US20070228008A1

US20070228008A1 - Medium pressure plasma system for removal of surface layers without substrate loss

Info

Publication number: US20070228008A1
Application number: US11/295,273
Authority: US
Inventors: John Wolfe; Aseem Srivastava; Ivan Berry; Palanikumaran Sakthivel
Original assignee: University of Houston
Current assignee: Axcelis Technologies Inc; University of Houston
Priority date: 2004-12-06
Filing date: 2005-12-06
Publication date: 2007-10-04

Abstract

A system and method for removing photoresist or other organic compounds from semiconductor wafers is provided. Non-fluorinated reactant gases (O₂, H₂, H₂O, N₂etc.) are activated in a quartz tube by a medium pressure surface wave discharge. As the plasma jet impinges on a substrate, volatile reaction products (H₂O, CO₂, or low molecular weight hydrocarbons) selectively remove the photoresist from the surface. The medium pressure also enables high gas temperatures that provide an effective source of heat in the reactive zone on the wafer that enhances etch rates and provides a practical means of removing ion implanted photoresist.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/633,673 filed on Dec. 6, 2004.

TECHNICAL FIELD

The present invention relates in general to semiconductor processing, and in particular, to the selective removal of surface layers from a workpiece, as for example, a semiconductor wafer in the manufacture of integrated circuits. It will be understood that while the following discussion is directed to semiconductor manufacturing proceses, the present invention may apply to various manufacturing processes and apparatus therefore such that the present invention shall not be limited to semiconductor manufacturing.

BACKGROUND INFORMATION

Photoresist masks define every layer of an integrated circuit (IC), from front-end-of-line (FEOL) ion implantation for isolation, P-or N-well doping, threshold voltage adjustment and source-drain contacts to back-end-of-line (BEOL) plasma etching or plating of metal and etching of interlevel dielectrics. These coatings must be removed efficiently and completely after each level in the semiconductor device is formed. In this context, resist removal may be variously described as resist ashing, stripping or etching. While the present discussion will make various references to “etching”, it will be understood that in the context of the present invention the term etching is used universally to refer to ashing, stripping or etching, and, where appropriate, may refer to various other processes where removal of a surface layer is implied. Presently, the utilization of a downstream plasma generating apparatus is the industry standard for removing resist. In this approach, a normally non-reactive gas, such as O₂, flows through a microwave or radio-frequency discharge, where it is transformed into a plasma, defined as a mixture of excited molecules, radicals, ions, and electrons. The charged species in the plasma may recombine as they flow through a downstream distribution system. However, many radicals may have sufficient lifetimes to reach the wafer. For the example using oxygen as the flow gas, singlet sigma metastable oxygen molecules may persist and ultimately interact with the wafer surface, (J. T. Jeong et al., Plasma Sources Sci. Technol. 7, 282-285, 1998). High energy ion bombardment may cause unwanted damage to components of the semiconducting device or to the wafer substrate itself. The absence of charged particles may thus prevent electrical damage to the integrated circuits (ICs) in downstream ash tools.
The present description provides a novel plasma source never before used in semiconductor manufacturing processes, based on surface waveguide discharge technology. Previous implementation of plasma systems have employed a source of electromagnetic power to activate a plasma gas, such as the Surfaguide device developed by Moisson et al., (Moisson et al., IEEE Trans. Plasma Sci., PS-12, 203-214, 1984). However, the limited cooling efficiency of this apparatus effectively limited the power densities of the resulting plasma. Previously, oil-cooled plasma sources have commonly been implemented. However, operating a plasma at high energy involves very high temperatures. Cooling oil decomposes under these conditions, depositing a carbonized layer on the outside wall of the plasma discharge tube within the waveguide. Once initiated, the oil-based carbon layer grows rapidly with increasing microwave exposure; eventually, catastrophic arcing takes place within the waveguide and destroys the plasma discharge tube. Thus, oil-cooled systems are unsuitable for high energy plasma discharges. Air-cooled high power plasma systems have been reported, but their operation was limited to atmospheric pressure, i.e. in the high pressure regime, such that the resulting plasma would not contain reactive species necessary for selective removal of organic surface layers, such as photoresist, (Y. Okamoto, High-Power Microwave-Induced Helium Plasma At Atmospheric Pressure For Determination Of Halogens In Aqueous Solution, Jap. Journ. Appl. Phys. 38, L 338, 1999).
Typically, the wafers are heated to enhance the reaction rate during downstream plasma ashing. The application time in conventional processes for an unimplanted resist layer may be as low as 15 seconds at 270° C. for O₂-based plasma chemistry. Once the resist layer has been subjected to ion implantation, as required for intermediate IC fabrication steps, the reaction mechanism using plasma becomes more complex. Ion implanted resist is much more difficult to remove than unimplanted resist, since the implantation process produces a carbonized crust mixed with metal ions that exhibits extremely low intrinsic etch rates, (G K. Vinogradova et al., J. Vac. Sci. Technol. B, 17, 1, January/Febuary 1999; S Fujimura et al. Nucl. Instrum. Methods B39, 1989, pp. 809; K J Orvek et al., Nucl. Instrum. Methods B7/8, 1985, P501; T Bausum et al., “Stripping High-Dose Implanted Resist for 300 mm Production,” Semiconductor International, Jun. 1, 2003; J. R. Wasson et al., “Ion Absorbing Stencil Mask Coatings For Ion Beam Lithography,” J. Vac. Sci. Technol. B, 15, 2214, 1997). The process throughput is further reduced because the wafer temperature must be kept below about 120° C. to prevent the ejection of particulates, which may occur when the crust explodes under the pressure of gas, mainly NH₃, that is evolved in response to heating above the hardbake temperature. This phenomena is known as popping, (D. Fleming et al., Manufacturing Improvements Realized through an Optimized pre-Implant UV/Bake Process, Future Fab International, 4, 1, 1977, p 177). Ion implanted resist films, unlike graphite or photoresist, are essentially inert; they do not adsorb atmospheric oxygen, nitrogen, or water vapor. The activation energy for an ion implanted resist film reacting with atomic oxygen has been reported to be 2.4 eV versus 0.17 eV for unimplanted resist, (A. Joshi et al., J. Vac. Sci. Technol. A, 8, 3, May/June 1990, pp. 2137). This additional activation energy explains why ion implanted resist films are essentially unetchable in conventional downstream plasmas. Further, RF-bias and fluorine chemistry have been used to enhance the etch rate for ion implanted films, (K J Orvek and C Huffinan, Nucl. Instrum. Methods B7/8 (1985) P501; JI. McOmber et al., Nucl. Instrum. Methods B74 (1993) pp. 266-270; K Reinhardt et al., IBM Technical Symposium, France October 1999). However, these more aggressive removal methods invariably cause some degree of erosion of unprotected surfaces. Increasingly, such losses on wafer surfaces are becoming economically unacceptable as the thickness of gate oxide and contacts continue to shrink with each new generation of ICs.
Thus, there is a critical need for a new etching paradigm that can remove ion implanted photoresist layers with essentially perfect selectivity over silicon dioxide, silicon or other thin dielectric films, and that provides total independence from fluorine chemistry. There is also a need for a new technology that provides commercially viable removal rates while maintaining low substrate temperatures that may be applied on materials coated with inorganic or organic materials, including either implanted or unimplanted surface layers.

