Method and apparatus for transferring heat from a substrate to a chuck

A chuck method of and apparatus (50, 150, 300) for supporting a substrate (W) during processing of the substrate, where the substrate has a lower surface (WL). The apparatus facilitates heat transfer away from the substrate during processing of the substrate. The apparatus comprises a chuck body (60) having an outer edge (70) and a rough upper surface (64U). The substrate is arranged adjacent the rough surface such that the substrate lower surface and the roughened upper surface form a gap (100) therebetween. The apparatus further includes a central gas conduit (80) passing through the chuck body. The central conduit has a second end (82b) open to the roughened upper surface and a first end opposite the second end connected to a gas source (86). The conduit is arranged such that a gas can flow through the conduit into the gap and toward the chuck body outer edge. The gas used has an atomic or molecular weight that is greater than that of helium. The surface roughness, the substrate lower surface and the flow of the heavier gas in the gap contribute to defining an accommodation coefficient α and a mean free path λ such that the ratio α/λ is higher than that of prior art apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a method and apparatus for transferring heat from a substrate to a chuck that supports the substrate.

2. Discussion of the Background

There is a never-ending demand for increasing throughput in semiconductor, display and other types of substrate manufacturing. Many of the processes in substrate processing involve placing the substrate, such as a semiconductor wafer, on a chuck and processing the substrate. During certain of these processes, the substrate heats up, and this heat needs to be dissipated quickly. Quick heat dissipation allows the substrate temperature to be maintained within certain limits determined for the process even at high processing tool power levels, and the quick initiation of the next process step. Both of these allow a high process throughput, which drives down the process cost per substrate.

One process used with substrates, such as in the fabrication of semiconductor devices (e.g., integrated circuits, or “ICs”) or displays, involves subjecting the substrate to a plasma for depositing material onto or etching material from the substrate surface. During this process, high-energy plasma particles bombard the substrate, and generate a large amount of heat, which is absorbed by the substrate. This heat needs to be quickly transferred from the substrate to the chuck and then quickly dissipated from the chuck, so that the substrate is kept at a steady temperature. If the transfer of heat from substrate to chuck, or the heat dissipation from the chuck itself is inefficient or inadequate, the temperature of the substrate increases rapidly. The accumulation of heat in the substrate can damage structures on the substrate (e.g., excess heat can cause unwanted diffusion of dopants in a semiconductor substrate, which can lead to leakage currents in transistors). This thermal buildup also impacts tool throughput because in the absence of an effective heat transfer mechanism, the plasma process needs to be operated at a lower power level or in an interrupted manner (to allow the substrates to cool) to achieve adequate process yield (e.g., fewer damaged devices). Control of substrate heating during plasma processing is also important because the substrate temperature affects the etch process itself (e.g., etch selectivity to photoresist, etc.).

FIG. 1is a schematic diagram of a prior art chuck apparatus10for supporting a substrate W in a low-pressure environment. Chuck10includes a chuck body12with an upper surface14and a conduit16formed within the chuck body that leads from a helium gas source (not shown) to surface14. During substrate processing, helium gas18is fed into conduit16and flows toward substrate W. Because of the low-pressure environment above the wafer, the helium gas18introduced between the wafer and chuck, and the roughness of chuck surface14, a gas gap forms between chuck surface14and the lower surface of substrate W. This gap separates the substrate and the chuck body by a low-pressure gas gap30having a mean gap width δ, which is typically a few micrometers wide. In this sense, substrate W is exposed to helium gas18flowing between chuck upper surface14and substrate W lower surface. The helium gas injected into gap30flows outwards to the outer edge of substrate W, its presence in the gap30thereby allowing the transfer of heat away from the substrate to the chuck (as indicated by arrows32). This heat transfer mechanism is known as low-pressure gas gap conduction and is widely used in the semiconductor industry. Helium gas is used to effectuate the heat transfer because it is inert and has a high thermal conductivity (only hydrogen has a higher thermal conductivity).

