Patent Application
20080072821
The present invention relates to a small volume symmetric-flow Atomic Layer Deposition (ALD) apparatus that improves ALD cycle times by minimizing the reaction space volume while maintaining symmetry of gas flow related to off-axis wafer transport slot valves and/or off-axis downstream pumping conduits.
ALD reactors may have a variety of design configurations. Conventional single wafer ALD reactor configurations include a cross-flow design (Suntola), wherein sequential chemical precursor exposures (pulses) and removals (purges) of injected gases flow substantially horizontally across the wafer surface and are pumped out in the horizontal direction as well. Wafer transport may be carried out in the same horizontal plane, at right angles to the gas flow direction. The term “traveling wave” has been used to refer to the movement of the time-dependent precursor pulses from injection orifice(s) to pump orifice(s). See, e.g., T. Suntola, “Atomic Layer Epitaxy,” in Handbook of Crystal Growth 3, Huerle ed., Ch. 14, pp. 601 et seq. (1994).
In so-called “pure” ALD processes, the first precursor is completely removed from the reaction space before the second precursor is introduced. See, S. M. Bedair, “Atomic layer epitaxy deposition processes,” J. Vac. Sci. Tech. B12 (1), January/February, pp. 179 et seq. (1994). However, as wafers scale to larger sizes, e.g., 300 mm and 450 mm, and as cycle times are pushed to lower limits, undesirable parasitic chemical vapor deposition (CVD) takes place at the edge of the wafer in the direction of the traveling wave. The parasitic CVD is due to undesirable chemical reactions from the simultaneous co-existence, in time and space, of the remnant precursor in the dispersion trailing tail of the first precursor and the onset of the second precursor. Parasitic CVD is undesirable in many ALD processes because it can lead to an increase in the within-wafer film thickness non-uniformity, reduced step coverage and uncontrolled changes in other film properties across the wafer surface. To avoid this parasitic CVD, precursor removal is used between pulses. Often, long removal times are needed. In the horizontal single wafer architecture, to avoid this parasitic CVD the concentration of the trailing edge of the first precursor pulse must be reduced to trace levels, for example an arbitrary figure of less than approximately 1% of the first precursor's peak value. See, e.g., U.S. Pat. No. 6,015,590 to Suntola.
Since ALD is a self-limiting process, it may be argued that the direction and symmetry of flow of the precursors does not matter because if enough time is used for the precursor removal periods—commonly referred to as the “purge period”—there will be no significant CVD. However, in the pursuit of high deposition rates (thickness/unit time or low cycle times), as purge times are pushed to the lowest possible times for value in commercial manufacturing, the symmetry of the flow becomes important. This is because, when operating at the practical limits of pulse and purge times, the flow symmetry will largely govern the deposition thickness symmetry on the wafer.
Alternative single wafer designs use injected precursor gases from axi-centric and axi-symmetric vertical gas distribution modules (GDM) (e.g., using an axi-centric orifice(s) or showerhead). An example of such a system 100 is shown in
The idealized vertical injection/radial flow/vertical downstream pumping design discussed with reference to
a and 2b help to illustrate this latter point.
Furthermore, the downstream exhaust pump 228 is commonly set at an azimuthal angle and range, θ2 and Δθ2, where θ2 is in general not necessarily the same as θ1. While this arrangement accommodates on-axis mechanical drive support hardware to achieve a vertically movable susceptor 126, together these asymmetries can lead to the formation of recirculation pockets, stagnation zones (206, 208) and/or pumping azimuthal non-uniformities. For example, if the residual precursor from the first precursor's flow are pumped or swept in a non-uniform azimuthal flow from the wafer, an additional mechanism is present for parasitic CVD to occur non-symmetrically or non-uniformly towards one azmuthal direction of the wafer. In this case, the onset of parasitic CVD occurs non-symmetrically and prematurely over particular azimuthal directions or angle(s) due to recirculations, stagnations and/or pumping effects.
The desirability of a small reaction space volume (the space above the wafer between the wafer and the precursor injection component (e.g., a showerhead)), is known in the art. See, e.g., M. Ritala and M. Leskela, “Atomic Layer Deposition” in Handbook of Thin Film Materials, H. Nalwa, ed., vol. 1, Ch. 2, pp. 103 et seq. (2002). The ALD reaction space volume should be minimized for reduced precursor removal time, reduced residence time (PV/flow) and therefore reduced ALD cycle time. With a vertically movable susceptor design (see, e.g., U.S. Pat. No. 5,855,675 of Doering, et. al., assigned to the assignee of the present invention and incorporated herein by reference), the distance between the wafer plane and the gas distribution orifices (showerhead) in the reactor lid may be optimized for uniformity of flow and residence time; that is, the volume of the reaction space may be minimized within the constraint of locally uniform exposures. Additionally, the reaction space may include the annulus region between the susceptor edge and the reactor's upper inner wall, which is parasitic reaction space volume.
