TWI495422B - System and method for transferring heat away from workpiece when processing workpiece - Google Patents

Description

System and method for transferring heat away from workpiece when processing workpiece

This invention relates to a semiconductor manufacturing, and more particularly to a cooling system for cooling a workpiece.

Ion implanters are commonly used in the manufacture of semiconductor wafers. The ion source is used to generate an ion beam, and the ion beam is then directed toward the wafer. When an ion strikes a wafer, it does a specific area of the wafer. The configuration of the doped regions defines their function, and these wafers can be converted into complex circuits via the use of conductive interconnects.

1 is a block diagram of a typical ion implanter 100. Ion source 110 produces the desired ion species. In some embodiments, these species are atomic ions that are best suited for high implant energies. In other embodiments, these species are molecular ions that are more suitable for low implant energies. These ions form a beam which is then passed through a source filter 120. The source filter is preferably located adjacent to the ion source. The ions in the ion beam are accelerated/decelerated in the column 130 to the desired energy level. A mass analyzer magnet 140 having a resolving aperture 145 is used to remove unwanted components from the ion beam such that the ion beam 150 having the desired energy and mass characteristics passes through the analytical aperture 145. .

In some embodiments, ion beam 150 is a spot beam. In this example, the ion beam passes through the scanner 160. Scanner 160 can be static Scanner or magnetic scanner. Scanner 160 deflects ion beam 150 to produce scan beams 155-157. In some embodiments, scanner 160 includes a separate scan plate that communicates with a scan generator. The scan generator generates a scan voltage waveform, such as a sinusoidal waveform having amplitude and frequency components, a sawtooth waveform, or a triangular waveform. These scan voltage waveforms are applied to the scan board. In a preferred embodiment, the scan waveform is typically very close to a triangular wave (fixed slope) such that the scanned beam stays at each location for approximately the same amount of time. Deviations from the triangle can be used to achieve uniform ion beam. The resulting electric field causes the ion beam to branch, as shown in Figure 1.

In another embodiment, the ion beam 150 is a ribbon beam. In this embodiment, no scanner is required and the ribbon beam has been shaped in a suitable manner.

An angle corrector 170 is used to deflect the bifurcated ion beams 155-157 into a set of ion beams having substantially parallel trajectories. Preferably, the angle corrector 170 includes a magnet coil and a plurality of magnetic pole pieces spaced apart from each other to form a gap, and the ion beam passes through the gap. The magnetic coil is energized to generate a magnetic field in the gap, and the ion beam is deflected according to the strength and direction of the applied magnetic field. The magnetic field can be adjusted by changing the current flowing through the magnetic coil. Alternatively, other structures such as a parallelizing lens can be used to perform this function.

After passing through the angle modifier 170, the scanned beam is aligned with the workpiece 175. The workpiece 175 is attached to the workpiece support. The workpiece holder provides a variety of moving angles.

The workpiece holder is used to hold the wafer in place and to orient the wafer to properly implant the ion beam into the wafer. In order to effectively hold the wafer in place, most of the workpiece supports (also known as platforms) typically use a circular force to hold the workpiece stationary on the workpiece support. Typically, the platform uses electrostatic forces to hold the workpiece in place. By creating a strong electrostatic force (also known as an electrostatic chuck) on the platform, the workpiece or wafer can be held in place without any mechanical fixtures. As a result, contamination is minimized and the cycle time is improved because the wafer does not need to be disassembled after the implantation is completed. These suction cups typically use one of two forces to hold the wafer in place: a coulomb force or a Johnson-Rahbeck force.

The workpiece support can generally move the workpiece in one or more directions. For example, in ion implantation, the ion beam is typically a scanned beam or a ribbon beam having a width greater than the height. It is assumed that the width of the ion beam is defined as the x-axis, and the height of the ion beam is defined as the y-axis, and the moving path of the ion beam is defined as the z-axis. The width of the ion beam is generally wider than the workpiece so that the workpiece does not need to move in the x direction. However, it is common to move the workpiece along the y-axis to expose the entire workpiece to the ion beam.

