Patent Application
20120241648
1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to a reliable seal used in cryogenic processing environments.
2. Discussion of Related Art
Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. One or more ion species may be implanted at different energy and dose levels to obtain desired device structures. In addition, the beam dose (the amount of ions implanted in the substrate) and the beam current (the uniformity of the ion beam) can be manipulated to provide a desired doping profile in the substrate. However, throughput or manufacturing of semiconductor devices is highly dependent on the uniformity of the ion beam on the target substrate to produce the desired semiconductor device characteristics.
It has been discovered that a relatively low target substrate or wafer temperature during ion implantation improves implant performance. In particular, lower wafer temperatures reduce the amount of damage caused when ions hit the substrate (“damage layer”) which improves device leakage currents. This allows manufacturers to create abrupt source-drain extensions and ultra-shallow junctions needed for today's semiconductor devices. When the temperature of the wafer is decreased, the thickness of the amorphous silicon layer increases because of a reduction in the self-annealing effect.
Typically, cooling of the target substrate to cryogenic temperatures is done by cooling the platen upon which the substrate is disposed in the range of below room temperature to about −100° C. Almost all existing low-temperature ion implanters cool wafers directly during ion implantation. In order to maintain sufficient contact between the cooling elements within the wafer processing vacuum and to provide for efficient thermo-coupling, a reliable gas seal must be maintained. In particular, seals are disposed between the cooling elements and the platen to prevent gas leakage and to maintain the vacuum chamber housing and the target wafer at the desired temperature. These seals are typically made from, for example, polyvinyl chloride, elastomers such as thermoplastic elastomers and/or other plastic materials. In order for the seals to provide a uniform seal between the cold sealing surfaces, they must be pliable. However, at cryogenic processing temperatures, PVC, TPE and other elastomer materials become brittle at approximately −20° C. and −60° C., respectively. Since wafer cryogenic processing is typically performed at temperatures down to −100° C., these seals may become brittle during processing. This may compromise thermocoupling between components. This may affect the vacuum environment for processing tools which may negatively impact manufacturing and device throughput. Consequently, there is a need to provide seals disposed between components in semiconductor processing equipment that maintain their sealing properties and pliability at cryogenic processing temperatures.
Exemplary embodiments of the present invention are directed to a seal used in cryogenic processing environments. In an exemplary embodiment, the seal includes an internal cavity defined by a pair of opposing side walls and a bottom wall disposed therebetween; and an electrically conductive spring disposed within the cavity and extending along a length of the seal. The spring is configured to provide a lateral biasing force against the pair of opposing side walls and to conduct an applied current from a first end of the spring to a second end of the spring. The current through the spring results in heat within the spring which emanates toward at least the pair of opposing side walls thereby maintaining their pliability and sealing function.
In another exemplary embodiment, an apparatus for use in an ion implanter includes a platen configured to receive a target substrate; a pad disposed beneath the platen where the pad has a plurality of channels configured to receive coolant material therethrough to reduce the temperature of the platen to approximately −100° C. The platen is capable of being displaced along a top surface of the pad. A lip seal is disposed between and in contact with a bottom surface of the platen and the top surface of the pad. The seal has an internal cavity defined by a pair of opposing side walls and a bottom wall disposed therebetween and an electrically conductive spring with is disposed within the cavity and extends along a length of the seal. The spring is configured to provide a lateral biasing force against the pair of opposing side walls and conducts an applied current from a first end of the spring to a second end of the spring. The current results in heat generated from the spring and emanates toward the pair of opposing side walls.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout.
The ion source chamber 102 typically includes a heated filament which ionizes a feed gas introduced into the chamber to form charged ions and electrons (plasma). The heating element may be, for example, a Bernas source filament, an indirectly heated cathode (IHC) assembly or other thermal electron source. Different feed gases are supplied to the ion source chamber to obtain ion beams having particular dopant characteristics. For example, the introduction of H2, BF3 and AsH3 at relatively high chamber temperatures are broken down into mono-atoms having high implant energies. High implant energies are usually associated with values greater than 20 keV. For low-energy ion implantation, heavier charged molecules such as decaborane, carborane, etc., are introduced into the source chamber at a lower chamber temperature which preserves the molecular structure of the ionized molecules having lower implant energies. Low implant energies typically have values below 20 keV.
The pads 208a and 208b include a plurality of channels 212 to accommodate the flow of coolant, therethrough. The coolant flowing through the channels 212 cools the pads 208a and 208b which in turn cools the platen 114 by contact with a gas heat transfer fluid in the contact area. The wafer 216 is cooled to a desired temperature for ion implantation by platen 114 due to contact therewith and the gas heat transfer fluid in the contact area between the wafer 216 and platen 114. The coolant may be, for example, N2 gas at −180 C. Alternatively, pads 208a and 208b may be integrally formed with respective support arms 210a and 210b and configured with the plurality of channels 212 to accommodate coolant flow. The channels 212 connect to one or more pipes 214 through shaft 201 that provide the coolant and the heat transfer gas to the pads 208a and 208b from a supply source.
Seals 220a, 220b are disposed between pads 208a, 208b (respectively) and platen 114 to form seals there between. Seals 220a, 220b may be formed from an elastomer or other pliable plastic material to form a deformable seal between these components within the vacuum environment. Seals 220a, 220b may be disposed within a channel in either the underside of platen 114 or pads 208a, 208b. The seals 220a, 220b may be referred to as “lip seals” since, as is typical in wafer processing, platen 114 and consequently wafer 216 are displaced in any one of several directions to allow for scanning of beam 95 toward wafer or substrate 216. Seals 220a, 220b may be lip seals which accommodate rotational movement of platen 114 with respect to thermal pads 208a, 208b.
Although seals 220a, 220b are described herein with reference to positioning between platen 114 and pads 208a, 208b, seals 220a and 220b may be disposed between any of the components within a cryogenic processing environment. For example, seals 220a and/or 220b may be used to provide a rotary seal between a shaft, such as shaft 201, and a rotating assembly such as, platen 114. In this configuration, the seal (220a, 220b) is disposed around the shaft 201 between the shaft 201 and the rotating assembly. The shaft 201 typically has passages or channels similar to channels 212 there through to carry cryogenic processing media (e.g. gas) from the stationary shaft 201 to the rotating assembly (e.g. platen 114).
Each of the seals 220a, 220b includes a spring (see
In this configuration, a single lip seal 320 is disposed between thermal pad 308 and platen 114. Seal 320 includes a spring 330 (see
It should be noted that although seals 220a, 220b, and 320 are being described herein with respect to cryogenic processing in an ion implanter, alternative uses of such a heated lip seal configuration may also be employed where pliability of a seal is required to maintain a gas seal between cryogenic sealing surfaces.
Spring 330 includes a first end 331 and a second end 332 each connected to a respective electrical wire 335, 336. One of the wires 335, 336 is connected to a power source and the other is connected to neutral to complete a closed loop circuit. When a voltage or current is applied to spring 330 via wires 335, 336, heat is generated through the spring which radiates along the walls 325a, 325b and 327 of seal 320 thereby heating the seal and preventing the seal material from becoming brittle. This heat allows seal 320 to remain pliable during low temperature cryogenic processing. The amount of current applied to spring 330 may be regulated based on the amount needed to generate sufficient heat to keep seal 320 pliable at particular cryogenic processing temperatures. This provides the necessary gas seal required by maintaining the temperature of the seal 320 to be compliant with cryogenic sealing surfaces within the vacuum processing chamber.
While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.