While the broken flow symmetry due to azimuthal placement of the wafer slot valve and wafer passage was substantially restored by using a vertically movable susceptor/heater configured so that when the wafer and its heater/susceptor were in the process position the wafer was above the wafer slot valve, this approach was still limited with respect to fine control of symmetric flow. For example, downstream gases may still form stagnation regions and eddies in the pocket associated with the wafer slot valve below the wafer plane when the susceptor is in its process position. Hence, what is needed is a reactor design that provides a minimal reaction space volume and improved symmetric flow, while maintaining the ability to work with conventional wafer (slot) transport mechanisms. The current invention provides a solution to these requirements, resulting in a small, confined volume with symmetric flow resulting in a high throughput, high performance (HP) single wafer reactor.
In one embodiment of the invention, a reaction chamber apparatus includes a vertically movable heater-susceptor, where the heater-susceptor is connected to an annular attached flow ring that performs as a gas conduit, with an outlet port of the flow ring extending below the bottom of a wafer transport slot valve when the susceptor is in the process (higher) position.
A further embodiment of the invention provides a reaction chamber apparatus containing a heater-susceptor connected to an annular attached flow ring conduit at the perimeter of the susceptor, the conduit having an external surface at its edge that isolates the outer space of the reactor above the wafer and below the wafer to the bottom of the flow ring from the confined reaction space when the heater-susceptor is in its process (higher) position with respect to its loading position.
In still another embodiment, the present invention provides a reaction chamber apparatus containing a heater-susceptor connected to an annular attached flow ring conduit at the perimeter of the susceptor, the conduit having an external surface at its edge that isolates the outer space of the reactor above the wafer from the confined reaction space when the heater-susceptor is in its process (higher) position with respect to the loading position, the outer edge being placed in proximity with an annular ring attached to the reactor lid and together the ring and conduit outer member acting together as a tongue-in-groove (TIG) configuration. In some cases, the TIG design may have a staircase (SC) contour, thereby limiting diffusion-backflow of downstream gases to the outer space of the reactor.
A further embodiment of the present invention provides a reaction chamber apparatus having a vertically movable susceptor (VMS) with respect to its loading (lower) position, said susceptor being connected to an annular attached flow ring (AFR) (or deep flow ring (DFR)) conduit at the perimeter of the susceptor, said annular AFR passing reaction gas effluent to a downstream pump orifice that is off-axis with respect to the axi-centric center of the reaction chamber. In some cases a downstream baffle may be placed between the lower orifice of the annular AFR and the downstream pump to attain symmetric gas flow at the edges of the wafer in the upstream wafer plane.
Still a further embodiment of the invention provides a TIG chamber configuration as described above and having a pump connected to remove gas streams from the reaction space, the AFR conduit and the lower chamber leading to the pump. A gas injection orifice that allows for by-passing the reaction space and the AFR conduit is placed such that gas injected through said orifice enters into the stream leading to the pump below the output orifice of the AFR. Hence, the orifice provides for directly injecting a gas into the pumping conduit leading directly to the input of the pump, without further restrictor(s) between the AFR output orifice and the pump input orifice, periodically during ALD cycling. In some cases the gas so injected may be injected at azmuthal points to achieve uniform exposure and uniform residence time. Also, the orifice of the AFR may have restrictors in the form of holes at the plane of its orifice, and the holes may be designed differently in different azmuthal directions to induce symmetric flow at the wafer plane. The TIG design may be such that the inner edge of the TIC lid element is curved to remove dead space in the reaction space.
The HP ALD design described herein may be further utilized in a “multi-single wafer” (MSW) reactor system as described for example in the above-referenced patent applications by Puchacz, et. al. and Strauch & Seidel. In that case, several (e.g., four) substantially independent HP reactors may be placed in a common vacuum housing system. In the application by Strauch & Seidel there is the added requirement of small gas flow (mostly via back-flow by diffusion as apposed to convective flow) between the otherwise substantially independently operating reactors placed within the same master vacuum housing.