SUMMARY OF THE INVENTION

The present invention addresses the foregoing needs by providing a new approach for removing surface layers from semiconductor wafers. The present invention provides a method wherein reactant gases are activated by a medium pressure surface wave discharge. The method further involves the formation of volatile reactants in the plasma gas that can strip photoresist from the surface of a wafer. The plasma gas forms a reactive plasma jet that impinges on a substrate from which surface layers may be selectively, thus safely, etched with high efficiency. The method may be practiced in a commerically viable manner for stripping applied materials from large wafers by scanning them in front of the jet.
In particular, the present invention can be characterized generally as an apparatus for selectively removing surface layers from a workpiece in a manufacturing process, wherein the apparatus comprises: a process chamber; a plama applicator; and a cooling system. The process chamber defines a subatmospheric environment for receiving the workpiece to be processed such that a surface layer can be removed. The plasma applicator generates a plasma and includes a pressurized supply of reactant process gas, a plasma discharge tube in fluid communication with the pressurized supply of reactant process gas, an electromagnetic power source for directing electromagnetic power to the plasma discharge tube to generate a plasma therein, and a nozzle opening situated at an end of the plasma discharge tube for jetting the plasma gas into the process chamber in a direction toward the workpiece. Finally, the cooling system includes a conduit substantially surrounding the plasma discharge tube for circulating a gaseous coolant therethrough, thereby forming a cooling channel around the plasma discharge tube.
An embodiment of the present invention is provided as an apparatus for performing medium pressure (between about 10 Torr and about 500 Torr) plasma material removal on semiconductor wafers. The apparatus provides a system wherein reactant gases, such as O₂, H₂, H₂O, N₂, etc., may flow through a narrow discharge tube made of quartz , sapphire or other electromagnetic insensitive material, and wherein surface wave activation by an electromagnetic power source, such as a microwave or RF power source, may be applied. Additionally, a cooling system for the discharge tube using a gaseous coolant is provided, further comprising an integral cooling flange on the discharge tube, which may be attached to a cooling channel. The apparatus may further comprise a discharge nozzle from which the gas emerges from the tube and impinges on a substrate, such that resultant volatile reaction products, such as H₂O, CO₂, or low molecular weight hydrocarbons, may selectively strip material layers from the surface of a substrate wafer. The apparatus may further comprise a positioning system for supporting a wafer chuck, that provides wafer heating and positioning, and provides for high speed scanning of a wafer with the plasma source.
The use of surface wave discharges has the unique advantage of being able to guide the discharge from the point where excitation power is applied, to the wafer where it is used. Also, the method of providing a surface wave discharge may be practiced over a wide range of pressure without significant changes to the electromagnetic power system.
The ideal operating pressure range of the present invention is in the medium pressure regime (greater than about 10 Torr, but less than about 500 Torr). Medium pressure plasmas have the advantage that very high rates of electron-ion recombination and energetic particle thermalization may eliminate the high energy charged species present in low pressure plasmas. Eliminating these high energy species eliminates the possibility of potentially damaging substrate currents and sputter erosion. Further, plasma gas temperatures in the medium pressure regime are extremely high compared to low pressure plasmas. Higher plasma gas temperatures provide an additional source of heat in the reactive zone on the wafer, specifically there where it is most required. This focused thermal energy positively contributes to the reactive removal of organic material, wherein the reaction rate of material removal is increased, thereby increasing the speed (and so the commercial viability) of the process. In contrast, the use of low pressures (less than about 10 Torr) for this plasma jet system may not be desirable because, as pressure decreases, the geometry of the plasma jet may flare out, thereby making the “spot size” less controllable. The use of high pressures (greater than about 500 Torr) may not be advantageous because the reactive species needed for surface removal may recombine before reaching the wafer, thus reducing the effectiveness of the plasma for highly selective removal. Operation of the present invention over a wide pressure window may, however, enable atmospheric wafer exchange while the plasma source is still operating. Since ignition of the plasma normally requires low pressure (close to 1T), cycling the process pressure may be avoided if the power source can be maintained during wafer exchange at about 760 Torr (atmospheric pressure). This may avoid additionally having to vaccum pump down to low pressure for plasma ignition, and then repressurizing to medium pressure for processing each semicondutor wafer, thereby further saving valuable process time in an industrial setting.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates a cross-sectional diagram of a plasma removal system in an embodiment of the present invention;
FIG. 2 is a photograph of an operating plasma removal system in an embodiment of the present invention;
FIGS. 3-7 illustrate empirical data collected on embodiments of the present invention;
FIG. 8 schematically illustrates a scanning pattern in an embodiment of the present invention;
FIG. 9 schematically illustrates heat flow in an embodiment of the present invention;
FIGS. 10-17 are photomicrographs of samples treated using embodiments of the present invention; and
