Integrated circuit fabrication requires one or more steps in which a material is deposited onto the surface of a specimen, typically a silicon wafer. It is important that this material be deposited as a uniform layer. At present, the speed with which this material can be satisfactorily deposited often limits the rate at which very large scale integrated circuits (VLSI) can be fabricated.

Chemical vapor deposition (CVD) is commonly used to form a layer or film of material on a substrate or other surface. The substrate may be heated to 1000°C or more to produce the desired chemical reactions on the surface needed to enable the film to form. Low pressure CVD is becoming more common utilizing thermal plasma enhanced low temperature methods (less than 500°C. Such methods are desirable because higher temperatures can damage substrate structures.

In CVD processes, a gaseous material is dispersed over the surface of the wafer while being heated and/or an RF induced plasma is used. The heated gas impacts on the surface of the wafer where it undergoes chemical reactions and becomes deposited on the surface.

Prior art CVD reaction chambers have been of many types, e.g. horizontal systems, where wafers are placed on a wafer holder and gas flows in one end of a quartz tube containing the wafers, the gas flows across

the wafers and out the other end; cylindrical, or barrel systems, where wafers are placed on an outer surface of a cylinder and gas flows into the chamber from the sides; and gas-blanketed downflow systems, where gas flows downward onto wafers on a horizontal surface.

A key concern in such configurations is that the flow of reactant gases must be uniform across the wafer in the reaction chamber in order to ensure uniform deposition of a desired layer. The problem with horizontal systems, for example, is the depletion of the reactant species in the gas as it flows across the wafers, causing the deposition rate to decrease as the gas approaches the vacuum pump end of the process chamber.

One solution to this problem in the gas-blanketed downflow type of system is to position a gas discharge head directly above each wafer being processed and orient the head as close as possible to be parallel with the surface of the wafer. Such a head acts as a shower nozzle to dispense the gas evenly over the wafer surface. The discharge head typically includes an interior chamber having a bottom surface which is parallel to the surface of the wafer. This bottom surface contains a large number of holes. When gaseous material is injected into this interior chamber, the chamber becomes pressurized, thereby causing the gaseous material to flow through these holes into the region between the bottom surface and the wafer.

When one attempts to increase the rate of deposition using such prior art gas discharge heads by increasing the rate of flow of the gas through the bottom surface, the uniformity of the resultant layer on the surface of the wafer deteriorates. Typically, ridges and other artifacts are produced on the surface.

Hence, prior art CVD devices have been limited in the rate at which material may deposited on a surface using such devices.

Broadly, it is an object of the present invention to provide an improved deposition apparatus which produces a more uniform layer on the surface of a specimen at higher gas flow rates.

Another object of the present invention is to provide a deposition apparatus which maximizes deposition rate without loss of film or layer uniformity.

These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of ^" the invention and the accompanying drawings.

Brief Description of the Drawings Figure 1 is a cross-sectional view of a prior art gas discharge head.

Figure 2 is a bottom view of the bottom surface of the gas discharge head shown in Figure 1.

Figure 3 illustrates the manner in which each orifice of a gas discharge head distributes material onto the surface of a specimen.

Figure 4 illustrates the manner in which each orifice collimates the gas molecules exiting through said orifice.

Figure 5 illustrates the distortions of the deposition pattern in prior art gas discharge heads when such heads are used at high flow rates. Figure 6 is a cross-sectional view of-a gas discharge head according to the present invention.

Figure 7 is a bottom view of the bottom surface of the gas discharge head shown in Figure 6. Figure 8 illustrates the manner in which the present invention avoids the distortions encountered with prior art gas discharge heads.

Figure 9 is a cross-sectional view of a conically shaped orifice according to the present invention.

Figure 10(a) is a side -view of an alternate gas deflection means.

Figure 10(b) is a top view of the gas deflection means shown in Figure 10(a).

Summary of the Invention The present invention consists of a gas discharge head apparatus for use in the deposition of materials on a surfa-ce using chemical vapor deposition methods. The apparatus defuses a gaseous material evenly over the surface in question. The apparatus consists of a pressurizable chamber having a planar bottom surface which is parallel to the surface on which the material is to be deposited. The bottom surface includes a plurality of orifices arranged in a hexagonal array. The orifices are preferably, but not limited to, conically shaped holes through said bottom surface. The chamber also includes an entrance orifice for injecting gaseous material into the chamber. This entrance orifice is preferably located in a wall of the chamber opposite to said bottom surface and located so as to deliver gaseous material toward the middle of said bottom surface. A baffle is placed in front of said entrance orifice. Said baffle deflects the gaseous material away from the middle of said bottom surface as the gaseous material enters the chamber. Detailed Dsscripfcion of the Invention The present invention can best be understood with reference to a typical prior art deposition apparatus which is shown at 10 in Figure 1. The gaseous material 12 to be deposited on a wafer 13 enters an interior chamber 14 through an opening 16 connected to a source 15 of gas through a valve 19.