Thus the HP ALD system described herein, is quantified with respect to the confined reaction space volume (minimized and optimized), with minimal re-circulations, symmetric flow, and small gas reactant transport outside the HP reaction zones. See
The present invention is illustrated by way of example, and not imitation, in the figures of the accompanying drawings, in which:
a and 2b illustrate the effects of a slot valve and off-axis downstream pump in breaking the symmetry of radial gas flow within an ALD apparatus;
a-4d are detailed views of ALD apparatus configured in accordance with various embodiments of the present invention, showing alternative designs for lid ring-flow ring interfaces and contoured fillets to remove dead zones in corners of a reaction chamber;
Described herein is a small volume symmetric flow (SVSF) apparatus defined for a minimal ALD reaction space volume with axi-symmetric flow with minimal chemical transport to the reactor walls. The description includes the reactor design and its functionality, as well as a discussion of the combined effects of small volume for the reaction space, generalized design for isolation of the reaction space from the reactor walls without stagnations and re-circulations, the minimization of gas expansion volume below the wafer plane, and a potential for time-phased multilevel choked downstream pump configuration suitably designed in all cases to achieve flow symmetry in the case of off-axis pumping conduits with maintainability and assembly features.
A consideration for the design of the small volume axi-symmetric flow ALD reactor is the requirement to deliver gas precursors rapidly and substantially uniformly across the semiconductor wafer or wafers or work piece or work pieces with high topology features. To achieve minimal exposure times and efficient precursor use, we desire chemical precursors to be brought to high aspect features in the center and the edge of the wafer in nearly the same timeframe, and with nearly the same concentration.
The benefit of same time exposure is to achieve efficient conformal coatings over the wafer area. The within-wafer non-uniformity of high topology features will be optimally small, while simultaneously using a minimal amount of precursor. To understand this, we refer to the theory for coating “holes” with high aspect ratios. R. Gordon, et al., “A Kinetic Model for Step Coverage by Atomic layer Deposition in Narrow Holes or Trenches,” Chem. Vap. Deposition, 9, No. 2, pp. 73 et seq. (2003). The exposure of a single ALD precursor proceeds by gas diffusion transport to the interior surfaces from the top down to the bottom of the hole. Holes placed near the location on the wafer having precursor arrival will coat first near the top and later coat at the bottom of the hole with a single pulse of suitably sufficient dosage. Holes farther away from the position of first precursor arrival to the wafer will coat to saturation at the bottom of their features at a later time. Reactors with distributed vertical injection are better suited to meet this condition efficiently, while reactors with horizontal injection perform poorly in this regard. In order to achieve an efficient coating, it is desirable to use a suitably designed showerhead or gas distribution manifold (GDM), where the gases are dispensed as simultaneously as possible over the entire wafer surface.
An optimal ALD system includes consideration of a rapid and efficient chemical precursor delivery into the GDM, and a GDM that, in turn, provides rapid precursor flow into the reaction space (see, e.g., U.S. patent application Ser. No. 11/278,700 of Dalton et al., filed 5 Apr. 2006, assigned to the assignee of the present invention and incorporated herein by reference). The detailed design of showerheads of uniform injection and low residence times (as well as chemical precursor source vaporizers of high partial pressure) are separate considerations from the design of the reactor itself, but must be optimized and integrated with best practices to obtain a fully competitive system.
In summary, a high performance system includes a chemical precursor source capable of rapidly delivering high partial pressures of precursor vapors by way of the GDM and optimized reactor chamber design. For the purposes of this disclosure, we consider the chemical source/delivery, GDM and reactor as modular with respect to each other and separately optimized. However, as mentioned above, for efficient uniform coatings of high topology features, one advantageously uses axi-symmetric exposure at the center and edge of the wafer at nearly the same time and a reactor design that is axi-symmetric with respect to flow at the edge of the wafer.
