AU650887B2 - A method and apparatus for treating a surface - Google Patents

Description

UNIFORM DEPOSITION OF A THEN FILM Ofl A SURFACE

The present invention relates to a method and apparatus for treating a surface and, more specifically, to a method and an apparatus for producing a thin film on a surface.

Production of thin films on surfaces, such as the surfaces of semiconductor substrates, is an art which continues to be the subject of considerable research and development. The art is also of significant technological importance as it is used during fabrication of a large number of products, including semiconductors. The thin film may be one of a number of materials, such as metals, oxides, chalcogenides, pnictides, superconductors, etc. Most of . the research and development has concentrated on providing a method and apparatus which produces a thin film having a thickness which is as uniform as possible. Uniformity of thickness is normally required to improve particular characteristics of the product containing the film and it is also advantageous for maximising yield during mass production of the product. It is also desirable to provide an efficient thin film production apparatus and method which does not rely on batch processing of the product.

A number of satisfactory techniques for depositing films on a surface, such as a substrate, are known and include chemical vapour deposition, physical vapour deposition, sputtering and spraying pyrolysis. These techniques provide the constituents of the thin film in gaseous or aerosol form. A number of thin film deposition apparatuses have been developed which employ the techniques and try to achieve substantial uniform thickness of the film. The apparatuses tend to fall into one of two classes, which correspond to two distinct directions in which development of the art has proceeded in the quest for obtaining the ideal uniform thin film production technique. One class of apparatus is those which employ elaborate mechanical devices to move the substrate relative to the flow of gas in a deposition chamber. The other class of apparatus is those which are configured to produce films having uniformity in a lateral direction across the substrate, which is substantially

SUBSTITUTE SHEET perpendicular to the direction of gas flow within a deposition chamber. To fully appreciate the advance made by the present invention, apparatuses of both classes are described herein.

The present invention provides a method of producing a thin film on a surface, comprising causing relative movement, at a constant rate, between said surface and a thin film deposition zone of a thin film deposition chamber; such that said surface passes through said zone, said chamber being configured such that deposition of film on said surface in said zone is uniform in a lateral direction across said surface, said lateral direction being perpendicular to the direction of said relative movement.

The present invention also provides an apparatus for producing a thin film on a surface, comprising a thin film deposition chamber having a thin film deposition zone, said chamber being adapted to deposit said film on said surface uniformly in a lateral direction across said surface in said zone, and means for causing relative movement, at a constant rate, between said surface and said deposition zone, such that said surface passes through said zone, said lateral direction being perpendicular to the direction of said relative movement.

Preferably the constituents of said film are provided in gaseous or aerosol form and flow through the deposition zone in substantially the same direction as the direction of said relative movement.

Preferably said chamber includes an upper susceptor below which said deposition zone is disposed.

Preferably a substrate including said surface is moved through said deposition zone.

Preferably said relative movement is reciprocal for the extent of said deposition zone.

SUBSTITUTE SHEET Alternatively, if the chamber is configured such that deposition of said film on said surface varies linearly in said lateral direction, said surface is also rotated relative to said zone about an axis of symmetry which is substantially perpendicular to said surface.

The present invention further provides a method of treating a surface, comprising causing relative movement, at a constant rate, between said surface and a treatment zone in a chamber, such that said surface passes through said zone, said chamber being configured such that said surface is treated in said zone uniformly in a lateral direction across said surface, said lateral direction being perpendicular to the direction of said relative movement.

The present invention also provides an apparatus for treating a surface comprising a chamber adapted to treat said surface in said zone uniformly in a lateral direction across said surface, and means for causing relative movement, at a constant rate, between said surface and sa^;d zone, such that said surface passes through said zone, said lateral direction being perpendicular to the direction of said relative movement.