The material in question is in the form of a gas which

when heated to a sufficient temperature will react with the surface of the wafer 13. The chamber 14 is typically cylindrical in shape. The bottom surface 17 of the chamber 14 is a plate having a plurality of orifices 18 through which the gas exits. The orifices are typically arranged in rows and columns, as exemplified by the rectangular pattern of orifices 18 illustrated in Figure 2, which is a bottom view of the bottom surface 17 of chamber 14. The wafer 13 is typically supported on a stand 22 which may include means for heating wafer 13. A vacuum chamber 23 surrounds chamber 14, wafer 13, and wafer stand 22. The vacuum chamber 23 is evacuated by a pump 24.

For CVD applications when plasma emhancement is desirable, an RF potential is preferably applied between the bottom surface 17 and the wafer 13. This RF potential heats the gas as it leaves the bottom surface 17. The heating is the result of collisions between the neutral gas molecules and a small fraction of the exiting gas molecules which are ionized by the

RF field created by said RF potential. The ionized gas molecules are accelerated by the RF potential and collide with the neutral gas molecules which results in the temperature of the neutral gas molecules increasing. The heated gas molecules impact the wafer 13. Those molecules with sufficient energy are deposited thereon. This results in a layer 20 of material being formed on the surface of the wafer 13. The gas molecules which are not immediately deposited upon the wafer 13 remain in the region between the bottom surface 17. and the wafer 13 for a period of time which depends on the pressure in said region. Some of the remaining gas molecules will be deposited on the wafer 13 at locations between those at which said gas molecules exited the bottom surface and the edge of the wafer 13. The remaining gas molecules

which are not deposited on the wafer 13 will eventually flow out of the region between the bottom surface 17 and the wafer 13 at the edges of this region. These gas molecules move under the influence of a pressure gradient which exists between the center of the bottom surface 17 and its edges.

It is important that the layer 20 be uniform in thickness. The layer 20 may be considered to be the combination of two layers. One layer results from those gas molecules which are deposited on the wafer 13 immediately after leaving an orifice 18. The other layer is the result of those gas molecules which are deposited far from the orifice through which they exited. These are the gas molecules which deposit before having a chance to exit from the region between the bottom surface 17 and the wafer 13 under the influence of the above mentioned pressure gradient. This second layer will in general be much more uniform than the first layer. The uniformity of the first layer depends on a number of factors. The distribution of material emanating from each of the orifices 18 is particularly important. An enlarged view of the bottom surface 17 and a portion of the wafer 13 is shown in Figure 3. The distribution of molecules on the wafer 13 resulting from gas molecules which pass through orifice 30 and are immediately deposited on the wafer 13 is shown at 32. This distribution will be referred to as a single orifice distribution. The single orifice distributions are the result of the impact of individual gas molecules on the wafer 13. An exemplary gas molecule leaving orifice 30 is shown at 34. It leaves orifice 30 having a velocity vector, v. This velocity vector may be decomposed into components, vz and vr, which are respectively parallel to the axis 36 of orifice 30 and orthogonal to said axis. The distance from the axis 36

at which this molecule impacts the surface of the wafer

13 determines the width of the distribution 32. As will be discussed in more detail below, it is important that the distribution 32 be as wide as possible. The distance from the axis 36 at which the molecule 34 impacts the wafer 13 is determined by the magnitude of vr and the distance, h, between the bottom surface 17 and the wafer 13. This distance is equal to vr times the time taken to traverse the distance, h. This time is related to the velocity of the gas molecule as it leaves the orifice 30, which in turn is related to the gas flow rate through the orifice 30. The distribution of orthogonal velocity components, vr, is determined by the shape of the orifice 30, the velocity distribution of gas molecules in the vicinity of the orifice 30, and the density of said gas molecules in this vicinity.