FIGS. 18-19 illustrate method steps in flowchart form of embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth such as specific process values or parameters, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well known components have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning specific semiconductor product applications and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.
Referring now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views:
FIG. 1 illustrates a schematic representation of the plasma applicator 101, process chamber 102, and high speed wafer scanning stage 103. The plasma applicator 101 may be mounted on the chamber wall 104 of a semiconductor process tool, comprising a process chamber 102, wherein the chamber defines a subatmospheric environment for processing the wafer or any other workpiece wherein surface layer removal is desired. An electromagnetic power supply feeds power 105 to a plasma discharge tube 106 through a thin-walled coupling aperture 110 in a reduced height section of waveguide 111. In one embodiment, microwave power at 2.45 GHz is applied to a 6 mm diameter quartz plasma discharge tube. Although, surface waves are launched in both directions along the interface between the plasma discharge tube 106 and the plasma 108, the directional flow 112 of process gas 114 effectively suppresses the discharge on the upstream side of the waveguide 111. This same flow 112, in conjunction with the downstream surface wave, produces a plasma jet 108 that emerges from the nozzle opening 119 in the base flange 118 which is attached to the plasma discharge tube 106. In this context, a plasma jet refers to the stream of pressurized plasma gas that emerges from the plasma applicator 101. In one example, the plasma jet causes the activated process gases to impinge on a semiconductor wafer 2 mm distant. In another example, the wafer is as much as about 20 mm from the plasma jet.
The high speed wafer scanning stage comprises a chuck 130 with a wafer holder which clamps the wafer. The wafer holder may be operated with the force of vacuum, chamber pressure, or electrostatically. The wafer holder may contact the wafer with a thermally conducting or insulating material, depending on the degree of contact conductance desired with the wafer. In one example, an insulating material layer is introduced between the wafer and the wafer holder to reduce thermal contact conductance, thereby increasing the wafer temperature by hindering dissipation of heat. Conversely, in one example, a conducting material layer is introduced between the wafer and the wafer holder to increase thermal contact conductance, thereby decreasing the wafer temperature by promoting dissipation of heat. Further, the chuck 130 may be connected to a power supply via coupling 133 for heating the wafer, or to an active cooling supply via coupling 132, such as water, for cooling the wafer. The chuck may also be equipped with a thermocouple sensor via coupling 135 or other temperature sensor for monitoring the chuck temperature.
The chuck and wafer holder may be mounted on a mechanical positioning system for scanning the wafer. In this regard, scanning the wafer refers to dynamicaly positioning the wafer while being impinged by the plasma jet, so as to expose a region of the wafer to the plasma treatment. The exposure by scanning may be uniform over an entire region on the wafer, or may involve selectively treating sections of the wafer to a differing level of exposure to the plasma. In FIG. 1, a dual axis orthogonal positioning system, comprising an x-axis linear drive 136 and a y-axis linear drive 134, is illustrated in an example embodiment. Other configurations for mechanical positioning, such as a polar coordinate arrangement with a rotating axis mounted on a radial linear drive is mounted, may also be practiced with the present invention. In one example, the mechanical positioning system comprises two orthogonal, motor driven, translation stages with accelerations exceeding 2.5 times the acceleration of gravity and scanning speeds greater than 100 cm/s. In one embodiment, the present invention may execute a scanning pattern under computer control so that each point of the wafer passes within the footprint of the jet, said footprint having a diameter equal to about the full-width-at-half-maximum of a lateral plasma jet profile of the etched track. In one particular example, the present invention may execute a scanning pattern under computer control providing lower scanning rates on the edge of the wafer to increase wafer temperature and compensate for reduced etch rates due to edge effects.
The present invention employs a cooling system using a gaseous coolant. A high velocity gas flowing in a direction 113 opposite the plasma gas 114 is used to cool the plasma discharge tube, whereby operation of the plasma applicator 101 at much higher power dissipation is made possible. In one example, a dry air or nitrogen coolant gas, confined by a concentric outer tube 116, cools the plasma discharge tube 106. As shown in FIG. 1, the plasma discharge tube 106 incorporates an integral base flange 118 to facilitate mounting to the applicator body. An important function of the base flange 118 is to displace the O-ring seals 140 from the immediate vicinity of the plasma discharge tube 106, which may be extremely hot. The O-ring seals 140 have a relatively low melting point and may easily be destroyed by excessive thermal loading. O-rings in direct contact with the downstream side of the plasma discharge tube 106 would inevitably melt. The construction and design of the cooling system of the present invention provides for a large enough temperature gradient across the a base flange 118, such that the hot plasma emerging from the center nozzle 119 of base flange 118 does not cause deterioration of the O-ring seals 140 on the edge of base flange 118. In one example embodiment, an aluminum spacer 142 separates the discharge tube flange 118 from a corresponding cooling flange 117 on an outer cooling conduit 116. The embodiment of the present invention depicted in FIG. 1 relies upon a concentric circular, coaxial cross-sectional geometry of the cooling system. Other cross-sectional geometries of a plasma discharge tube surrounded by a cooling conduit, such as rectangular, square, oval, or eccentric arrangements may be practiced within the scope of the present invention. Various cooling systems employing liquid or gaseous coolants, providing the same cooling performance, so as to enable the power regimes practiced in the present invention, may also be implemented in embodiments of the present invention.