Another important function of the workpiece holder is to provide a heat sink to the workpiece. For example, during ion implantation, a large amount of energy (in the form of heat) is delivered to the workpiece. Disorderly heat can affect the characteristics of the implanted workpiece. Therefore, it is preferred to transfer heat away from the workpiece and to the workpiece support. The workpiece holder then removes heat. In certain embodiments, the fluid flows through conduits in the workpiece support that cause heat to be transferred to the flow Body and leave the workpiece support. Other methods of cooling the workpiece support are also well known in the art.

In some embodiments, heat is simply transferred from the workpiece to the workpiece support via physical contact between the two elements. However, tests have shown that due to the flaws and roughness of the abutment surface, even though the workpiece is in physical contact with the workpiece support, there is only a relatively small amount of actual contact between the two elements at a microscopic level.

The ion implantation system described above is preferably arranged in an environment close to a vacuum state. In fact, the pressure in this environment is typically less than 10 ^-5 Torr. Since the surrounding environment is close to an absolute vacuum, there are no other media that can transfer heat. Therefore, heat transfer is much less than expected.

One technique to improve heat transfer from the workpiece to the workpiece support is to use "back side gas." Figure 2 is a schematic diagram of this technique. Briefly, the workpiece 200 is secured to the workpiece support using mechanical or electrostatic means. The conduit 220 in the workpiece support 210 then transfers the gas 250 to the space between the workpiece 200 and the workpiece support 210, i.e., the wafer/platform interface.

Figure 3 is a schematic diagram of the heat transfer mechanism. Heat transfer occurs when gas molecules collide with the workpiece 200 to absorb heat from the workpiece 200. The gas molecules then collide with the workpiece support 210 to transfer heat to the workpiece support 210. The workpiece holder 210 acts as a heat sink and maintains an acceptable temperature. In some embodiments, the workpiece support 210 is cooled by passing fluid flowing through the cooling conduit 230. The flow of the back end gas can be controlled by a mass flow controller (MFC) 240.

Heat transfer is improved due to an increase in the amount of these hot gas molecules (e.g., by increasing the pressure). However, the pressure of the back end gas has The upper limit, and as the back end gas pressure increases, begins to overcome the clamping forces, thus causing the workpiece to be pushed away from the workpiece support. This reduces the actual physical contact between the two surfaces and significantly reduces heat transfer. This reduction occurs at very low pressures, such as pressures in ion implantation environments of less than 50 Torr. Excessive pressure can also cause damage to the workpiece. In addition, increasing the number of molecules in order to increase collisions between molecules can also result in reduced heat transfer between the solids.

As described above, the back end gas facilitates heat transfer as the gas molecules receive heat from the workpiece and transfer heat to the workpiece support. As is well known, there is a heat transfer effect at the gas-solid interface depending on the type of gas molecule and the type of solid. This effect is represented by an accommodation coefficient with values between 0 (no heat transfer) and 1 (best heat transfer). The adjustment factor (α) is generally defined as: α = (T _r -T _i ) / (T _s -T _i )

T _r is the temperature of the reflective elements (i.e., reflected off the solid surface of the gas molecules); T _i is the incident molecules (i.e., gas molecules before striking a solid surface) temperature; T _s is the temperature of the solid surface.

Lighter gases, such as helium and hydrogen, generally have lower adjustment factors than heavier gases such as nitrogen, argon, and air. In addition, the solid surface facilitates the adjustment factor since some solids provide better heat transfer than others. Referring to FIG. 3, it is assumed that the adjustment coefficient between the gas molecules and the workpiece 200 is α ₁ , and the adjustment coefficient between the gas molecules and the workpiece support 210 is α ₂ . When molecules collide with the workpiece 200, these molecules absorb heat from the workpiece 200 (proportional to the adjustment factor α ₁ ). These molecules then collide with the workpiece support 210 to transfer heat (proportional to the adjustment factor α ₂ ). Therefore, the actual heat transfer between the workpiece and the workpiece support is proportional to α ₁ × α ₂ . For example, if the adjustment factor is 0.9 at one interface to a particular gas and the adjustment factor is 0.7 at the other interface with the gas, then the heat transfer between the two interfaces is only 63% effective. Heavier gases can increase these coefficients, however, lighter gas molecules move faster and therefore transfer heat more quickly. This may result in a bias toward using a lighter gas rather than a heavier gas, regardless of the difference in the adjustment factor.