Unlike the case of heat conduction in normal atmospheric (e.g. high) pressure conditions, under low pressure conditions, the extent of gas-surface energy exchange (and, hence, the effectiveness of cooling) is characterized by the so-called accommodation coefficient α, in addition to the gas thermal conductivity.

Low-pressure (i.e., of the order of 10 to 50 Torr or less) gas gap conduction is used for cooling in many types of substrate processing equipment. For instance, in most etch tools, there is a gap between the substrate and the lower electrode. This gap is filled with low-pressure helium or argon and is used to cool the substrate. To guide the helium or argon flow in a certain direction, various channels on the upper surface of the chuck or on the backside of the substrate may be used.

The heat flux q″ of thermal conduction for a low-pressure gas gap between a substrate and a chuck is given by the product of the heat transfer coefficient hgand the temperature difference ΔT=Tw−Tcbetween the proximate surfaces of the substrate and the chuck, or q″=hgΔT. In general, the chuck temperature is controlled by its cooling system. The substrate temperature, on the other hand, is constrained by the desire to maintain a high process yield and by the type of devices being fabricated on the substrate. For a given process and device type, this essentially fixes the temperature difference ΔT.

BRIEF SUMMARY OF THE INVENTION

The present invention pertains to methods and apparatus for transferring heat from a substrate to a chuck that supports the substrate.

The present invention utilizes a chuck upper surface that is rough and may optionally utilize a gas having an atomic/molecular weight larger than argon. For example, the surface may have a roughness (Ra) of 0.4 μm or more. Alternatively, the roughness (Ra) may be between 0.4 and about 10 μm or about 1 to about 4 μm. The roughness may be such that the temperature difference between the chuck upper surface and the substrate may be within 5 percent, 2 percent or 1 percent of the minimum, or at the minimum. The roughness may be such that the ratio of accommodation coefficient α to the gas mean free path λ is at a maximum, or within 1 percent, 2 percent or 5 percent of the maximum. The present invention may optionally include providing a substrate having a lower surface with a surface roughness, e.g., a Ra of 1 μm or more.

Accordingly, a first aspect of the invention is a chuck apparatus for supporting a substrate having an upper and lower surface and for facilitating heat transfer away from the substrate during processing of the upper surface of the substrate. The apparatus comprises a chuck body having an outer edge and a roughened upper surface. The substrate, which can be a silicon wafer, display substrate or the like, is arranged adjacent the roughened surface such that the substrate lower surface and the roughened chuck body upper surface form a gap therebetween. The apparatus further includes a gas conduit passing through the chuck body. The conduit has a first end open to the roughened upper surface and a second end opposite said first end. The conduit is arranged such that a gas can flow through the conduit from a gas source and into the gap and toward the chuck body outer edge.

A second aspect of the invention is the apparatus as described above, but having different regions of surface roughness on the upper surface.

A third aspect of the invention is the apparatus as described above, having a plurality of gas holes which can be connected to different gas sources that allow different types of gas to be injected into different zones within the gap, thereby providing for spatially varying amounts of heat transfer. A multi-channel gas mixer can be used to efficiently mix the gases and also inject the gas (mixed or otherwise) into the gap. In addition, a main control unit electronically connected to the different gas sources and the multi-channel gas mixer allows for dynamically changing the flow of gas (or gases) into the gap to adjust the heat transfer from the substrate to the chuck body.

A fourth aspect of the invention is a method facilitating the transfer of heat from the substrate to the chuck body. The method comprises the steps of first, providing the upper surface with a rough surface. This surface roughness may be uniform across the surface, or may vary spatially in the form of different regions. Alternatively, the surface roughness may vary smoothly as a function of position, e.g., radially outwardly decreasing from the center of the surface. The next step is forming a gap between the roughened upper surface and the substrate lower surface by arranging the substrate adjacent the roughened surface. The last step then involves flowing at least one gas into said gap such that the gas flows towards the outer edge. The gas (which may be a gas mixture) flowing through the gap may have an atomic or molecular weight greater than that of argon, and thus a higher ratio α/λ, where α is the accommodation coefficient of the surface and λ is the gas mean free path.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to methods and apparatus for transferring heat from a substrate to a chuck that supports the substrate.