The present invention may further comprise a trap 120 incorporated below the flange of the inner tube so as to eliminate leakage of electromagnetic power into the processing chamber. In one example, a ¼ γ transformer based microwave trap is employed. The gaseous coolant may flow radially inward through channels in the lower surface of the trap 120 toward the plasma discharge tube 106 and enter the narrowed space 105 between the plasma discharge tube and the outer cooling conduit 116. The velocity of the coolant gas increases substantially as it enters this region, as the flow cross-section is reduced. The result is a signficantly enhanced cooling of the plasma discharge tube 106, particularly in the extremely hot zone within the waveguide 110. In one example embodiment, a 1 mm wide gap between the plasma discharge tube and the cooling conduit results in coolant gas velocities approaching Mach 1, whereby high microwave power levels near 2.5 kW may be sustained on a continuous basis. In contrast to oil-based cooling systems, the air cooling of the present invention does not leave deposits on the discharge tube and does not cause damage to the plasma discharge tube, even after extended, continuous operation of the plasma jet at high power levels.
FIG. 2 is a photograph of an operating plasma removal system in an embodiment of the present invention. The visible plasma jet is about 20 cm in length and is luminous. The process gas used in the example embodiment shown in FIG. 2 is a reactive O₂:N₂mixture at a ratio of about 9:1, wherein a pressure of about 80 Torr was supplied at a flow rate of about 2 slpm. The electromagnetic discharge power is about 1 kW.
The thermal power of the plasma jet provides the ability to heat the wafer locally, thus increasing etch rates by increasing reaction rates, while simultaneously delivering reactive species to feed the etching reaction of organic surface layers. The total thermal power P delivered to the substrate from the impinging plasma jet was determined by measuring the rate of temperature T rise vs. time t, dT/dt, of a thermally isolated aluminum block placed under the jet by the following equation:
P=Cρ V(dT/dt)
where the heat capacity is C=0.9 J/K.g, the density is ρ=2.7 g/cm³, and the volume is V=104.04 cm³. The results of these measurements are illustrated in the data plot of FIG. 3, which shows temperature versus time for a thermally isolated aluminum block. In this example, the block is heated by an O₂:N₂=9:1 reaction gas supplied at a pressure of 80 Torr and at a flow rate of 3 slpm, under 1.8 kW of microwave power applied for creating a plasma jet impinging at a distance of 3 cm from the aluminum block, which exhibited an initial temperature of 25° C. The graph in FIG. 3 shows a linear relationship between temperature and time, whereby dT/dt is the slope of the line. In this example, the total thermal power is P=312 W. This measurement may repeated at different microwave powers, gas compositions, and substrate distances for verifying the enhanced thermal performance of the present invention.
FIG. 4 illustrates a data plot of measurements of plasma jet power versus applied microwave power under the otherwise same process conditions for the example as in FIG. 3. As shown in FIG. 4, plasma jet power depends linearly on microwave power. In this example, the conversion efficiency is about 19% and 21%, as measured by the slope of the linear interpolation of the measured data points, when the target (the semiconductor wafer) is 0.9 cm and 2.9 cm away, respectively, from the plasma source, measured from nozzle opening 119.
FIG. 5 illustrates a data plot of measurements of plasma jet power versus distance from the plasma source under the otherwise same process conditions for the example as in FIG. 3. As shown in FIG. 5, the jet power decreases as the distance from the source is increased from about 1 cm to about 5 cm. This may be a result of cooling of the plasma jet by mixing with the ambient temperature gas in the process chamber 102.
FIG. 6 illustrates a data plot of measurements of plasma jet power versus concentration of O₂, at a target distance of 2.9 cm from the plasma source, under the otherwise same process conditions for the example as in FIG. 3. FIG. 6 shows a relatively constant of plasma jet power over an O₂concentration of about 20% to about 90%. This result shows that plasma jet power is essentially independent of O₂/N₂gas composition.
FIG. 7 illustrates a data plot of measurements of contact conductance versus the space between the wafer and the chuck. The ability to control wafer temperature under the dynamic conditions during scanning by a plasma jet may determine the success of an ashing process. Controlling wafer temperature is constrained by the thermal contact conductance (K) between the wafer and the chuck. The values for K were measured for various gaps between the wafer and the chuck, by determining the steady state temperature of an aluminum block mounted on a constant temperature chuck. The spacing between block and chuck was maintained with thin mica spacers. K is given by the following formula:
K=A (T−T ₀)/P
where A is the area of contact between the block and the chuck, T_ois the chuck temperature; and P is the power. The measured value for thermal conductance was found to be K=55 mW/cm²K when the chuck and block are in intimate contact, in good agreement with other reported values. As shown in FIG. 8, the conductance decreases significantly as the gap between the wafer and the chuck increases. The time constant T for heat transfer between the chuck and the wafer is given by:
