Drying and gas or vapor contact with solids – Process – Gas or vapor pressure varies during treatment
Reexamination Certificate
2000-07-10
2001-07-24
Gravini, Stephen (Department: 2162)
Drying and gas or vapor contact with solids
Process
Gas or vapor pressure varies during treatment
C034S410000
Reexamination Certificate
active
06263587
ABSTRACT:
FIELD
The present invention relates generally to the field of semiconductor device fabrication, and specifically to degassing semiconductor wafers.
BACKGROUND
Semiconductor substrates and the layers deposited thereon (collectively referred to herein as a “wafers”) absorb water vapor and other gases and impurities (e.g., hydrocarbons) when exposed to the same (e.g., when a wafer is removed from a vacuum chamber). These gases and impurities degrade film properties and therefore must be desorbed and driven off the wafer (i.e., the wafer must be degassed) before further films are deposited thereon.
One conventional degas module uses infrared radiation to heat the wafer to a desired temperature. The infrared radiation originates from an array of lamps positioned above the wafer. The wafer's temperature is measured using infrared pyrometry, a measure of the infrared radiation emitted from the wafer and, if the emissivity of the wafer is known, a measure of the wafer's absolute temperature. A major disadvantage of heating a wafer by infrared radiation and measuring its temperature by infrared pyrometry is the substantial transparency of most common substrate materials (e.g., silicon) to infrared wavelength radiation at the temperature range of interest for degassing (150-500° C.). Because most substrates are transparent to infrared wavelengths, the rate at which a wafer heats is dependent on the presence of other non-transparent (i.e., energy absorbing) layers, and device patterning placed on the wafer before it enters the degas chamber. Furthermore, any layers or device patterns which may be present from previous process steps affect the wafer's emmissivity making it difficult to obtain an accurate wafer temperature measurement by infrared pyrometry.
In order to achieve heating rates independent of wafer patterning some conventional degas methods employ a heated substrate support. However, due to surface roughness of both the wafer and the substrate support, small interstitial spaces may remain between the substrate support and the wafer which effectively decreases the contact area and heat transfer rate. Particularly in vacuum environments, this decreased contact area causes heat transferred to be dominated by radiation, which is slow at the temperatures of interest. These spaces interfere with and cause non-uniform heat transfer from the substrate support to the wafer. To promote more uniform heat transfer, a heat transfer gas such as argon, helium or nitrogen is often used to fill the interstitial spaces between the wafer and the substrate support. This gas has better heat transfer characteristics than the vacuum it replaces and therefore acts as a thermal medium for heat transfer from the substrate support to the wafer. The heat transfer coefficient of such a system is dependent on the spaces between the wafer and the substrate support and on the pressure, the atomic mass and the accommodation coefficient of the heat transfer medium. Small spaces and high pressures generate the best heat transfer.
In an effort to achieve smaller spaces, more efficient heating and more uniform wafer temperatures, conventional degassing apparatuses mechanically clamp the wafer to the substrate support using a clamp ring which contacts the outer edge of the wafer's frontside (i.e., a side that faces into the chamber). The clamp ring holds the wafer against the substrate support to maintain the necessary gas pressure between the substrate support and the wafer's backside (i.e., a side that faces the substrate support) a lower pressure is therefore maintained on the wafer's frontside. However, the opposing forces applied to the wafer by the clamp ring and by the backside gas pressure may cause the wafer to bow. A 10 Torr backside pressure causes an 8 inch wafer to bow about 1 mm at the wafer's center, and causes a 12 inch wafer to bow about 5 mm at the wafer's center. This bow increases the space between the substrate support and the wafer's backside, thereby decreasing the backside pressure and deteriorating heat transfer. Moreover, backside pressure can cause the stress in the substrate to exceed the substrate's yield strength and break the wafer.
In addition to the disadvantages described above, mechanical clamping of the wafer is undesirable because the clamp ring consumes otherwise patternable surface area and because the surface contact between the wafer and the clamp ring promotes particle generation particularly as the wafer heats and expands. Accordingly, a need exists for a degassing apparatus and method that heats wafers independent of individual wafer patterning, that reduces wafer bowing, that reduces particles generated by contact between moving parts, and that increases patternable surface area.
SUMMARY OF THE INVENTION
The present invention provides a degassing apparatus and method for effectively clamping and heating a wafer without using moving parts, and without exposing a wafer to external stress. In the present invention, very high backside wafer pressures are effectively offset by a high frontside wafer pressure, which may be higher than or lower, than the backside wafer pressure. The high backside pressure provides very efficient heat transfer from the heated substrate support to the wafer; the high frontside pressure offsets the backside pressure, reducing wafer stress and when the frontside pressure exceeds the backside pressure, pressing the wafer toward the heated substrate support.
Various features of the invention include configurations that generate a vacuum pressure differential between a wafer's frontside and a wafer's backside. Because vacuum levels and pressure levels are inversely related, for simplicity, consistent reference is made herein to pressure levels rather than vacuum levels. The invention also features a configuration that provides a continuous flow of dry gas across the wafer's surface. In such a configuration, during the degas process the dry gas is continuously flowed into the vacuum chamber via one or more gas inlets and continuously pumped out of the vacuum chamber via one or more gas outlets, creating a continuous gas purge. Thus, as the wafer heats desorbed contaminants are swiftly carried away in a viscous gas flow and do not reabsorb in chamber surfaces. Further features include heating elements (e.g., reflecting elements) within a manifold positioned opposite the substrate support to minimize he heat loss due to radiation. These features preferably heat the frontside of the wafer from a position within the vacuum chamber and preferably are positioned in close proximity with the wafer's frontside. With the use of such heating elements wafer heating and degassing continues to occur even as the chamber is being pumped out for wafer transfer.
In a first aspect a lower pressure (i.e., greater vacuum) is generated along a wafer's backside than the pressure generated along the wafer's frontside. This pressure differential (having greater frontside pressure) uniformly presses the wafer toward the substrate support, eliminating the need for a clamp ring and the particle generation associated therewith. The uniform pressure along the wafer's frontside and the uniform pressure along the wafer's backside promotes uniform heat transfer from the substrate support to the wafer while minimizing the wafer's exposure to external stress. A pressure differential and a continuous gas purge is achieved by coordinated operation of two gas inlets (a frontside gas inlet and a backside gas inlet) and two gas outlets (a frontside outlet and a backside outlet) each outlet having its own gas pump operatively coupled thereto.
Similarly in a second aspect of the invention a greater pressure is generated along a wafer's frontside than the pressure generated along the wafer's backside. In this aspect, both a pressure differential and a continuous gas purge are achieved by coordinating operation of a single gas inlet (a frontside gas inlet) and two gas outlets (a frontside g
Marohl Dan
Raaijmakers Ivo
Applied Materials Inc.
Dugan & Dugan, L.L.P.
Gravini Stephen
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