In considering the merits of axi-symmetric flow, we discuss the benefits of symmetric flow during wafer exposure as well as the removal of the reactants and byproducts from the reaction chamber. The importance of removing precursors with azmuthal symmetry is related to the minimization of the onset of parasitic CVD at all azmuthal points located either near the center circular zone of the wafer or around the edge, in a donut-like or toroidal zone, of the wafer. The signature of parasitic CVD near the center will occur if the upstream remnants of the “A” precursor are dominant at the time that the “B” precursor pulse is switched on. In such cases, the GDM region has not been adequately cleared. Conversely, the signature of parasitic CVD near the edge of the wafer will occur if the downstream remnants of the “A” precursor are dominant when the leading edge of the “B” precursor arrives near the edge of the wafer. In these cases, the region down-stream from the wafer has not cleared. Additionally, whether having azmuthal symmetry or not, if the design permits flows to re-circulate in the pocket regions associated with the wafer slot valve, or stagnate in unnecessary dead-space corners, then eddies can result and precursor remnants may exist in the precursor removal/purge periods and give rise to parasitic CVD
Thus, the starting constraints of the design challenges are:
The ALD cycle time (CT) consists of exposure of a first precursor, followed by removal (or “purge”) of unused portions of the first precursor and first precursor's reaction by-products, followed by exposure of a second precursor and removal of the unused portions of the second precursor and second precursor's reaction by-products. The sum of these four cycle time elements are the ALD CT.
In the present invention, to minimize the reaction space volume (305 in
The flow ring 345 (with inner surface element 354 and outer surface element 355) has a conduit with an input orifice 346 at nominally the same height as the susceptor. The lower orifice 348 of the flow ring is below or substantially below the lower edge 502 of the slot valve 204 when the wafer (i.e., the susceptor) is in the processing position (see, e.g.,
When the vertically movable susceptor 326 with the deep flow ring 345 is elevated into its “up” or processing position, the outer surface element 355 of the DFR 345 is placed in close proximity and overlapped with respect to a bottom of an inner surface element 376 of a “lid-ring” 375 (made up of inner element 376 and an outer element 377) that is attached to inside of the lid 380 of the reactor 314. The basic design is illustrated in
By iterative simulation, it has been found that (still) some small amount of reactants back-diffuse upstream within the conduit of the flow ring and the intended isolated region outside of the lid ring. This results in unwanted deposits on the reactor wall and, in the case of, for example, a multi-single-wafer reactor (e.g., as described by Puchacz et al. in the above-cited U.S. patent application Ser. No. 11/224,767), results in excessive diffusive cross-talk between intended independent reactors.
Hence, referring now to
As shown in
Still further reduction of diffusive back-flow can be obtained by using a TIG design with matched staircase surface contours on the flow ring and lid ring, as shown in
Returning now to
Importantly, if the DFR were not defined as extending deeper than the slot valve, then the re-circulations and isolation with respect to the slot valve would be poor. These poor alternative results imply a certain uniqueness to the attached extended depth annular DFR combined with a staircase-TIG design.
CFD simulations indicate that the design as shown in
The above-described ALD system can be operated using a multilevel flow process, such as that described in U.S. patent application Ser. No. 10/791,030 of Liu et al., assigned to the assignee of the present invention and incorporated herein by reference (which application also discusses a bi-level flow system proposed by Sneh in WO 03/062490), wherein for the assignee's case there is no downstream restrictor. Hence, embodiments of the invention may provide a TIG chamber configuration as described above and having a pump connected to remove gas streams from the reaction space, the DFR conduit and the lower chamber leading to the pump. A gas injection orifice that allows for by-passing the reaction space and the DFR conduit is placed such that gas injected through said orifice enters into the stream leading to the pump below the output orifice of the DFR. The orifice thus provides for directly injecting a gas into the pumping conduit leading directly to the input of the pump, without further restrictor(s) between the DFR output orifice and the pump input orifice, periodically during ALD cycling. In some cases the gas so injected may be injected at azimuthal points to achieve uniform exposure and uniform residence time. Also, the orifice of the DFR may have restrictors in the form of holes at the plane of its orifice, and the holes may be designed differently in different azimuthal directions to induce symmetric flow at the wafer plane.
Thus, a small volume symmetric flow apparatus defined for a minimal ALD reaction space volume with axi-symmetric flow with minimal chemical transport to the reactor walls has been described. Although discussed with reference to certain illustrated embodiments, however, the present invention is not limited to these embodiments. For example, as shown in
This application is a non-provisional of, claims priority to and incorporates by reference U.S. Provisional Patent Application 60/820,042, filed 21 Jul. 2006; and is also related to and incorporates by reference German patent application DE 102005056326.6 by Strauch and Seidel, filed 22 Nov. 2005; U.S. patent application Ser. No. 11/224,767 by Puchacz et al., filed 12 Sep. 2005, which is a non-provisional of, claims priority to and incorporates by reference U.S. Provisional Patent Application 60/609,598, filed 13 Sep. 2004; each of which is assigned to the assignee of the present invention.
| Number | Date | Country | |
|---|---|---|---|
| 60820042 | Jul 2006 | US |