In many environments, it is important to keep the workpiece at a predetermined temperature range. Therefore, it is indispensable to efficiently transfer heat from the workpiece to the workpiece support. Accordingly, it would be advantageous to develop systems and methods for enhancing the cooling of workpieces, particularly semiconductor wafers in ion implantation systems.

The problems of the prior art can be overcome by the workpiece cooling system and method of the present application. Typically, heat is transferred to the workpiece support or platform. In an embodiment, the required operating temperature is determined. Based on this, a gas having a vapor pressure (for example, 10 torr to 50 torr) in a desired range is selected. This range must be low enough to be below the fixed force. This condensable gas is used to fill the space between the workpiece and the workpiece support. Based on adsorption and desorption, heat transfer occurs, thereby providing improved transfer characteristics compared to conventionally used gases such as helium, hydrogen, nitrogen, argon, and air.

As noted above, it is necessary to maintain the temperature of the workpiece, such as a semiconductor wafer in an ion implantation process. Current techniques for maintaining workpiece temperature rely on transferring heat from a workpiece, such as a platform, to a workpiece support that is physically in contact with the workpiece. Some embodiments increase the heat transfer mechanism by transferring "back end gas" in the space between the workpiece and the workpiece support. These gas molecules are used to transfer heat (or a portion of the heat) from the workpiece to the workpiece support. However, as mentioned above, this heat transfer mechanism is not as effective as imagined.

Referring to FIG. 2, a cross section of the workpiece support 210 and the workpiece 200 is shown. The workpiece holder 210 can have two types of conduits. The conduit 220 directs the gas 250 to the rear end of the workpiece 200, the space between the workpiece 200 and the workpiece support 210. Gas 250 is preferably stored in a central reservoir, such as a tank, and may pass through a mass flow controller or pressure regulator 240 to regulate its flow through conduit 220. In some embodiments, a small groove 260 is provided in the upper surface of the workpiece support 210 to provide an unobstructed path to allow the gas 250 to approach the space. The MFC or pressure regulator 240 controls the flow of gas to achieve the desired gas pressure. As noted above, since excessive pressure may cause the workpiece 200 to exit the workpiece support 210 or may damage the workpiece 200, it is preferred to carefully control the pressure.

In some embodiments, the second conduit 230 is used to circulate fluid used to cool the workpiece support 210. For example, water, air, or a suitable coolant can be circulated through the conduit 230 inside the workpiece to direct heat away from the platform.

Each ion implantation process has a predetermined operating temperature range. For example, many ion implantations are carried out in the temperature range of 0 °C to 50 °C, and more generally at room temperature (15 °C to 30 °C). Others can also be carried out at low temperatures, for example below -50 °C. Others can also be carried out at elevated temperatures, for example above 100 °C. Once you have decided on the required operating range, choose the right gas. The gas should have a sufficiently low vapor pressure at the desired operating temperature. For example, at room temperature, water has a vapor pressure of about 20 Torr. Propane has a similar vapor pressure for low temperature implantation at -100 °C. Ammonia (NH ₃ ) is also suitable for low temperature implantation. At -80 ° C, the vapor pressure of ammonia is about 30 Torr. For high temperature implantation, a substance such as glycerine may be used, which has a vapor pressure of about 40 Torr at 200 °C.

As noted above, the vapor pressure of the gas in the work area must be lower than the fixed force applied to the workpiece so that the workpiece does not suffer damage and remains in contact with the workpiece support. In other words, the pressure exerted by the gas (multiplied by the area of the workpiece) determines the force applied to the workpiece in a direction away from the workpiece support. The opposite of this force is the fixed force. In order to maintain the workpiece in contact with the workpiece support, the holding force must be greater than the gas pressure (multiplied by the area of the workpiece). Since the area of the workpiece is fixed, the gas pressure must be controlled to ensure that the above conditions are met.