To increase the amount of heat transfer from a substrate to a chuck body in a chuck apparatus, the heat transfer coefficient hgcan be increased. At low pressures, the heat transfer coefficient hgis actually a fairly complex function of a number of other parameters, including the atomic and/or molecular sizes of the gas and solid surfaces, the pressure of the gas used, and the conditions of the surfaces (e.g., roughness or contamination) that define the gap.

To account for the above effects on the heat transfer coefficient hg, an effective gap width δ′ that is greater than the physical mean gap width δ is introduced.FIG. 2shows the known manner in which the effective thickness δ′ of the gas gap relates to the physical gas gap width δ and the temperature T within the gap. When the gas pressure is large and δ>>λ, where λ is the gas mean free path (i.e., the average distance between collisions between gas atoms or molecules), the temperature is a linear function of position between the surfaces comprising the gap of thickness δ. Under these conditions the temperature difference is ΔT, the slope of the temperature profile curve is ΔT/δ, and the heat flux is q″=hgΔT=kΔT/δ. However, if the pressure in the gas gap is lowered to a value such that δ˜λ, or δ<λ, which is the case for typical substrate chuck gas gaps, and the heat flux q″ is kept the same, the temperature difference ΔT′ becomes much larger, as shown by curves C1and C2. The phenomenon of deviation of the temperature profile in the vicinity of solid walls from a linear profile is called “temperature jump.” It is a result of the effects of solid walls and gas atomic and/or molecular weights, wall surface roughness, and low gas pressure, which all reduce the heat transfer coefficient.

With continuing reference toFIG. 2, the effective length of the heat transfer path also increases due to temperature jumps (i.e., steep temperature gradients), resulting in an effective gas gap width of δ′=δ+g1+g2, where g1and g2are called “temperature jump distances.” The size of temperature jump distances gidepends on the accommodation coefficient αiat each gas-solid interface. The accommodation coefficient is defined as α=[Tscat−Tg]/[Ts−Tg], where Tsis the solid surface temperature, Tgis the gas temperature before collision, and Tscatis the temperature of the gas scattered after collision with the solid surface. Heat transfer is most efficient when Tscat=Ts, which means that the gas “fully accommodates” its temperature to that of the solid wall. In this case, α=1. In general, α<1 and can be anywhere between 0.01 and 1.0, depending on the gas pressure, the gas and solid wall chemical species atomic and/or molecular weights, and the conditions of the solid surface (e.g. roughness and contamination).

Lighter, monatomic gases have smaller accommodation coefficients; molecular gases have larger accommodation coefficients; up to a point, clean or smooth surfaces have lower accommodation coefficients than contaminated or rough surfaces. After some manipulation, in the limit of low pressure when λ>>δ, temperature jump distances can be shown to approximately be gi≈λ/α, and the effective gas gap width becomes δ′≈2λ/α. The heat transfer coefficient (conductance) now becomes hg=kg/δ′≈kα/2λ, no longer a simple function of gas conductivity kg, but also a very strong function of the ratio α/λ. Indeed, it has been found that the ratio α/λ for some gases with low conductivity kgcompared to helium, in the presence of solid surfaces of certain roughness, and at certain pressures, is such that it makes the overall heat transfer coefficient hggreater than that of helium under similar gap conditions (e.g., gap distance and gas pressure).

