Methods and apparatus for fabricating a high purity thermally-conductive polymer layer

Abstract: Thermal transfer between a semiconductor wafer and a platen during vacuum processing is provided through a soft, thermally-conductive polymer layer having a thin, hard surface film. The soft polymer layer, which is preferably silicone rubber containing thermally-conductive particles, conforms to surface irregularities on the wafer and has low thermal resistance. The surface film is preferably silicon dioxide in the form of a multiplicity of flat plates integrally formed on the silicone rubber. Adherence of the water and of foreign matter to the polymer layer is prevented by the surface film. In addition, the underlying polymer layer is protected by the surface film. A high purity polymer layer is fabricated by evacuating the mold cavity and the resin container prior to injection of resin. The mold for the polymer layer utilizes a hard, smooth mold release surface and a resilient gasket between the platen and the mold release surface. The gasket establishes the dimensions of the polymer layer and seals the mold cavity. The surface film is formed by oxygen plasma treamtent of the silicone rubber layer.

Description: This invention relates to moulding a thin polymer layer and includes a mould for such a layer and a method for moulding such a layer. JP-A-58062017 discloses a mould for a fibre-reinforced resin item having two members which can be secured together. The present invention in one aspect provides a mould as set out in claim 1. The invention also includes a mould for moulding a polymer layer using such a mould and comprising the steps as set out in claim 7. An example of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a block diagram of moulding apparatus in accordance with the present invention shown during evacuation; Fig. 2 is a block diagram of moulding apparatus in accordance with the present invention shown during injection of polymer resin into the mould cavities; Fig. 3 is a cross-sectional view of a mould for fabricating a polymer layer; and Fig. 4 is a block diagram illustrating the polymer layer fabrication process. In an ion implantation system for treating a semiconductor wafer, the wafer is mounted on thermal transfer apparatus including a thermally conductive polymer layer 30 affixed to a heat transfer surface of a platen. Prior art polymer layers have included voids or trapped gas bubbles which reduce the thermal transfer rate and can outgas during vacuum processing of the wafer. Apparatus for moulding the present polymer layer 30 is illustrated in block diagram form in Figs. 1 and 2. A preferred mould structure is illustrated in Fig. 3. The apparatus of Figs. 1 and 2 can be utilized to fabricate polymer layers 30 for one or more wafer sites. A mould 70 is utilized for forming the polymer layer 30 on aluminum platen 34. Moulds 72 and 74 are utilized for forming additional polymer layers on additional platens (not shown) in a single moulding process. The moulds 70, 72, 74 and sealed vacuum-tight A container 76 is sealed utilizing a cover 78 and an O-ring 80. A tube 82 passes through an opening 84 in cover 78 and is connected to a mold cavity (not shown in FIG. 1) in each of molds 70, 72, 74. The tube 82 is sealed to cover 78 by an O-ring 86. Preferably, the tube 82 is flexible and is disposable after each molding process. Within container 76 is a vessel 87 which contains a liquid resin 88 used to form the polymer layer 30. Typically, the resin 88 is formed as a mixture of two or more reacting components which are kept separate until the fabrication procedure begins. As used herein, the term "resin" is used to refer to the liquid form of the polymer layer prior to curing, in which form it can be pumped or injected into a mold. It will be understood that the resin 88 can be placed in the container 76 without using vessel 87. However, a separate vessel 87 provides the capability to keep the container 76 clean and to easily replace the resin 88 when desired. The sealed container 76 is coupled by a conduit 90 through a valve 92 to a pressure source 94. The pressure source 94 can be an air pump or other means for pressurizing the container 76 to a desired pressure level of about 3.4 x 10⁴ Pa. The container 76 is also coupled by conduit 90 through a valve 96 to a vacuum pump 98 which provides the capability to evacuate the sealed container 76 to a pressure level of about 1 torr. The tube 82 is coupled vacuum-tight to one or more molds 70, 72, 74 and the end of tube 82 within container 76 is positioned outside the liquid resin 88. Valve 92 is closed and valve 96 is opened, causing vacuum pump 98 to evacuate both the sealed container 76 and the mold cavities in molds 70, 72, 74 as shown in FIG. 1. The reduction in pressure