τ=C/K
where C is the heat capacity of the wafer per unit area. This time constant is about 2 seconds for a 300 mm silicon wafer in intimate contact with the chuck, increasing to about 10 seconds for a 0.01″ gap. The strong variation of contact conductance, and hence time constant, requires very precise gap control, implying the need for electrostatic or vacuum clamping of the wafer on the chuck. Thus another benefit of the present invention operable in the medium pressure regime, over conventional low pressure systems is the ability to allow the use of vacuum clamping of the wafer on the chuck instead of requiring electrostatic clamping.
To completely remove the resist, the wafer may be scanned in a serpentine raster pattern 1014 as shown in FIG. 8. In FIG. 8, line scans 1014 along the x-axis 1010 are alternated in each direction between short translations, i.e. track spacings, along the y-axis 1012, relative to the semiconducting wafer 1016. For such a pattern the track spacing may be less than the diameter of the jet to provide a uniform etch profile across the wafer. In one example embodiment of the present invention, the track spacing was set to 0.7 cm. With a track spacing of 0.7 cm, the variation in etch depth between the track centers and the midpoints between the tracks is less than 2% of the initial resist thickness.
The thermal processes involved in photoresist removal with a scanning plasma jet 1521 in an embodiment of the present invention are illustrated in FIG. 9. The plasma jet 1521 is scanned over a semiconductor wafer substrate 1530 that is coated with a surface layer of organic resist 1531. As the wafer is scanned at high speed in the x-direction 1511, the plasma jet 1521, emerging from the plasma source 1520 with high energy under medium pressure in the direction 1523 of the wafer 1530, creates a heated track 1522 that cools laterally by heat conduction through the wafer and vertically through the contact conductance to the wafer holder, i.e., the chuck. The relevant thermal fluxes, F _lateral 1524 and F _vertical 1526, correspond to lateral heat flow and vertical conduction, respectively.
The chuck may be heated to increase resist etch rates on the wafer. The chuck may also dissipate the excess heat imparted by the plasma jet. The heat of the jet diffuses rapidly through the wafer, the diffusion length corresponding to the dwell time of the jet being greater than the wafer thickness for even the highest scanning speeds. In one example of the present invention, the lateral diffusion length is only 0.5 cm during a track scan time of about 0.2-0.4 seconds, increasing the width of the heated zone by about 50%. Thus, to a first approximation, the high speed scanning may be understood as the thermodynamic equivalent of a line heater moving across the wafer in the y-direction 1510, perpendicular to the high-speed scanning direction. In one example, vertical heat flow is a slow process with a 2-10 second time constant for a silicon substrate, and is negligible during the time required to scan a single track. There may be instances, however, where vertical heat flow becomes an important thermal factor after several tracks have been scanned.
The balance between jet power, scanning speed, and vertical heat flow may determine the effectiveness of a particular ashing process. To maximize throughput, embodiments of the present invention are operated using a high level of electromagnetic power to activate the plasma jet, which translates directly into a higher etch rate. Increased power also maximizes the generation of reactive gases in the plasma and provides the heat for activating the ashing reaction between the resist and etching gas.
In the case of ion implanted resist, the initial chuck temperature may be set just below the hardbake temperature of the resist. In one example, the initial chuck temperature is set to about 10° C. below the resist hardbake temperature, which may be about 125° C. The resist would be stable at this temperature and no popping should occur. Contact conductance between the wafer and chuck may be maximized, for example, with helium backside cooling to minimize wafer temperature for a given input power density. Finally, the scanning speed may be increased, thereby reducing effective power density in the wafer, to the point where the wafer can be scanned indefinitely without popping. The required speed may be significantly greater than 1 m/s. As scanning proceeds, the wafer temperature gradually rises and the scanning plasma jet creates minute holes in the implanted photoresist crust, making the crust permeable to the gases released from the base resist. Once permeability is achieved, the temperature may be allowed to rise by reducing the scanning speed or by reducing the contact conductance between the wafer holder and the chuck, thereby reducing the amount of heat dissipated through the wafer holder. The result of the pre-scanning process for permeating the photoresist crust may be a rapid removal of the resist from the wafer surface during a secondary scanning operation.
Etching unimplanted resist involves fewer thermal constraints; the initial chuck temperature may be higher, in one example around 200-350° C., and contact conductance and scan speed may be set to be much lower, all leading to higher wafer temperatures and, thus, higher etch rates. In the case of unimplanted resist, contact conductance could be reduced significantly. In one example, the wafer may be raised off the chuck by a few ten thousands of an inch.