First Embodiment

With reference toFIG. 3, there is shown a first embodiment of a chuck apparatus50for supporting a substrate W with a lower surface WLand an upper surface WU, for providing enhanced gap conduction of heat during substrate processing. In a preferred embodiment of the present invention, lower surface WLis optionally roughened to enhance the effects described below. For example, lower surface WLmay be roughened so that the average roughness Ra, as defined by ANSI standard ANSI-B46.1-1985, may be 1 μm or more. Chuck apparatus50is in a low-pressure environment, such as within a chamber54having an evacuated interior region56. Chuck apparatus50comprises a chuck body60. Chuck body60has upper portion64with a rough upper surface64Uand a lower surface64L, and may possibly, but not necessarily, have a base portion68depending from lower surface64Land having an upper end68Uand a lower end68L. Upper surface64Umay have a roughness Ra, as defined by ANSI standard ANSI-B46.1-1985, of 0.4 μm or more. The roughness Ra of upper surface64Umay be 0.4 to about 10 μm or about 1 to about 4 μm. Upper portion64and base portion68may be cylindrical or any other shape. Base portion68is typically narrower and more elongate than upper portion64, so that chuck body60has a T-shaped cross-section with a central axis A. Base portion68may be a separate member attached to lower surface64L, or may be integral with upper portion64. The latter has an outer edge70. Chuck body60(or at least upper portion64) is typically made of aluminum, stainless steel, or other materials compatible with the chamber process, and that are good heat conductors. Upper surface64Uis also referred to herein generally as the “chuck body upper surface.”

Chuck body60further includes a central gas conduit80which may be aligned along central axis A and pass through upper cylindrical portion64and lower cylindrical portion68. Alternately, portion64and/or portion68may be non-cylindrical. Conduit80has a first end82aopen at lower end68Land a second end82bopen to upper surface64U.

Chuck apparatus50also comprises a gas source86connected to gas conduit80at first end82avia a gas line90. Gas source86is shown to be exterior to chamber54, but may be interior.

Chuck apparatus also may include a main control unit92electronically connected to gas source86and is used to dynamically control the delivery of gas from the gas source to control the heat transfer process. In one embodiment, main control unit92is a computer having a memory unit MU with both random-access memory (RAM) and read-only memory (ROM), a central processing unit CPU (e.g., PENTIUM™ processor from Intel Corporation), and a hard disk HD, all electronically connected. Hard disk HD serves as a secondary computer-readable storage medium, and may be, for example, a hard disk drive for storing information corresponding to instructions for the main control unit to control the uniformity of the substrate temperature via control of the heat transfer, as described below. Main control unit92also may include a disk drive DD, electronically connected to hard disk HD, memory unit MU and central processing unit CPU, wherein the disk drive is capable of accepting and reading (and even writing to) a computer-readable medium CRM, such as a floppy disk or compact disk (CD), on which is stored information corresponding to instructions for main control unit92to carry out the present invention. It is also preferable that main control unit92have data acquisition and control capability. Main control unit92may comprise a computer, such as a DELL PRECISION WORKSTATION 610™, available from Dell Corporation, Dallas, Tex. and a number of peripherals.

Lower surface WLof substrate W is arranged adjacent upper surface64Uduring processing. Gas94from gas source86flows through central gas conduit80from first end82ato second end82band then flows between substrate lower surface WLand upper surface64Utowards outer edge70. The flow of gas94in the low-pressure environment of interior region56results in the formation of a low-pressure gap100of width δ between substrate lower surface WLand upper surface64Uof chuck body60.

As discussed above, the larger the ratio α/λ, the larger the heat transfer coefficient, and, therefore, the larger the heat flux between substrate W and upper cylindrical portion64of chuck body60through gap100. The heat flux is illustrated by arrows98. The flow of gas94into gap100can be dynamically controlled by main control unit92to control the heat transfer between substrate W and chuck body60.

The surface roughness of upper surface ΓUmay be selected so that the ratio α/λ, of the accommodation coefficient α to the mean free path λ of the gas has a maximum value, or is within 1 percent, 2 percent or 5 percent of the maximum. The amount of surface roughness affects the efficiency of the exchange of energy between the gas atoms/molecules and the substrate and chuck surfaces. The interaction of the gas atoms/molecules with a rough surface is more like an inelastic collision as compared to that with a smooth surface, which means more energy is exchanged between the gas atoms/molecules and the surfaces. Thus, the use of one or more roughened surfaces (i.e., on upper surface64Uand/or lower surface WL) increases the accommodation coefficient α, thereby increasing the ratio α/λ. The result is a more efficient exchange of heat between the gas molecules and the substrate and chuck surfaces. The surface roughness can be uniform across upper surface64U, can comprise various regions of surface roughness (discussed further below), or can vary smoothly (e.g., radially outwardly decreasing, or increasing, from central axis A).