Referring again to Figure 1, if the portion of the layer 20 resulting from those gas molecules which exit an orifice 18 and are immediately deposited on the wafer 13 is to be uniform, threέ conditions must be met. First, the distribution of gas molecules in the chamber 14 in the vicinity of each of the orifices 18 shown in Figure 1 must be the same. If different velocity distributions or different density distribution exist in the vicinity of each orifice 18, the resulting single orifice distributions such as the one shown at 32 in Figure 3 will be different. Such differences will result in thickness variations in the layer 20. Prior art gas discharge heads rely on the even diffusion of the gas molecules through the chamber

14 prior to said gas molecules leaving chamber 14 through the orifices 18 in bottom surface 17. If the average residence time of a gas molecule in the chamber 14 is sufficiently long, the distribution of gas molecules will be the same in the vicinity of each of

the orifices 18. However, such a long residency time is not compatible with high deposition rates. High deposition rates require high gas flow rates through the chamber 14, which in turn restrict the residency time. As a result, prior art gas discharge heads could not provide both uniformity and high deposition rates.

Second, the distance between the bottom surface 17 and the wafer 13 must be constant. Similarly, the shape and orientation of each of the orifices 18 must be the same. If this distance varies, variations in the with of the single orifice distributions will result. These variations will in turn produce thickness variation in the layer 20. Referring again to Figure 3, if the orientation of an axis 36 of an orifice 30 varies, the distribution 32 will not be the same for that orifice as for the other orifices even if the distance from the orifice to the wafer is the same. This too will result in a non-uniform layer on the surface of the wafer 13. Such alterations in orientations may result from warpage in the bottom surface 17 if this plate is too thin.

Finally, the width of the single orifice distributions must be greater than, or on the order of, the separation, S, between adjacent orifices, if the portion of layer 20 resulting from the superposition of the single orifice distributions is to be uniform. If the tails 33 of these distributions do. not extend sufficiently into the regions between the orifices, "valleys" will be produced in these regions. If this occurs, the resulting layer 20 will have a "beaded" pattern.

In principle, this third condition could be met by decreasing the ratio of the diameter of the orifices 18 to the spacing of the orifices, S..

However, there is a practical limit to the spacing of

the orifices 30. If the orifices 30 are placed too close together, the bottom surface 17 of the chamber 14 will have insufficient structural strength to prevent warpage as the temperature of the apparatus is changed. The bottom surface 17 is subjected to a significant temperature gradient, since the side of said bottom surface 17 interior to said head is cooled by the incoming gas molecules, while the outside surface is heated by the heated gas between the bottom surface 17 and the wafer 13. If the bottom surface 17 warps, variations in the single orifice distributions will result. These variations can result from different distances between a warped bottom surface 17 and the wafer 13. Variations may also result from changes in the angle between the axis of an orifice in a warped region and the wafer surface.

Attempts to prevent such warping by increasing the thickness, • w, of the bottom surface 17 in conjunction with decreasing the spacing between the orifices 30 are counter-productive as may best be understood with reference to Figure 4. The tails of the single orifice distributions are produced by gas molecules which pass through an orifice at an angle with respect to the orifice axis. A typical orifice having an axis 39 is shown at 40 in Figure 4. A gas molecule having a trajectory with the angle 43 relative to the axis of the orifice 40 is shown at 41. A gas molecule with an angle greater than the one shown at 43 will strike the wall of the orifice 40. Such- a collision will result in a change of direction or in the deposition of the gas molecule on the wall of the orifice. Although some of these large angle gas molecules will manage to pass through the orifice after multiple elastic collisions, gas molecules with angles larger than that shown at 43 are, on average, prevented from exiting along a trajectory with an angle greater

than that shown at 43 by such collisions with the orifice walls.

In effect, the orifice 40 collimates the gas molecules leaving it into a beam. The amount of collimation depends on the ratio of the diameter of the orifice to its length. If the thickness of the orifice 40 is increased as shown at 46, the maximum angle 47 of the trajectory of a gas molecule which can just pass through the orifice 46 without striking the wall of said orifice will be less than angle 43. The thicker orifice shown at 46 actually collimates the gas molecules into a beam having a narrower distribution than the thinner orifice shown at 40. As a result, increasing the ^' thickness of the bottom surface 17 results in narrower single orifice distributions. Thus, no improvement is obtained by increasing the thickness of the bottom surface 17 shown in Figure 1 and reducing the spaces between the orifices 18. Hence, prior art gas discharge heads have been limited by such effects in the degree of uniformity of layer 20 as well as the rate at which material may be deposited.