As a result of the foregoing, the ashing of high-dose, ion-implanted photoresist may occur as a two-step process in which the crust is first rendered permeable by a low temperature pretreatment process followed by a high temperature resist removal process. The pretreatment process may take place with chuck temperatures below the bake temperature of the resist, in one instance 120° C. This relatively low temperature is required for preventing the ejection of particulates when the crust explodes due to gases evolved by thermal decomposition of the resist in the event that the carbonized crust has not been removed/punctured, a process also known as popping. Once the photoresist crust has been rendered permeable to gases by the pretreatment scanning of the present invention, the temperature of the wafer may be safely raised to enhance the rate of resist removal. Measurements have established and verified the conditions of pretreatment and resist removal in scanned plasma jet ashing of heavily implanted (P, 40 keV, 5×1015/cm2) I-line photoresist from silicon wafers.
FIGS. 10-17 are photomicrographs of sample semiconductor wafers, having a surface layer of photoresist, treated by embodiments of the present invention. FIG. 10 shows a network of fine gas-filled blisters formed by the heat of a 1 kW plasma jet at 15 cm/s. In FIG. 11, a blister fractured when the substrate was cleaved, showing that the pressure of the evolved gas has delaminated the crust from the underlying unimplanted resist base. In this example, the height of the blisters is 3-4 times the original resist thickness. This blistering effect is related to, but distinct from, the popping phenomenon, whereby large plates of crust are blown off the surface. Blistering, however, may be acceptable in instances whereby no particulate debris is generated. Once the blister is formed, heat conduction from the crust to the substrate decreases dramatically and its temperature may rise several hundred degrees C. above that of the substrate. The localized high temperature due to delamination accelerates the etching of the crust as shown in FIG. 12. As these openings appear in the crust 1901 , the base resist is exposed to the jet and lateral etching 1902 takes place beneath the crust, as illustrated in FIG. 13. FIG. 14 shows a later stage of etching where most of the crust 2011 has been removed and blisters 2010 have merged.
To maximize throughput rates of material removal using embodiments of the present invention, the plasma jet may be operated at the maximum possible elecromagnetic power that can be applied. In order to prevent popping during a pretreatment process, it may be then necessary to scan the jet fast enough prevent excessive temperature rise. In one exemplary instance, multiple pretreatment scans may be required to achieve sufficient permeability to prevent popping during the resist removal step. In one example embodiment of the present invention, resist can be removed completely in a single scan at a speed on the order of 50-100 cm/s without changing the substrate temperature. Other settings for process parameters may be used to achieve similar results in differing, but related, embodiments of the present invention.
Other processes besides blister formation may lead to permeability of a crust in an ion implanted photoresist. FIG. 15, for example, shows a reticulated resist surface that may develop during the early stages of the pretreatment process. This surface becomes permeable, as shown in FIG. 16, in the late stages of the pretreatment step.
In an illustrative example embodiment for practicing the present invention, an optimized process was developed for implanted I-line photoresist (1.2 microns I-line base resist, hardbaked at 120° C., then implanted with phosphorus at an energy of 40 keV, and at a heavy implantation density of 5×10¹⁵/cm²) with multiple pretreatment scans (microwave power=2.15 kW, substrate temperature=100° C., scan speed=105 cm/s, O₂:N₂=9:1, flow=3 slpm, pressure=80 T). This pretreatment was followed by subsequent resist removal scans at 2.5 kW, and 40 cm/s (whereas the other conditions were kept the same as the pretreatment). All crust and base resist was removed off the wafer, and no residue was seen under the scanning electron microscope, as evident in FIG. 17.
FIG. 18 illustrates a method 2401 for practicing the present invention in flowchart form. The method may begin with step 2402 of introducing the wafer into a process chamber equipped with a plasma applicator apparatus, as described in FIG. 1. If the plasma has previously been ignited at low temperature, step 2402 may be performed while the plasma is activated at ambient pressure. In step 2404, the clamping interface between the wafer and the wafer holder may be adjusted for the desired thermal conductivity, either for high or low conductance. In step 2406 the semiconductor wafer may be clamped into the chuck, using an atmospheric or vacuum force, or electrostatically. In step 2408 operation of the cooling system, as previously described, for cooling the plasma discharge tube may be initiated. Next, the activation of the reactant process gas may be initiated in step 2410. After activation of the plasma, the wafer surface may be treated by the impinging plasma jet in step 2412. The wafer may be scanned by the plasma jet beam in step 2414. Note that the method 2401 is illustrative of one embodiment of the present invention and may be equally practiced with various combinations of the illustrated process steps, with omission of certain steps, or in a different order of the given, depending on process and equipment requirements. For example, in one embodiment of method 2401, a pretreatment of an ion implanted resist is performed for rendering the resist crust permeable to gases. In another embodiment of method 2401, unimplanted or pretreated ion implanted resist is treated for selective ashing and removal of the photoresist layer only.
FIG. 19 illustrates one exemplary method 2501 for practicing the method step 2410 shown in FIG. 18. First an electromagnetic power source, as previously described, may be activated in step 2502. The electromagnetic radiation is transmitted through a waveguide to a plasma discharge tube in step 2504. In step 2506, electromagnetic power is contained in a trap for protecting the wafer from uncontrolled radiation.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. An apparatus for selectively removing surface layers from a workpiece in a manufacturing process, comprising:

a process chamber for defining an atmospheric to subatmospheric environment and receiving the workpiece therein;

a plasma applicator for generating a plasma, including

a pressurized supply of reactant process gas;

a plasma discharge tube in fluid communication with said pressurized supply of reactant process gas;

an electromagnetic power source for directing electromagnetic power to said plasma discharge tube to generate a plasma therein; and

a nozzle opening situated at an end of said plasma discharge tube for jetting the plasma gas into said process chamber in a direction toward the workpiece; and

a cooling system including a conduit substantially surrounding said plasma discharge tube for circulating a gaseous coolant therethrough, thereby forming a cooling channel around said plasma discharge tube.

2. The apparatus of claim 1, further including a waveguide for transmitting the electromagnetic power to said plasma discharge tube.

3. The apparatus of claim 2, further including a microwave trap for containing the electromagnetic power within said plasma applicator.

4. The apparatus of claim 1, wherein the reactant process gas comprises O₂, H₂, H₂O, N₂or a combination thereof.

5. The apparatus of claim 1, wherein the reactant process gas consists of O₂, H₂, H₂O, N₂or a combination thereof and wherein said reactant process gas does not comprise fluorine.

6. The apparatus of claim 1, wherein said plasma discharge tube is made of quartz or other electromagnetically insensitive ceramic materials

7. The apparatus of claim 1, whereby said cooling system has a thermodynamic performance that provides for operation of said electromagnetic power source at a power dissipation of at least 2.5 kW, or at a power density of at least 1.5 kW/cm³.

8. The apparatus of claim 1, wherein said electromagnetic power source operates at frequencies between about 100 kHz and 2.45 GHz.

9. The apparatus as recited in claim 1, further comprising:

a mechanical positioning system including a chuck for receiving and maintaining the workpiece thereon to scan the workpiece relative to said nozzle such that a surface layer of said workpiece is exposed to said plasma.