The amount of surface roughness depends on the gas that is being used. In prior art chucks, the top surfaces14are typically mirror polished, which means the roughness is generally less than 0.1 to 0.4 μm. Chucks with different roughnesses of the top surface64Ucan be tested and the one that reduces the temperature difference ΔT between the chuck surface and the substrate can be used. For example, a chuck surface roughness that minimizes the temperature difference or is within 5 percent, 2 percent or 1 percent of the minimum can be selected. The chucks should be tested at the same power setting, and the temperatures of the wafer and chuck may be measured using, for example, thermocouples or fluoroscopic temperature sensors. In general, one does not need to find and know α, or even λ, as long as one knows that the heat transfer coefficient hgincreases (e.g. temperature difference ΔT decreases). Changing the gas would, in general, require that the exercise be repeated, and a different roughness may be chosen. The chuck top surface may be ground with the appropriate grit size to obtain the desired roughness.

Gas94supplied by gas source86may be helium or argon, or have an atomic or molecular weight greater than that of helium or argon, or be a mixture of gasses. Examples of such atomic gases include noble gases neon, argon, krypton, and xenon. Examples of such molecular gases include those typically used in etch processes, such as C4F8, SF6, as well as C5F8, C2F6, etc.

Second Embodiment

With reference now toFIGS. 4A and 4B, there is shown a second chuck apparatus150for supporting a substrate W as a second embodiment of the present invention. A x-y plane in gap100parallel to surface64Uis shown for the sake of orientation. Chuck apparatus150is essentially the same as chuck apparatus50described above, except that chuck apparatus150includes some additional features, described below. Those elements in chuck apparatus150that are the same as in chuck apparatus50are given the same reference numbers. Also, chuck apparatus150is in a low-pressure environment (region56) as well, though chamber54is not shown inFIGS. 4A and 4B.

Thus, chuck apparatus150includes the same elements as chuck50, and further includes a plurality of gas injection holes160on upper surface64Uarranged in concentric gas rings162aand162b. Alternatively, holes160may be arranged as rays emanating from the center of the chuck. Each concentric gas ring162a,162bis in pneumatic communication with a corresponding gas ring conduit166aand166bconnecting upper surface64Uand lower surface68L. Thus, gas injection holes160are the openings at one end of conduits166aand166bat surface64U. Though two concentric gas rings and two corresponding gas conduits are shown for the sake of illustration, in practice one or more such concentric gas rings and gas conduits can be employed.

Chuck apparatus150additionally includes secondary gas sources186A and186B (i.e., second and third gas sources, where gas source86is the first or primary gas source) pneumatically connected to corresponding gas conduits166aand166bvia respective gas supply lines190A and190B, for supplying second and third gases194A and194B to upper surface64Uthrough gas injection holes160. Thus, upper surface64Ureceives gas94from first gas source86, second gas194A from gas source186A, and third gas194B from third gas source186B. Second and third gases194A and194B may be helium or argon, may each have an average atomic/molecular weight greater than helium or argon, or may be a mixture of gases.

With continuing reference toFIGS. 4A and 4B, chuck apparatus150may include a multi-channel gas mixer200connected to some or all of gas sources86,186A and186B, for supplying to upper surface64Uthrough conduits80,166aand166b, one or more different gas mixtures. This allows chuck apparatus150to have different volume zones Zi(e.g., Z1, Z2, Z3) within gap100, wherein the ratios (kα/λ)idiffer due to the introduction of different compositions of gas mixtures into gap100in the different zones. The configuration of chuck apparatus150thus allows for spatially varying the heat transfer coefficient in the x-y directions within gap100, thereby providing a way of controlling the temperature uniformity of substrate W. This feature is particularly useful for processes that heat the substrate in a spatially varying manner.