Preferred embodiments of the present invention are hereinafter described and explained, by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a schematic diagram of barrel reactor; Figure 2 is a plan view of part of the barrel reactor; Figure 3 is a diagram of a horizontal reactor;

Figure 4 is a diagram of a horizontal reactor with a susceptor employed at the top of the deposition chamber of the reactor;

Figure 5 is a diagram of a Cambridge Instruments Limited inverted horizontal reactor; and Figure 6 is a graph of growth rate v. susceptor position for the horizontal reactor of Figure 5;

SUBSTITUTE SHEET Figure 7 is a diagram of a preferred embodiment of a deposition chamber according to the present invention; and

Figure 8 is a diagram of another preferred embodiment of a deposition chamber.

In recent years, most improvements in thin film uniformity have been achieved by making empirical iterative modifications as a result of measuring properties of the films, and mathematical modelling. A discussion of the modifications is provided in H. Tanaka, N. Tomesakai, H. Itoh, T. Ohori, K. Makiyama, T. Okabe, M. Takikawa, K. Kasai and J. Komeno, "Large-area MOVPE growth of AlGaAs/GaAs Heterostructures for HEMT LSIs" Jap. J. Appl. Phys. 1990, 29, L10; C. Takenaka, T. Fujii, A. Kuramata, S. Yamazaki and K. Nakajima, "Design of the optimum reactor chamber for uniform InP epilayer thickness profiles grown by MOVPE" J. Crystal Growth 1988, 91, 173"; and W.H. Johnson, WA Keenan and A.K. Smith, "Controlling an epitaxial reactor via thickness and resistivity measurements" Microelectronic Manufacturing and Testing 1987, November, pl7. The use of mathematical modelling is discussed in J. Ouazzani and F. Rosenberger, "Three- dimensional modelling of horizontal chemical vapor deposition. I. MOCVD at atmospheric pressure" J. Crystal Growth 1990, 100, 545; D.I. Fotiadis, M. Boekholt, K.F. Jensen and W. Richter, "Flow and heat transfer in CVD reactors: Comparison of Raman temperature measurements and finite element model predictions" J. Crystal Growth 1990, 100, 577; W.L. Holstein, J.L. Fitzjohn, E.J. Fahy, P.W. Gilmour and E.R. Schmelzer, "Mathematical modelling of cold-wall channel CVD reactors" J. Crystal Growth 1989, 94, 131; Yu.N. Makarov and A.I. Zhmakin, "On the flow regimes in VPE reactors" J. Crystal Growth 1989, 94, 537; and W.L. Holstein and J.L. Fitzjohn, "Effect of buoyancy forces and reactor orientation on fluid flow and growth rate uniformity in cold-wall channel CVD reactors" J. Crystal Growth 1989, 94, 145. Modelling in two or three dimensions is extremely complex and is currently limited by the memory and speed of supercomputers, as discussed in the first two modelling papers mentioned above, and in K.F. Jensen, D.I. Fotiadis, D. R. McKenna and H.K. Moffat, "Growth of compound semiconductors and superlattices by organometallic

SUBSTITUTE SHEET chemicai vapor deposition. Transport Phenomena" Chapter 19 in ACS Symposium Series Volume 353, pρ353-75.

Reaction chambers which are used to effect deposition of thin films usually include either vertical (also known as barrel) or horizontal basic cells. For both types of chambers, a reaction gas is passed at atmospheric or reduced pressure through the cell.

Two types of vertical cell chambers are described in a recent paper, H. Tanaka, N. Tomesakai, H. Itoh, T. Ohori, K. Makiyama, T. Okabe, M. Takikawa, K. Kasai and J. Komeno, "Large-area MOVPE growth of AlGaAs/GaAs Heterostructures for HEMT LSIs" Jap. J. Appl. Phys. 1990, 29, L10 and the specification of UK Patent Application 2,168,080 and both serve to illustrate the level of complexity designers are willing to employ to achieve good uniformity in thin film deposition. Both cells have a configuration which is similar to that of the cell 2 illustrated in Figures 1 and 2. The cell 2 includes a deposition chamber 4 where a source gas inlet 6 is provided at the top of the chamber 4 and exhaust outlets 8 are provided at the bottom of the chamber 4. A plurality of semiconductor wafers 10 are mounted substantially vertically on side facets 12 of a susceptor 14 mounted within the chamber 4. The susceptor 14 is heated by a high frequency coil (not shown) which in turn heats the wafers 10 to a predetermined temperature. Gas injected into the inlet 6 reacts with the heated wafers 10 so as to cause deposition of a desired thin film on the wafers. To achieve substantial uniformity of film thickness on the wafers 10, the facets 12 rotate each respective wafer 10 about the axis of symmetry of the wafer which is perpendicular to the plane of the wafer. In addition, the susceptor 14 is also rotated about a vertical axis 16 which is an axis of symmetry with respect to the susceptor 14. Although substantial uniformity is achieved, a fundamental disadvantage of this configuration is a uniform flow of source gas can not be provided over the surfaces of the wafers 10, even in a chamber 4 having a curved bell-jar shape. The distance between the edges 18 of the wafers 10 and the wall of the chamber 4 is always less than the distance between the wall and the centre 20 of the wafers 10, as shown in