These collimation effects become more pronounced in systems having high deposition rates. To obtain a high deposition rate, the rate of flow of the gas through the bottom surface 17 shown in Figure 1 must be increased. This can only be accomplished by increasing the porosity of the bottom surface 17 or the flow rate of the gas through the orifices 18 therein. As noted above, increasing the porosity in question would weaken the bottom surface 17 leading to undesirable warpage. Hence, the flow rate can be increased only by increasing the velocity of the gas molecules through the orifices 18.

However, increasing the velocity of the gas molecules leaving the orifices 18 results in two undesirable effects which have prevented prior art

designs from obtaining high deposition rates. First, increasing the gas molecule exit velocities results in a narrowing of the single orifice distributions. As pointed out above with reference to Figure 3, the width of the single orifice distributions is related to the time needed for a gas molecule to traverse the distance between the bottom surface 17 and the wafer 13. This time is inversely related to the velocity of the gas molecules as said molecules leave the orifice in question. Hence, increasing the velocity of the gas molecules through the orifices decreases the width of the single orifice distributions which leads to a "beaded" pattern in the layer 20.

Second, increasing the velocity in question results in an increase in the pressure differential in the region between the bottom surface 17 and the wafer 13. As pointed out above, there is a pressure differential between the center of the wafer 13 and the edges of said wafer. This pressure differential "sweeps" the gas molecules which do not react with surface of the wafer 13 out of the region between the bottom surface 17 and the wafer 13. The exit velocity may only be increased by increasing the pressure differential across the bottom surface 17. Since the wafer 13 is ideally in a low pressure environment, this can only be accomplished by increasing the pressure in chamber 14 which in turn results in an increase in pressure in the region between the bottom surface 17 and the wafer 13. Increasing the pressure difference between the center of the wafer and the edges thereof results in two undesirable effects which result in a non-uniform layer 20. First, increasing said pressure difference reduces the residence time of the gas molecules which do not immediately combine with the surface of the wafer 13. As pointed out above, these gas molecules

are trapped for a period of time between bottom surface 17 and the wafer 13. During this period of time, some of these molecules will combine with the wafer 13. If the resistance time of said molecules is decreased, the probability of such secondary reactions is reduced. In addition to reducing the efficiency of deposition, this loss of secondary reactions accentuates any non-uniformity resulting from the single orifice distributions. As pointed out above, the layer 20 may be thought of as resulting from the superposition of two layers, one resulting from the single orifice distributions and one resulting from the these secondary reactions. The layer resulting from the secondary reactions is in general more uniform than that resulting from the single orifice distributions. Hence reducing the thickness of the layer resulting from the secondary reactions accentuates any variations resulting from the single orifice distributions. Second, increasing the pressure difference between the center of the wafer 13 and the edges thereof resulting in a distortion of the single orifice distributions which can lead to further non-uniformities in layer 20. This pressure differential gives rise to a radial flow of gas molecules outward from the center of the bottom surface

17. The gas molecules in this radial flow undergo collisions with the gas molecules leaving the orifices

18. As a result of these collisions, the gas molecules leaving the orifices 18 acquire a radial velocity component which distorts the shape of the resulting single orifice distributions.

This distortion in the single orifice distributions gives rise to ridges in layer 20. This phenomenon can best be understood with reference to Figure 5, which illustrates the distorted single

orifice distributions. A typical orifice 76 gives rise to an ellipsoidal single orifice distribution. A typical contour on the distribution produced by orifice 76 is shown at 74. Each of the orifices in the bottom surface gives rise to an ellipsoidal distribution having its major axis along a line joining the center of the orifice in question with the center of the bottom surface 17. These ellipsoidal distributions overlap more along the the horizontal and vertical rows of orifices which pass through the center of the bottom surface 17. These rows are shown at 72 in Figure 5. This overlapping produces a ridge of material along these lines with a low trough on each side of the lines. If the single orifice distributions were not distorted, material from the neighboring lines would have tended to fill in this trough. Similarly, material from the orifices along the lines in question would also have tended to fill in these troughs.

The apparatus of the present invention avoids the above mentioned non-uniformities even at high deposition rates. The preferred embodiment of a gas discharge head apparatus according to the present invention is shown at 50 in Figure 6. The gas 52 which produces the layer on a wafer 53 enters a pressurizable interior chamber 54 through an orifice 56. The chamber 54 has a bottom surface 57 which contains a plurality of orifices 58 through which the gas exits prior to heating by an RF potential, V, which is applied between the bottom surface 57 and the wafer 13. The pattern of orifices 58 in the bottom surface 57 is shown in Figure 7. In contrast to prior art designs which employ a rectangular pattern, the pattern is hexagonal. The hexagonal pattern has the same number of orifices per unit area as that shown in Figure 2. As a result, the spaces between some of the orifices are less than those shown in Figure 2, and

some are further apart.