in sealed container 76 promotes outgassing of gas bubbles in liquid resin 88, thereby providing a uniform and gas-free material. Next, the end of tube 82 within container 76 is lowered below the surface of liquid resin 88 by sliding the tube 82 through O-ring 86, valve 96 is closed and valve 92 is opened. Pressure source 94 provides the prescribed pressure within container 76, causing liquid resin 88 to be forced through tube 82 into still-evacuated mold cavities in molds 70, 72, 74 as shown in FIG. 2. Since the mold cavities have been evacuated and the liquid resin 88 has been outgassed, the mold cavities are filled with liquid resin 88 which is substantially free of trapped gas bubbles. The resin 88 in the mold cavities is allowed to cure, either at room temperature or at an elevated temperature, to provide the polymer layer. The polymer layer formed by this process is highly uniform and free of surface cavities and trapped gas bubbles. The details of one embodiment of the molds 70, 72, 74 are illustrated in FIG. 3. The aluminum platen 34, on which the silicone rubber layer is to be formed, is provided with a hole 102 between heat transfer surface 32 and the opposite surface. A flexible, resilient gasket 104 having an opening 106 is positioned on the heat transfer surface 32. The gasket 104 has a thickness equal to the desired thickness of the polymer layer, and the opening 106, typically circular, has a diameter equal to the desired diameter of the polymer layer. Preferably, the gasket 104 is formed of polyurethane. Then, an upper mold member 110, having a mold release surface 112, is placed over gasket 104 so as to form a sealed mold cavity 114 which defines the size and shape of the polymer layer to be molded. As noted hereinabove, mold release surface 112 can be glass, hardened stainless steel, polyester film or another material having sufficient hardness to prevent thermally-conductive particles in resin 88 from prodtruding above the surface of the polymer layer, while avoiding adherence of the polymer layer thereto during curing. Preferably, the mold release surface 112 is made optically flat. When necessary, a mold release material in the form of an aerosol spray can be applied to mold release surface 112. A preferred aerosol spray is Teflon having particle sizes in the range of 1-5 micrometers. In one preferred embodiment, the mold release surface 112 can be polyester film which transfers to polymer layer 30 during molding and which is retained on the polymer layer 30 after molding for protection during the remainder of the system assembly and test procedure. The polyester film is subsequently removed by peeling it from the polymer layer 30. The use of a resilient gasket 104 seals the mold cavity 114 and defines the thickness and the diameter of the polymer layer 30, while preventing flashing (leakage between the mold elements) and other irregularities at the peripheral edge of the polymer layer. The tube 82 from sealed container 76 is coupled by a suitable vacuum-tight fitting (not shown) of conventional construction through hole 102 to mold cavity 114 for evacuation and subsequent filling of mold cavity 114 as described hereinabove. The resin in mold cavity 114 is allowed to cure, typically at room temperature. Subsequent to curing, the mold member 110 and gasket 104 are removed from platen 34, leaving soft polymer layer 30 adhered to the heat transfer surface 32. Next, the platen 34 and the polymer layer 30 are placed in a plasma etcher such as a Drytek etcher commonly used to remove photoresist from wafers. The chamber of the etcher is evacuated and oxygen is introduced into the chamber at a pressure level of about 400 microns. Radio frequency energy at a level of about one kilowatt is applied to the oxygen by the etcher so as to form an oxygen plasma in the chamber. While the platen 34 and the polymer layer 30 are not intentionally heated, the applied radio frequency energy raises their temperature to about 100°C. The platen 34 and the polymer layer 30 are left in the oxygen plasma for about 30 minutes, causing formation of the thin, relatively hard surface film 50 on polymer layer 30. The oxygen plasma treatment of the polymer layer 30 produces an extremely clean surface. In addition, the oxygen reacts with the silicon in the silicone rubber to form a silicone dioxide, or quartz, layer, preferably having thickness in the range of 0.1 to 5.0 micrometers. The thickness of the quartz layer depends on the oxygen plasma treatment time. Since silicone rubber consists of hydrogen, carbon and silicon, it is believed that the oxygen plasma erodes the hydrogen and carbon from the upper portion of the silicone rubber layer to form H₂O and CO₂, leaving an upper layer of almost pure silicon. This silicon then reacts with the oxygen in