In many embodiments, the desired vapor pressure is between 1 Torr and 50 Torr, although other ranges are possible and are within the scope of the present application. The selected gas is passed through conduit 220. For example, as described above, water has a vapor pressure of between 10 Torr and 20 Torr at room temperature. For ion implantation occurring at room temperature, water vapor is delivered to the space between the workpiece and the workpiece support. This can be achieved using the catheter 220 of Figure 2. Pressurizing water vapor using MFC or pressure regulator 240 to bring the vapor phase to the liquid phase Achieve a balance. When this phenomenon occurs, the film 205 of water vapor is adsorbed on the back surface of the wafer 200. The film 215 is also adsorbed on the top surface of the workpiece support 210. The heat transfer mechanism can be altered by creating a film of gas vapor on each surface.

Figure 4 is a schematic diagram of a heat transfer mechanism. In this arrangement, gas vapor molecules are adsorbed to the membrane 205 on the surface of the workpiece. Different water vapor molecules (already at high temperatures) are displaced and desorbed from the membrane 205. The removed molecules are then adsorbed to the film 215 on the top surface of the workpiece support 210. Again, at reduced workpiece support temperatures, the different molecules are then removed. Since the molecules are adsorbed to the solid at a temperature at or near the solid temperature (i.e., approximately equal to T _r T _s), and therefore can be adjusted to achieve near 1 coefficients.

Figure 5 is a flow chart of the aforementioned process steps. As described above, first, the required operating temperature is determined (block 400). Then, based on this operating temperature, a suitable gas is selected (block 410). The vapor pressure of this gas is preferably low enough at the desired temperature without damaging the workpiece or overcoming the fixing force. As noted above, MFC or pressure regulator 240 can be used to reduce the operating pressure at the vapor pressure of the working fluid, if desired. The selected gas is then delivered to the space between the workpiece and the workpiece support (block 420). Preferably, sufficient time is provided to allow the gas to reach steady state conditions in space (block 430). When the gas pressure is equal to the vapor pressure, the steady state condition is met. This allows gas to be adsorbed on the back side of the workpiece and adsorbed on the top surface of the workpiece support. Once the steady state condition is reached, the ion implantation process can begin (block 440).

As indicated by block 430, it is preferred to steam prior to the ion implantation process. A steady state condition is reached. This can be achieved in a variety of ways. In an embodiment, the process cycle time is reduced to allow steady state conditions to be reached. In other words, once a new workpiece or wafer is placed on the platform, the flow of steam begins. A lot of time is consumed before the ion implantation process begins. This time allows the vapor pressure to reach a steady state value with the adsorbed film. This method is simple, but can affect productivity, depending on the time required to reach equilibrium.

Other methods can be used to reduce the time required for the vapor pressure to reach a steady state condition. For example, the vapor film adsorbed on the workpiece support can be maintained during wafer exchange by reducing the temperature of the workpiece support. Lower temperatures will cause the film to liquefy or freeze. Additionally, steam can be directed through the porous media (which is part of the workpiece support). Finally, it is possible to reduce the time required by coating the workpiece with the selected gas, liquid or material before the workpiece is placed on the workpiece support. For example, the workpiece may be exposed to water vapor prior to being placed in the workpiece support and then chilled to retain water until placed on the workpiece support. In one embodiment, a wafer orient station is used to simultaneously coat the water vapor and freeze the wafer (during the positioning process). After this is done, the wafer is placed on the workpiece support and a steady state vapor pressure established as the wafer and workpiece support temperatures are established.

Although the present application discloses ion implantation, the application is not limited to this embodiment. The methods and systems described herein can be used in any application that uses workpieces and workpiece supports, particularly in a vacuum environment.

