This invention relates to process chambers for chemical vapor deposition or other processing of semiconductor wafers and the like. More particularly, the invention relates to a process chamber capable of withstanding stresses associated with high temperature, low pressure processes, and having improved wafer temperature uniformity and gas flow characteristics.
Process chambers for thermally processing semiconductor wafers are desirably made of quartz (vitreous silica) or similar material because quartz is substantially transparent to radiant energy. Thus, radiant heaters may be positioned adjacent the exterior of the chamber, and a wafer being processed in the chamber can be heated to elevated temperatures without having the chamber walls heated to the same level. On the other hand, quartz is desirable because it can withstand very high temperatures. Quartz is also desirable because of its inert characteristics that enable it to withstand degradation by various processing gases and because of its high purity characteristics.
For applications in which the pressure within a quartz chamber is to be reduced much lower than the surrounding ambient pressure, cylindrical or spherical chambers are preferred from a strength standpoint because their curved surfaces can best withstand the inwardly directed force. However, when positioning a flat wafer for chemical vapor deposition purposes where the deposition gases flow parallel to the wafer, it is desirable that the chamber wall be parallel to the facing flat surface of the wafer, to obtain even deposition on the wafer surface. Uniform deposition is critical to obtain a high yield of acceptable products to be made from such wafer. However, a flat wall will collapse inwardly with reduced interior pressure sooner than will an outwardly convex wall of similar size and thickness.
To handle the inwardly directed forces on flat wall chambers, gussets have been provided on the exterior of the walls extending generally perpendicular to the walls to which they are joined, as may be seen in U.S. Pat. No. 4,920,918. That patent also illustrates gussets on the exterior of a chamber having upper and lower outwardly convex elliptical walls having a large radius of curvature, thus providing a somewhat flattened, but curved, configuration. This compromise provides some additional strength from the curved walls while not affecting the evenness of deposition appreciably. One significant disadvantage of such design is that the external gussets complicate and interfere with the external positioning of radiant heat lamps. Furthermore, the complexity and mass of the quartz gussets increases material and fabrication expense.
Of course, flat walls can be made thicker to increase strength, but that adds cost and adversely affects heating and cooling characteristics of the chamber.
U.S. Pat. No. 5,085,887 discloses a chamber which includes a circular, slightly domed, or curved upper chamber wall to accommodate the load of reduced chamber pressure. The circular wall is provided with a greatly thickened peripheral flange that radially confines the upper wall to cause the domed wall to bow outward due to thermal expansion, helping to resist the exterior ambient pressure in vacuum applications. The chamber requires a complex mechanism for clamping the thickened exterior flanges of the upper and lower chamber walls.
Due to the high temperatures associated with thermally activated chemical vapor deposition processes, the walls of the process chamber often heat up to a certain degree, and chemical particulates are deposited thereon. These particulates can cause serious problems with the purity of the resulting processed wafer. As a result, there has been a large effort to reduce the buildup of particulate matter on reaction chamber walls. One solution is to periodically etch the insides of the process chambers to remove the particulate matter before it accumulates to a harmful level. Unfortunately, quartz process chambers take a long time to heat up due to their high transparency to radiant heat. These periodic slow etch cycles thus reduce the maximum throughput of the machine.
There has also been attempts at controlling the gas flow profile in parallel across the wafer to be processed so as to create a more uniform deposition. For example, U.S. Pat. No. 5,221,556 discloses a system in which the apertures through a gas inlet manifold are varied in size to allow more gas through one section, typically the center section, as opposed to others. U.S. Pat. No. 5,269,847 includes valves for adjustment of pairs of gas flows merging into a number of independent streams distributed laterally upstream of the wafer to be processed. This system emphasizes the importance of channeling the various gas flows separately until just before the wafer leading edge so as to prevent premature mixing and enable greater control over the flow and concentration profiles of reactant and carrier gases across the wafer.
Another problem which has not been sufficiently addressed in the prior art is that of recirculation of the process gas in parallel flow reactors. More particularly, after the gas travels in parallel over the wafer and susceptor, it may experience temperature gradients between the hot susceptor and cooler chamber walls. This can lead to recirculations as the gas rises toward the walls and is subsequently cooled. Also, the gas flow may be constricted proximate an exhaust conduit which may create turbulence and recirculations. Recirculations from either source may migrate upstream to impact the uniformity of flow in the area of the wafer thus reducing the uniformity of film deposition.