This hexagonal pattern produces a more uniform layer on the wafer 57. In particular, it has been found experimentally that this pattern significantly reduces the ridging described above. This may be best understood by comparing the pattern shown in Figure 5 with that shown in Figure 8, which illustrates the effects of the distorted single orifice distributions obtained at high gas flow rates on this hexagonal distribution. The ridging along lines 72 in Figure 5 was the result of the overlapping of the single orifices in a region having a clear area on each side thereof. The hexagonal pattern reduces this ridging by- providing two lines of orifices close together on all radial lines from the center 82 of the bottom surface

57, except along those lines shown at 80. It should be noted that the area separating the lines shown at 80 from the neighboring lines is substantially smaller than that separating the lines shown at 72 in Figure 2 from their neighboring lines. Hence, these "single" lines produce less ridging, since material from the neighboring lines tends to fill this smaller space. Furthermore, only every other single orifice distribution overlaps along these lines. Hence, the amount of material in the ridge area is significantly less than is the case with a rectangular orifice pattern.

Further improvements in the uniformity of the layer deposited on the wafer 53 may be obtained by employing conical shaped orifices. Prior art designs employ cylindrical orifices which are obtained by drilling holes in the bottom surface 57. As noted above with reference to Figure 4, cylindrical orifices tend to collimate the exiting gas molecules into a narrow beam which produces a "beaded" pattern on the wafer 13. It is preferred that the orifices 58 have a

conical cross-section. Such an orifice is shown at 90 in Figure 9. A conically shaped orifice produces a much wider single orifice distribution than a cylindrical orifice having the same hole size. The maximum width of the single orifice distribution produced by orifice 90 is shown at 92. For comparison, the maximum width of a cylindrical orifice having the same hole diameter is shown at 94. The walls of this exemplary cylindrical orifice are shown at 96. Referring again to Figure 6, as noted above with reference to prior art designs, it is important that the pressure above each of the orifices 58 be the same. If one region of the bottom surface 57 has a higher pressure than a another region, the resultant layer on the wafer 53 will be thicker in the region closest to the high pressure area. At high gas flow rates, prior art devices have such a high pressure region in the center of the bottom surface 57, sincώ this area is directly under the gas input orifice 56. The preferred embodiment of the present invention substantially reduces such pressure differentials. A deflector plate 62 is suspended by rods 64 from the top surface of chamber 54 to provide a baffle at the input orifice 56. It is positioned as close as possible to be centered under orifice 56. The incoming gas is deflected by baffle 62 to the sides of chamber 54. This diffuses the gas molecules in chamber 54, thereby reducing the pressure at the middle of the bottom surface 57 while increasing the pressure at the edges thereof. As a result, a substantially uniform pressure is maintained across the bottom surface 57.

An exemplary embodiment of the present invention which is suitable for depositing material on 6 inch diameter wafers has the following dimensions and tolerances. Interior chamber 54 is cylindrical with a circular cross-section. The bottom surface of the

chamber 54 is constructed from steel having a thickness between .060" and .080". The bottom surface 57 should be flat to within .001". The chamber 54 has a diameter of 7" and a height of .375". The deflector plate 62 consists of a 2" diameter circular plate having a thickness of 0.125" suspended from three 0.125" diameter rods at a distance of 0.125" from the input oriface 56. The input orifice is 0.375" in diameter. The chamber is pressured to a pressure which yields a flow rate of 2.2 liters/minute into a vacuum of 2 Torr. Chambers with different dimensions and operating parameters will be apparent to those skilled in the art.

Alternate deflection means may be used in place of deflector plate 62. The essential feature of the deflection means is that it uniformly deflects the incoming gas stream away from the center of the bottom surface. An alternate deflection means is shown at 100 in Figure 10. The incoming gas 52 enters a deflection chamber 101 contained within chamber 54. The deflection chamber 101 includes a plurality of orifaces 102 connecting the input oriface 56 to the periphery of said deflection chamber. This is shown in more detail in Figure 10(b) which is a top view of the deflection chamber 101. At least 6 orifices 102 are needed to obtain a uniform gas distribution at the bottom surface 57. The deflection chamber 101 is preferrably circular in cross-seetion. However, any cross-section which is symmetrical about the input orifice 56 and which allows at least 6 orifaces will function properly.

Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.