SUBSTITUTE SHEET Figure 2, with the distance decreasing from the centre 20 to the edges 18. This gives rise to a build up in film deposition near the edges 18 of the wafers 10. These limitations are discussed in P.-H. Shih, K. Chen and YXiu, "Finite element analysis of circumferential flow and temperature characteristics in a barrel-type CVD reactor" AICHE Symposium series 1988, 84(2), 96 and in HA. Lord, "Convective transport in silicon epitaxial deposition in a barrel reactor" J. Electrochem. Soc. 1987, 134, 1227. A paper by M. De Keijser, C. van Opdorp and C. Weber, "Peculiar asymmetric flow pattern in a vertical axisymmetric VPE reactor" J. Crystal Growth 1988, 92, 33 indicates it can be demonstrated by flow visualisation and modelling that gas flow distribution in vertical cell reactors can be also asymmetric, rather than symmetric as would be predicted on intuitive grounds.

The standard horizontal cell 22, as shown in Figure 3, includes a chamber 24 having a susceptor 26 on which a wafer 28 to be treated is mounted. It includes a gas inlet 30 at one end and a gas outlet 32 at the opposite end so the direction of flow of the reaction gases is substantially parallel to the plane of the wafer 28. The susceptor 26 is heated by a high frequency coil 34. It has been shown, however, that the flow of gas within the horizontal cell 22 is not substantially uniform and is affected by gravity, heat rising from the susceptor 26 and to a lesser extent the cell geometry. These factors give rise to a thin film profile which is not uniform.

An advantage of horizontal cells, however, is that they can be reconfigured to enable continuous processing of wafers 28 by moving wafers 28 to be treated horizontally through the deposition chamber. These configurations, however, do not attend to the inherent uniformity problems discussed above and therefore are only used for the production of films which do not need to be substantially uniform. For instance, the configurations are suitable for the production of films having graded composition profiles.

To improve the uniformity of deposition for horizontal cell reactors development has concentrated on trying to achieve lateral uniformity, i.e.

SUBSTITUTE SHEET perpendicular to the direction of flow, which has produced the "inverted" horizontal reactor discussed in N. Puetz, G. Hillier and A.J. SpringThorpe, "The inverted horizontal reactor: growth of uniform InP and GalnAs by LPMOCVD" J. Electronic Mater. 1988, 17, 381. The inverted reactor falls within the second class of reactors discussed previously and includes a cell 40, as shown in Figure 4, which has a susceptor 42 mounted at the top of the chamber 44 of the cell 40. The wafer 46 to be treated is mounted on the bottom of the suscepter 42. The top of the cell 40 is therefore made the hottest part of the cell and the configuration eliminates the problems previously caused by heat rising from the susceptor 42 and the effects of gravity on deposition of the film. Although the cell 40 necessitates the use of complex mounting designs for the wafer 46 it has a number of advantages over the standard horizontal cell 22. The cell 40 yields stable laminar flow over a wide range of gas flow rates and isotherms are essentially parallel to the direction 48 of gas flow after stabilisation over an entrance length, as discussed in D.I. Fotiadis, M. Boekholt, KF. Jensen and W. Richter, "Flow and heat transfer in CVD reactors: Comparison of Raman temperature measurements and finite element model predictions" J. Crystal Growth 1990, 100, 577. The cell 40 provides very good lateral uniformity and modelling indicates the cell dimensions perpendicular to the direction 48 of gas flow can be increased to allow a larger area of deposition. The cell 40, however, still has the disadvantage that deposition in the direction 48 across the wafer 46 is not uniform.