Additionally, the temperature gradient across the wafer is nonuniform from the leading edge to the trailing edge. That is, the temperature of the gas is primarily determined by its proximity to the heat-absorbing susceptor underneath the wafer. As the gas approaches and passes over the susceptor, it heats up fairly quickly to a maximum temperature towards the downstream edge of the susceptor, and then drops off after traveling past that point. This temperature nonuniformity may further negatively affect film deposition uniformity.
A need exists for an improved chamber for chemical vapor deposition purposes, and other high temperature processes, that can be made of quartz or similar materials and yet withstand the stresses incident to reduced pressure processes. There is also a need for a more uniform temperature and flow environment surrounding the wafer to ensure more uniform deposition thereon. Also, a more responsive flow control system is needed. Finally, there is a need for a more energy efficient chemical vapor deposition system with higher throughput.
Briefly stated, the invention provides a process chamber having thin upper and lower curved walls forming a flattened configuration. The upper and lower curved walls have a convex exterior surface and a concave interior surface. These walls are joined at their side edges to side rails, thus giving the chamber a generally flattened ellipsoidal or lenticular cross section, wherein the internal height of the chamber is less than the width or distance between the side walls. An internal support extending across and joined to the side rails provides the strength necessary to prevent collapse of the chamber when operating in a mode in which the interior of the chamber is at a pressure lower than that outside the chamber.
In a preferred form, the chamber upper and lower walls are generally rectangular in shape, and the spaced side rails extend the length of the walls. This produces an elongated configuration. The internal support is in the form of a plate that includes an inlet section extending to an inlet flange and an outlet section extending to an outlet flange, with a large opening between the two sections. The support plate essentially divides the chamber into an upper and lower region. A susceptor is positioned in the opening in the plate, and is supported on a shaft that extends through a tube depending from the lower wall of the chamber. A semiconductor wafer or other element to be processed can be inserted through the inlet flange and supported on the susceptor approximately aligned with the inlet section of the support plate so that processing gases may flow smoothly over the inlet support plate section and across the surface of the wafer to be processed. In this respect, the upper region of the chamber is preferably exclusively assigned to the task of wafer processing.
The chamber upper and lower walls are preferably made of quartz and are constructed by cutting segments from a large diameter cylindrical tube, or otherwise formed into curvilinear plates. These segments are welded to sidewalls which may be molded or cut to shapes to facilitate welding to the edges of the upper and lower walls. It is also possible, but not preferred, to build this structure with elements having elliptical, parabolic, or slumped plate cross sections, which are not well defined by simple circular, elliptical, or parabolic geometries.
The support plate is also preferably made of quartz and located centered between the upper and lower walls so that the stress on those walls is uniform.
The chamber disclosed thus has the advantages of being able to withstand reduced pressure processing, being made into an integral unit, and not requiring external support elements that interfere with the positioning of radiant heaters for transmitting radiant energy through the thin quartz upper and lower walls. Also, the internal support plate does not interfere with the flow of process gases through the chamber; and in fact, assists in providing the desired gas flow by conducting greater gas flow at the center of the flow path than at the edges. Further, the internal support does not interfere with the insertion or removal from the chamber of wafers, susceptors, or susceptor rings.
In a still further aspect of the present invention, an apparatus for chemical vapor deposition is provided which comprises walls defining a. deposition chamber having a chamber gas inlet and outlet. A generally horizontal quartz inlet wall extends from the inlet of the chamber to a downstream edge defining part of an opening for receiving a susceptor. A generally circular susceptor is horizontally positioned in the opening and receives a semiconductor substrate for vapor deposition purposes. The apparatus further includes a sacrificial quartz plate having a horizontal portion and a vertical lip extending into the opening closely adjacent to the downstream edge of the inlet wall to minimize vapor deposition on and devitrification of the downstream edge. In a particular embodiment, the horizontal portion of the quartz plate rests on the inlet wall. The portion of the opening defined by the downstream edge is curved and the vertical lip may be curved to conform to the curved portion of the opening and is sized to protect one half of the downstream edge of the inlet wall. A second sacrificial plate may be provided having a vertical lip curved to conform to a curved edge of the susceptor or a ring positioned around the susceptor to protect a second half of the downstream edge.
