saturation.
• Base resistance R,bx should also be reduced but will not have as much effect on speed as reducing base-collector capacitance Ccb.
• the present improvement essentially decouples Rex and Ccb by allowing the 29 structural features that determine these two parameters to be defined independently.
• the collector feature is defined by a keyhole shape having a collector contact portion 15OB that is enlarged widthwise and a base contact and emitter portion that is narrowed relative to one another and to the rectangular embodiment described above.
• This arrangement allows a greater width between the collector contact and the emitter regions (e.g. 4.6 um. ) , thereby reducing collector resistance Rex, without increasing the area of base-collector interface.
• the base and emitter regions and interconnecting low-resistivity region can be narrowed (e.g., to 3.0 um.), thereby reducing base-collector interface area and the resultant base-collector capacitance Ccb.
• the contact features identified by numerals 154B, 156B, are spaced closer together (e.g., 1.8 um vs. 2.0 um. ) and reduced in the dimension lengthwise of the collector feature (e.g., 1.4 um. for feature 156B and 1.2 um for feature 154B vs 2.0 um. ) .
• the overall length of the collector feature can also be reduced, e.g. to 10 um.
• the polysilicon defining features are enlarged to increase the tolerance for misalignment with prior features.
• the emitter feature 164B is not reduced with the spacing between the contact features but is preferably lengthened to overlap the contact features.
• This provides guardbands 165 over oxide layers 154B, 156B (e.g. total overlap of 0.5 um. divided about equally on each side of the emitter region) for protecting the area of 30 polysilicon deposited on top of the surfaces of the silicon substrate 22 while leaving the polysilicon deposited on the field oxide 34, 36 and base oxide films 86, 88 exposed for later oxidation.
• This 5 guardband is sized to approximate or exceed the typical range of variation in mask alignment, e.g. 0.3 micrometer.
• a similar guardband 169 is provided to overlap the end of the collector region adjoining the base contact region.
• FIG. 21 shows the surface topography produced by the collector and contact feature masks of FIG. 20, at a stage corresponding to FIGS. 11 and 12, but without the implant masking. As discussed above in connection with the embodiment of FIGS. 1-18, the undercutting and
• the first nitride undercutting step reduces the width of the collector region about
• the field oxide 34 is formed by oxidation at 1000°C for five hours to produce a thickness of about 11,600 Angstroms and a bird's beak intrusion 36 of about 7000 Angstroms. These steps reduce physical collector length to about 8 um., reduce physical
• FIGS. 10 and 11 which produce the oxide ovals 86, 88 shown in FIG. 21, reduces the physical width of the emitter region 64B to about .8 um. Increasing the f ⁇ eld oxide thickness also reduces capacitance between s ⁇ irface metallization and the substrate.
• the area of the collector contact region 66B is substantially greater than the area of the emitter region 64B by about an order of magnitude.
• the electrical width of the N-type low resistivity region 8OB near the collector contact region is greater than the electrical length of the emitter region by a factor of about two and then, approaching the emitter region, tapers down by nearly one-half to about the electrical width of the P-type base in the emitter region. This provides a conduction path from the collector contact region to the emitter region immediately beneath oxide layer 80B that is wider than the corresponding linear structure in the first embodiment. This reduces collector resistance Rex.
• the emitter region 64B, base contact region 68B and P-type low resistivity region 74B are narrower, and thereby provide a lesser area of base-to-collector interf ce.
• the combined mask-defined area of the base contact, emitter and intervening portions of the initial collector region is about 60% of the overall area of the collector region in the first, linear embodiment. This combined area is reduced to just under 50% of the overall collector area in the keyhole design. These proportions are further reduced as the as-masked collector region is reduced by the undercutting and field oxidation steps prior to implanting the P-type low resistivity region, base contact and intrinsic base. This reduces collector- base capacitance Ccb.
• Table 1 shows a comparison of performance characteristics of the keyhole transistor configuration with the linear configuration of the first embodiment.
• Double precision ALU 33 MFLOPS 40 MFLOPS

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