10. The apparatus of claim 9, wherein said mechanical positioning system comprises a plurality of mechatronic translation stages for scanning said surface of the workpiece, operable such that said chuck may be accelerated greater than about 2.5 times the acceleration of gravity and positioned at a linear velocity greater than about 100 cm/s.

11. The apparatus of claim 1, wherein the distance between said nozzle and the workpiece is greater than about 2 mm and less than about 20 mm.

12. The apparatus of claim 9, wherein said plurality of mechatronic translation stages are arranged for positioning according to Cartesian or polar coordinates.

13. The apparatus as recited in claim 9, wherein said chuck further includes a layer of thermal material on a surface thereof having thermally insulating or thermally conducting characteristics for modifying a thermal contact conductance between said chuck and the workpiece.

14. The apparatus as recited in claim 9, further comprising:

means for removing and introducing a workpiece onto said chuck with a pressure pressure within said process chamber in a raised state to ambient pressure while said plasma applicator is maintained operational, thereby eliminating a need for extinguishing and reigniting the plasma for each workpiece to be processed in said process chamber.

15. The apparatus as recited in claim 9, further comprising:

means for clamping a workpiece onto said chuck by a force supplied by process atmosphere or a vacuum, or by an electrostatic force.

16. A method for selectively removing surface layers from a wafer in a semiconductor manufacturing process, comprising the following steps:

introducing the wafer into a process chamber defining an atmospheric to subatmospheric processing environment;

exposing a reactant gas flowing through a discharge tube to a surface wave discharge provided by an electromagnetic power source to generate an activated reactant gas flowing through said discharge tube; and

jetting the activated reactant gas onto into said chamber and onto a surface of the wafer, whereby a surface layer thereof is selectively removed without substantial loss of substrate material.

17. The method of claim 16, further including the step of cooling said plasma discharge tube by forming a cooling channel thereabout, and circulating a gaseous coolant through said cooling channel.

18. The method of claim 16, wherein said reactant process gas comprises O₂, H₂, H₂O, N₂or a combination thereof.

19. The method of claim 16, wherein the surface layer comprises an unimplanted photoresist or other organic or inorganic material.

20. The method of claim 16, wherein the surface layer comprises an ion implanted photoresist material or other organic or inorganic material.

21. The method as recited in claim 16, further comprising the step of:

scanning the wafer via relative motion of the wafer with respect to the jetted reactant gas at a first speed whereby an implanted photoresist crust is rendered permeable to gases.

22. The method as recited in claim 21, further comprising the step of:

scanning the wafer via relative motion of the wafer with respect to the jetted reactant gas at a second speed, whereby photoresist and/or crust is removed from the wafer.

23. The method as recited in claim 16, further comprising the step of:

exposing the reactant gas to the surface wave discharge at a first power level whereby an implanted photoresist crust is rendered permeable to gases.

24. The method as recited in claim 23, further comprising the step of:

exposing the reactant gas to the surface wave discharge at a second power level, whereby photoresist and/or crust is removed from the wafer.

25. The method as recited in claim 16, further comprising the step of:

scanning the wafer via relative motion of the wafer with respect to the jetted reactant gas at a first temperature, whereby an implanted photoresist crust is rendered permeable to gases.

26. The method as recited in claim 25, further comprising the step of:

scanning the wafer via relative motion of the wafer with respect to the jetted reactant gas at a second temperature, whereby photoresist and/or crust is removed from the wafer.

27. The method as recited in claim 16, wherein said activating step further comprises:

exciting an electromagnetic power source for generating a surface wave on said plasma discharge tube;

transmitting electromagnetic power to said plasma discharge tube through a waveguide engaged to said plasma discharge tube; and

containing electromagnetic radiation using a trap within said plasma applicator.

28. The method of claim 16, wherein said electromagnetic power source operates at frequencies between about 100 kHz and 2.45 GHz.

29. The method as recited in claim 16, further comprising the steps of:

placing a wafer on a chuck; and

scanning the chuck relative to the jetted reactant gas via a mechanical positioning system mounted to said chuck, whereby the chuck having the wafer placed theron is positioned such that a surface layer thereof is exposed to the jetted reactant gas.

30. The method of claim 29, wherein said mechanical positioning system is operated such that said chuck may be accelerated greater than about 2.5 times the acceleration of gravity and positioned at a linear velocity greater than about 100 cm/s.

31. The method as recited in claim 29, further comprising the step of:

varying the temperature of the wafer via a layer of thermally insulative or conductive material situated between the wafer and a wafer holder mounted on said chuck, whereby a thermal contact conductance between said wafer holder and the wafer is modified.

32. The method as recited in claim 29, further comprising:

removing and introducing a wafer onto said chuck when a pressure within said process chamber is at ambient pressure, thereby eliminating a need for igniting the activated reactant gas for each wafer processed in said process chamber.

33. The method as recited in claim 31, further comprising:

clamping semiconducting wafers onto said chuck by a force supplied by process atmosphere or a vacuum, or by an electrostatic force.