In one particular embodiment the sacrificial plate is supported beneath the inlet wall, and its vertical lip extends upward adjacent the downstream edge of the inlet wall. The sacrificial plate may be in the form of a tray that extends beneath the susceptor and has a central hole for receiving a shaft rotatably supporting the susceptor. The apparatus may include a generally horizontal quartz outlet wall extending downstream from the susceptor and spaced therefrom, whereby an inlet end of the tray is supported beneath the inlet wall and a downstream edge of the tray is supported beneath the outlet wall.
In another aspect, the present invention provides a method of using a chemical vapor deposition chamber, the chamber having a quartz horizontal inlet wall with a downstream edge defining a portion of an opening in which a horizontally extending susceptor is positioned, adapted to receive a substrate. The method includes positioning a vertical lip of a sacrificial quartz plate between the downstream edge of the inlet wall and the susceptor to minimize the vapor deposition on and the devitrification of the downstream edge of the inlet wall. A temperature compensation ring surrounding the susceptor and extending between the susceptor and the downstream edge of the inlet wall may be provided. A vertical lip of the sacrificial plate is preferably positioned in a gap between the temperature compensation ring and the downstream edge of the inlet wall. The method may include providing a short, horizontally extending flange on the upper edge of the vertical lip, with the flange extending upstream over the downstream edge of the inlet wall.
In a still further embodiment, the chamber also includes a quartz horizontal outlet wall with an upstream edge which, in combination with the downstream edge of the inlet wall and chamber, defines the opening in which the susceptor is positioned. The ring is circular and the sacrificial quartz plate has an inner diameter which closely conforms with the outer diameter of the ring. The quartz plate has an outer shape which conforms with and desirably abuts against the opening, which preferably has a rounded rectangular shape. In this manner, the edges of the opening are protected from devitrification from repeated heating of the reaction chamber. The sacrificial quartz plate is desirably shaped to closely fit within the opening with a minimum of clearance therebetween, and may be supported by modified fingers or support elements attached to the inlet and outlet walls.
In accordance with a further preferred embodiment, the present invention provides an apparatus for use in a chemical vapor deposition chamber comprising a temperature compensation ring having an interior edge defining a generally circular opening for receiving a susceptor adapted to support a semiconductor, and a generally rectangular exterior edge. Preferably, the ring has rounded exterior corners, a leading edge, a trailing edge, and a pair of exterior side edges. The shortest distance between the leading exterior edge and the interior edge is desirably less than the shortest distance between the trailing edge and the interior edge. The ring is preferably made of graphite and has an inner generally hollow portion adapted to receive one or more temperature sensors therein. The ring has a generally flat, nontubular leading edge portion extending forwardly from the hollow portion, and a generally flat, nontubular trailing edge portion extending rearwardly from the hollow portion.
In another preferred embodiment, the present invention includes a process chamber gas injector having multiple tuned ports distributed laterally across the width of the injector to control the velocity profile of the process gas over the wafer to be processed. The injector is preferably formed of two juxtaposed plates with a plurality of flow control valves mounted in one. A single gas input supplies a plenum common to the flow control valves so that an equal pressure of gas is provided upstream of the valve mechanisms of each of the valves. Narrow channels lead from each valves to separated expansion chambers formed in the injector before the independently metered flows are mixed while passing through a slit-like opening forming an outlet of the injector. A smoothed ribbon of process gas with a predetermined gas velocity profile is thus directed into the chamber and over the wafer. This ribbon of gas flow is formed a significant distance upstream of the wafer leading edge so as to provide adequate distance and time for the separate streams of flow to blend together by diffusion, thereby smoothing the gas density profile laterally across the wafer.
In another form, the present invention provides a quartz process chamber having a support structure for a susceptor and wafer thereon. A temperature compensation ring surround the susceptor and is preferably made of the same high thermal mass material as the susceptor to help maintain the temperature of the susceptor more uniform. Upper and lower banks of heat lamps are positioned outside of the chamber to heat the susceptor and ring. Desirably, the lamps are sized the same as the exterior dimension of the ring to focus radiant energy where it is needed and conserve energy used by the system. In one embodiment, the ring has a circular interior edge sized to closely surround the susceptor and a rounded rectangular exterior edge sized to fit closely within a similarly shaped aperture in an inner chamber support plate.