An alternative inverted horizontal cell 60 is described in the specification of UK Patent Application 2,196,019 to Cambridge Instruments Ltd, herein incorporated by reference. The specification relates to a cell 60, as shown in Figure 5, which includes an upper susceptor 62 and a lower susceptor 66 onto which the wafer 64 to be treated is placed. H e highest temperature zone of the cell 60 is maintained at the top of the chamber 68 of the cell 60 by heating the upper susceptor 62 so it is maintained at a temperature which is considerably higher than the temperature to which the lower susceptor 66 is heated. Deposition material in the gas phase is driven by thermal and concentration gradients to the lower cooler part of the cell 60 onto the wafer 64. Experiments have shown that growth rates in the lateral direction

SUBSTITUTE SHEET perpendicular to the direction of flow 69 are very uniform at the bottom of the cell 60, however, the cell 60 still suffers from a non uniform growth rate across the wafer 64 in the flow direction 69.

The variation of growth rate (μm/hr) with respect to position on the susceptor

66 in the direction 69 is illustrated by the curve 50 of the graph 52 shown in Figure 6. The y axis 54 of the graph 52 represents the growth rate and the x axis 56 represents position on the susceptor 66 as a distance from the edge 71 closest to the gas inlet 73 of the cell 60. As can be seen from the graph 52, the variation is non linear.

An apparatus 100, as illustrated in Figure 7, employs a deposition method which attends to the thin film uniformity problems of both the horizontal and the barrel reactors described previously. The apparatus 100 includes a deposition chamber 102 having an upper susceptor 104 and a lower susceptor 106, as is the case for the cell 60 of Figure 5. The susceptors 104 and 106 are heated in the same manner as the susceptors 62 and 66 of the Cambridge Instruments Ltd cell 60. The cell 100 includes a gas inlet 108 and outlet 110 at opposite ends of the chamber 102 and a gas flow between the susceptors 104 and 106 is established having a flow direction 112. The cell 100 further includes a movable belt or platform 114 on which a wafer 116 to be treated is placed. The belt 114 moves the wafer 116 through the chamber 102 in a direction 118, which is the same as the flow direction 112. During its transition through the chamber 102 the wafer 116 is disposed adjacent the upper surface of the lower susceptor 106.

The cell 100 is configured so as to produce the same lateral uniformity and growth rate variation 50 in the flow direction 112, as the Cambridge Instruments Ltd cell 60, if the wafer 116 is held stationary in the deposition zone 105 between the susceptors 104 and 106, as illustrated in Figure 7. The apparatus 100, however, is such that the wafer 116 is moved in the direction 118 at a constant rate so each point on the wafer in the direction 118 traverses the curve 50 and experiences the same

SUBSTITUTE SHEET growth rate both in the direction 118 and the lateral direction, perpendicular to the direction 118. The amount of material deposited at each point of the substrate 116 is the integral of the curve 50 with respect to time. Therefore, longitudinal and lateral thickness uniformity of the deposited thin film is provided by moving the substrate 116 through a deposition chamber 102 at a constant rate, as long as the chamber 102 is configured so that deposition of the film on the substrate is uniform in the lateral direction across the substrate, the lateral direction being perpendicular to the direction traversed by the substrate.

Although the embodiment illustrated in Figure 7 employs a belt 114 to move the wafer 116 with respect to a stationary deposition chamber 102, it is the relative movement of the wafer 116 at a constant rate with respect to the treatment zone 105 which is important. For example, the wafer 116 could be held stationary and the chamber 102 or the laterally uniform deposition zone 105 moved at a constant rate with respect to the wafer 116. In a further embodiment of the present invention, the wafer 116 undergoes a reciprocal motion across the deposition zone 105 before being removed from the chamber 102. The motion involves moving the water 116 at a constant rate through the entire deposition zone 105 in one direction 118 and then moving the wafer 116 at a constant rate back through the entire deposition zone 105 in the opposite direction, once or continuously, until deposition is completed.