In chuck apparatus150, main control unit92is electrically connected to first gas source86, secondary gas sources186A and186B, and to multi-channel gas mixer200. This allows for dynamic control of the mixing of gases from these gas sources and for controlling the flow of the mixed gas into the gap to control the heat transfer from substrate W to chuck body60.

Third Embodiment

FIG. 5is a plan view of chuck apparatus300of the present invention. A radial coordinate r is shown for reference. Chuck apparatus300is identical in most respects to chuck apparatus150described above in connection withFIGS. 4A and 4B. The difference between chuck apparatus150and chuck apparatus300is that surface ΓUof chuck apparatus300has annular regions Ri(i.e., R1, R2and R3) having different surface roughnesses σi(e.g., σ1, σ2and σ3, respectively). Regions R1, R2and R3may be defined by concentric gas rings162aand162b, each of which comprise a plurality of gas holes160, as described above in connection with chuck apparatus150.

In chuck apparatus300, the ratios (α/λ)iof the accommodation coefficient α to the mean free path λ of the gases in different zones Zicorresponding to regions Rimay be very different due to two factors. The first factor is the different gases injected into different zones Zicorresponding to regions Riof upper surface64U. The second factor is the different surface roughnesses σiregions Ri. Exemplary values for surface roughnesses σ1, σ2and σ3are an Ra of 2, 8 and 16 μm. Therefore, the heat transfer due to gas gap conduction in zones Zimay be quite different, even more so than in chuck apparatus150. Chuck apparatus300thus provides a more versatile system for cooling substrate W and for controlling substrate temperature uniformity. Greater or fewer zones Zican be formed, depending on the need for controlling the temperature uniformity.

Multi-channel gas mixer200in the second and third embodiments is used to mix two or more component gases from two or more gases from primary gas source86and secondary gas sources186A and186B into gas mixtures of varying composition. It will be apparent to one skilled in the art that the use of three gasses sources as described above and shown inFIGS. 4A and 4Bis for illustration, and that any reasonable number of gas sources can be connected to gas mixer200to achieve a wide variety of gas mixtures. In the present invention, two or more different gases with different values of kα/λ may be used so that a gas mixture having a wide range of kα/λ, and thus a wide range of heat transfer coefficients, can be achieved. For example, helium might serve as a first mixing gas, while a heavy process gas like C4F8or SF6, or a heavy inert atomic gas such as xenon, might serve as a second mixing gas. Multi-channel gas mixer200can then provide mixtures of the two (or more) gases that have a heat transfer coefficient between a low value (e.g., for pure helium gas) and a high value (e.g. for the pure heavy gas).

The roughness of surface64Uin chuck apparatus150and the roughness of surface64Uin regions Riis not easily changed during processing of substrate W. If the range of heat transfer coefficients obtainable with the surface roughness values (σ or σi) and the range of gas mixture compositions is not sufficiently wide, then upper cylindrical portion64of chuck body60can be replaced with another having a different surface roughness σ or σi. This allows for a different range of heat transfer coefficients to be obtained.

With either of the last two embodiments, the different regions Rimay be of a shape that corresponds to the heating of the substrate. For example, where the substrate process induces radial heating of the substrate, annular regions Risuch as described may prove most advantageous. However, other shapes can be used, particularly where the substrate heating distribution is non-radial. Also, as mentioned above, surface roughness σ can be a smooth function of position (i.e., σ(x,y) or σ(r), where r is the radial measure (x2+y2)1/2) so that there are no discrete regions R, but one region R where σ varies continuously.

The many features and advantages of the present invention are apparent from the detailed specification and thus, it is intended by the appended claims to cover all such features and advantages of the described method which follow in the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those of ordinary skill in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the invention as claimed.