the plasma to form the oxide layer. It is further believed that oils in the rubber bubble up during the oxygen plasma treatment and react with the oxygen to form silicon dioxide. It has been found that the oxide (quartz) surface film 50 is easily fractured into multiple plates varying in dimension from about 5 to 1,000 micrometers across. The plates can be created by applying a mechanical stress, a thermal stress or a combination thereof to the film 50. The quartz plates do not flake off the silicone rubber surface since they are formed as an integral part of it. Since each of the multiple plates moves independently, they conform to surface irregularities on the back of the wafer and to particles inadvertently trapped between the wafer and the polymer layer. As a result, cooling performance is substantially enhanced. The method for fabricating the polymer layer 30 is summarized in FIG. 4. Initially, gasket 104 and mold member 110 are attached to platen 34 to form mold cavity 114, as indicated in step 120, and resin 88 in vessel 87 is placed in container 76 and cover 78 is sealed thereto. Next, valve 96 is opened, and mold cavity 114 and container 76 are evacuated in step 122. Tube 82 is immersed in resin 88, valve 96 is closed and valve 92 is opened so that pressure source 94 pumps resin into mold cavity 114, as indicated in step 124. The resin is permitted to cure in step 126, and gasket 104 and mold member 110 are removed in step 128. Finally, the platen 34 and polymer layer 30 adhered thereto are subjected to oxygen plasma treatment to form surface film 50 in step 130. It will be understood that other techniques for forming the surface film 50 are included within the scope of the present invention. For example, deposition of silane gas and oxygen on the silicone rubber surface can be utilized to form the oxide layer. In addition, the surface film can be formed of another smooth, plate-like material such as graphite. Graphite, mechanically deposited and rubbed so as to adhere to the surface of the silicone rubber layer, produces enhanced thermal transfer and does not adhere to wafers. The primary requirement is to provide a thin, smooth and relatively hard surface film on a relatively soft, thermally-conductive polymer layer. The surface film prevents sticking and permits intimate contact with the semiconductor wafer. The soft polymer layer and its surface film both conform to the back surface of the wafer and both have high thermal conductivity so that overall thermal transfer performance is greatly improved. Thermal contact resistances have been measured with the thermal transfer apparatus disclosed herein at about 10°C/watt/cm². By contrast, a good performance with gas cooling is about 40°C/watt/cm² and typical performance with prior art polymer layers is about 50°-60°C/watt/cm².

Claims: A mould for a thin polymer layer comprising a first member having a first mould surface, a second member having a second smooth, hard mould release surface opposed to said first mould surface, a gasket between said first member and said second member, said first and second members and said gasket together defining a mould cavity, said gasket having a thickness equal to the desired thickness of said mould cavity and having an opening that defines the outer periphery of said mould cavity, and means for evacuating said mould cavity and for injecting resin for a polymer layer into said mould cavity, said first surface being adapted to adhere to the polymer layer formed by curing said resin, said second surface being adapted to prevent adherence of said polymer layer thereto. A mould as claimed in claim 1 wherein said gasket is resilient. A mould as claimed in claim 2 wherein said gasket comprises polyurethane. A mould as claimed in any one of claims 1 to 3 wherein said second member has a polyester film mould release surface. A mould as claimed in any one of claims 1 to 3 wherein said second member has a hardened stainless steel mould release surface. A mould as claimed in any one of claims 1 to 3 wherein said second member has a glass mould release surface. A method as claimed in any one of claims 1 to 6 for moulding a polymer layer using the mould of claim 1 comprising the steps of (a) evacuating said mould cavity; (b) injecting a polymer resin into said mould cavity; (c) causing said polymer resin to cure; (d) removing said second member from said mould to expose a polymer layer attached to said first surface of said first member; and (e) treated said polymer layer in an oxygen plasma while said polymer layer remains attached to said first surface.

Inventors: MEARS ERIC LORING, PIERCE KEITH D, MEARS, ERIC LORING, PIERCE, KEITH D.
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1989
Main classification: B29C39
IPC codes: B29C39, H01L21, H01J37