The above method can be extended to instances where a chamber does not provide lateral uniformity if the chamber is able to provide lateral uniformity which varies linearly across a substrate. In this instance, uniformity across the full surface of the substrate can be achieved by rotating the substrate about a perpendicular axis of symmetry of the substrate as it is moved through the deposition zone 105 at a constant rate, as described above.

An apparatus 100 which has been modified to effect rotation of the substrate 116 is illustrated in Figure 8. The belt 114 includes a rotatable platform 120 having a lower part 122 attached to the belt 114 and an upper part 124 on which the substrate

SUBSTITUTE SHEET 116 to be treated is mounted. The upper part 124 of the platform 120 rotates about a central axis 126 with respect to the lower part 122. The platform 120 therefore rotates the substrate 116 as it is moved by the belt 114 in the direction 118 through the deposition zone 105. To provide uniform deposition on the substrate 116, the substrate 116 is mounted on the platform 120 so that its axis of symmetry is perpendicular to the plane of the substrate 116 and coincides with the central axis 126.

The deposition method employed by the apparatus 100 is particularly useful for deposition of multi-layered films where each layer is required to be uniform in width and depth. The films can be produced either by passing the substrate, as described above, through a number of deposition zones 105, each zone depositing a unique layer, or by changing the composition of the source gas in a single zone 105. For the film to be uniform in depth, uniform gas composition throughout the zone needs to be maintained after effecting a change in composition. Graded composition with depth can be achieved by either changing the gas composition uniformly with time or by varying the gas composition at particular points in the direction of the gas flow by varying temperature and depletion.

The method is also well suited for production of superlattices and films derived by solid state interdifiusion of multi-layers, as described in the specifications of

International Application PCT/GB85/00504 (WO 86/02951) and UK Patent

Application 2,146,663. For the production of such superlattices and films it is necessary to also establish lateral uniformity of the temperature within a zone 105 as well as growth rate uniformity in the lateral direction, in order to ensure complete interdiffusion. Calculations provided in D.I. Fotiadis, M. Boekholt, K.F. Jensen and

W. Richter, "Flow and heat transfer in CVD reactors: Comparison of Raman temperature measurements and finite element model predictions" J. Crystal Growth

1990, 100, 577 indicate that the laterally uniform temperature condition is met by both of the inverted horizontal reactors described previously. In the method described above each point on the substrate 116 experiences identical thermal histories and thus the technique of moving the substrate at a constant rate is also useful for heat treating

SUBSTITUTE SHEET by annealing even in the absence of deposition.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.

SUBSTITUTE SHEET

Claims (22)

CLAIMS:

1. A method of producing a thin film on a surface (116), comprising causing relative movement, at a constant rate, between said surface (116) and a thin film deposition zone (105) of a thin film deposition chamber (102); such that said surface (116) passes through said zone (105), said chamber (102) being configured such that deposition of film on said surface (116) in said zone (105) is uniform in a lateral direction across said surface (116), said lateral direction being perpendicular to the direction (118) of said relative movement.

2. A method as claimed in claim 1, wherein the constituents of said film are provided in gaseous or aerosol form and flow through the deposition zone (105) in substantially the same direction (112) as the direction (118) of said relative movement.

3. A method as claimed in claim 1 or 2, wherein said chamber (102) includes an upper susceptor (104) below which said deposition zone (105) is disposed.

4. A method as claimed in claim 3, wherein a substrate (116) including said surface is moved through said deposition zone (105).

5. A method as claimed in any one of claims 1 to 4, wherein said relative movement is reciprocal for the extent of said deposition zone (105).

6. A method of producing a thin film of a surface (116), comprising causing relative movement, at a constant rate, between said surface (116), and a thin film deposition zone (105) of a thin film deposition chamber (102), such that said surface (116) passes through said zone (105), said deposition chamber (102) being configured such that deposition of film on said surface (116) in said zone (105) varies linearly in a lateral direction across said surface (116), said lateral direction being perpendicular to the direction (118) of said relative linear movement, and rotating said surface (116) about an axis of symmetry thereof, which is substantially perpendicular

SUBSTITUTE SHEET to said surface (116), as said surface (116) passes through said zone (105).