100‧‧‧Ion implanter

110‧‧‧Ion source

120‧‧‧Source filter

130‧‧‧Cylinder

140‧‧‧Quality Analyzer Magnet

145‧‧‧analysis hole

150‧‧‧Ion Beam

155~157‧‧‧Scanning beam

160‧‧‧Scanner

170‧‧‧Angle Corrector

175, 200‧‧‧ workpiece

205, 215‧‧‧ film

210‧‧‧Workpiece support

220, 230‧‧‧ catheter

240‧‧‧Flow controller or pressure regulator

250‧‧‧ gas

260‧‧‧Small trench

400, 410, 420, 430, 440‧‧‧ squares

Figure 1 is a schematic illustration of a conventional ion implanter.

2 is a cross-sectional view of a workpiece and a workpiece support in accordance with an embodiment.

3 is a schematic diagram of a prior art heat transfer mechanism.

Figure 4 is a schematic illustration of the heat transfer mechanism of the present invention.

Figure 5 is a flow diagram of process steps used in accordance with an embodiment.

400, 410, 420, 430, 440‧‧‧ squares

Claims (17)

A method of transferring heat away from a workpiece while the workpiece is being processed, the workpiece being mounted on a workpiece support, the method comprising: a. determining an operating temperature range for processing; b. selecting a gas at the operation The gas has a vapor pressure in a desired range at a temperature range; c. transporting the gas to a space between a back surface of the workpiece and a top surface of the workpiece support; and d. processing the workpiece Where a force is applied to hold the workpiece on the workpiece support and the desired range of vapor pressures produces an opposing force that is less than the force that holds the workpiece. The method of transferring heat away from the workpiece when the workpiece is processed as described in claim 1, further comprising the step of waiting for the gas to reach equilibrium in the space before processing the workpiece. A method of transferring heat away from a workpiece when the workpiece is processed as described in claim 2, wherein a liquid film is produced on the back surface of the workpiece and the top surface of the workpiece holder. A method of transferring heat away from a workpiece when the workpiece is processed as described in claim 1, wherein the gas is delivered at a pressure equal to the vapor pressure. The method of transferring heat away from a workpiece as described in claim 1 of the patent application, further comprising the step of cooling the workpiece support prior to moving the processed workpiece. A method of transferring heat away from a workpiece as claimed in claim 1 wherein said operating temperature range is between 0 ° C and 50 ° C and said selected gas comprises water vapor. A method of transferring heat away from a workpiece when the workpiece is processed as described in claim 6 wherein the vapor pressure is between 10 torr and 50 torr. A method of transferring heat away from a workpiece as claimed in claim 1 wherein said operating temperature range is below -50 ° C and said selected gas comprises ammonia. A method of transferring heat away from a workpiece as claimed in claim 1 wherein said operating temperature range is above 100 ° C and said selected gas comprises glycerol. A method of transferring heat away from a workpiece when the workpiece is processed as described in claim 1, wherein the processing comprises ion implantation. A system for transferring heat away from a workpiece, the workpiece being processed at a predetermined operating temperature range, the system comprising: a. a workpiece support, the workpiece being located on the workpiece support to a top surface of the workpiece holder in contact with the back surface of the workpiece; b. means for holding the workpiece on the workpiece holder, the device applying a force to the workpiece; c. a conduit for Providing a gas to a space defined by the back surface of the workpiece and the top surface of the workpiece support; and d. a reservoir for holding the gas, wherein the operating temperature range The gas has a vapor pressure in which a pressure is applied to the workpiece The vapor pressure of the opposing force is less than the force applied by the device to hold the workpiece. A system for transferring heat away from a workpiece as described in claim 11 wherein said operating temperature range is between 0 ° C and 50 ° C and said gas comprises water vapor. A system for transferring heat away from a workpiece as described in claim 11 wherein said operating temperature range is below -50 ° C and said gas comprises ammonia. A system for transferring heat away from a workpiece as described in claim 11 wherein said operating temperature range is above 100 ° C and said gas comprises glycerol. A system for transferring heat away from a workpiece as described in claim 11, wherein the conduit is located in the workpiece support and the gas passes through the workpiece support to the space. A system for transferring heat away from a workpiece as described in claim 11 further includes a mass flow controller or pressure regulator between the reservoir and the space. A system for transferring heat away from a workpiece as described in claim 16 wherein the mass flow controller delivers the gas at a pressure equal to the vapor pressure.