7. A method as claimed in claim 6, wherein the constituents of said film are provided in gaseous or aerosol form and flow through the deposition zone (105) in substantially the same direction (112) as the direction of said relative linear movement.

8. A method as claimed in claims 6 or 7, wherein said chamber (102) includes an upper susceptor (104) below which said deposition zone (105) is disposed.

9. A method as claimed in claim 8, wherein a substrate (116) including said surface, is moved through said deposition zone (105).

10. A method as claimed in any one of claims 6 to 9, wherein said relative linear movement is reciprocal for the extent of said deposition zone (105).

11. An apparatus (100) for producing a thin film on a surface (116), comprising a thin film deposition chamber (102) having a thin film deposition zone (105), said chamber (102) being adapted to deposit said film on said surface (116) uniformly in a lateral direction across said surface (116) in said zone (105), and means (114) for causing relative movement, at a constant rate, between said surface (116) and said deposition zone (105), such that said surface (116) passes through said zone (105), said lateral direction being perpendicular to the direction (118) of said relative movement.

12. An apparatus as claimed in claim 11, wherein said chamber (102) includes a constituent inlet (108) and outlet (110) through which the constituents of said film are provided in gaseous or aerosol form such that the constituents flow through the deposition zone (105) in substantially the same direction (112) as said direction (118) of said relative movement.

13. An apparatus as claimed in claims 11 or 12, wherein said chamber (102)

SUBSTITUTE SHEET includes an upper susceptor (104) below which said deposition zone (105) is disposed.

14. An apparatus as claimed in claim 13, wherein said moving means (114) moves a substrate (116) including said surface through said deposition zone (105).

15. An apparatus as claimed in any one of claim 11 to 14, wherein said relative movement is reciprocal for the extent of said deposition zone (105).

16. An apparatus (100) for producing a thin film on a surface (116), comprising a thin film deposition chamber (102) having a thin film deposition zone (105), said chamber (102) being adapted to deposit said film on said surface (116) linearly in a lateral direction across said surface (116) in said zone (105), and means (114) for causing relative linear movement, at a constant rate, between said surface (116) and said zone (105), so said surface (116) passes through said zone (105), and means (120) for rotating said surface (116), relative to said zone (105) as said surface (116) passes therethrough, about an axis of symmetry of said surface (116) which is substantially perpendicular to said surface (116), said lateral direction being perpendicular to the direction (118) of said relative linear movement.

17. An apparatus as claimed in claim 16, wherein the chamber (102) includes a constituent inlet (108) and outlet (110) through which the constituents of said film are provided in gaseous or aerosol form such that said constituents flow through the deposition zone (105) in substantially the same direction (112) as the direction (118) of said relative linear movement.

18. An apparatus as claimed in claims 16 or 17, wherein said chamber (102) includes an upper susceptor (104) below which said deposition zone (105) is disposed.

19. An apparatus as claimed in claim 18, wherein said moving and rotating means (114,120) is adapted to move a substrate including said surface through said deposition zone (105).

SUBSTITUTE SHEET

20. An apparatus as claimed in any one of claims 16 to 19, wherein said relative linear movement is reciprocal for the extent of said deposition zone (105).

21. A method of treating a surface (116), comprising causing relative movement, at a constant rate, between said surface (116) and a treatment zone (105) in a chamber

(102), so said surface (116) passes through said zone (105), said chamber (102) being configured such that said surface (116) is treated in said zone (105) uniformly in a lateral direction across said surface (116), said lateral direction being perpendicular to the direction (118) of said relative movement.

22. An apparatus (100) for treating a surface (116) comprising a chamber (102) adapted to treat said surface (116) in said zone (105) uniformly in a lateral direction across said surface (116), and means (114) for causing relative movement, at a constant rate, between said surface (116) and said zone (105), so said surface (116) passes through said zone (105), said lateral direction being perpendicular to the direction (118) of said relative movement.

SUBSTITUTE SHEET