Drying and gas or vapor contact with solids – Process – Diverse types of drying operations
Reexamination Certificate
1997-07-10
2001-02-06
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
Diverse types of drying operations
C034S092000, C034S412000
Reexamination Certificate
active
06182376
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 exist between the substrate support and the wafer which effectively decreases the contact area and heat transfer rate therebetween. Particularly in vacuum environments, this decreased contact area causes heat transfer to be dominated by radiation, and thereby to be slow, (as little radiation is produced at typical degassing temperatures). 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. Accordingly 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 along the wafer's frontside than the pressure along the wafer's backside. However, the opposing forces applied to the wafer by the clamp ring and by the backside gas pressure may cause the wafer to bow. For example, 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 reducing heat transfer. Moreover, a 10 Torr 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 flowing gas into a degassing chamber until the degassing chamber reaches a pressure at which wafer heating occurs primarily via gas conduction rather than radiation. Thus, wafer heating and degassing occur uniformly regardless of pre-existing wafer patterning. Gas is preferably gradually flowed into the degassing chamber via a needle valve or flow controller so as to prevent the wafer from being unseated from the heated substrate support
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. Thus, the wafer need not be clamped. Because pressures as low as a few Torr provide adequate heat conduction (the heat conductivity of Argon, for example, gas varying from near zero at high vacuum levels to full conductivity at 4 Torr.), after a degassing process the degassing chamber may be evacuated via a cryo-pump, without the need for an additional rough pumping step. Thus the configuration of the present invention provides faster evacuation times and increased throughput as compared to conventional systems that require both rough and high vacuum pumping. A roughing pump may nonetheless be employed with the present system to increase the time between cryo-pump regenerations. Alternatively the cryo-pump can be replaced with both a roughing pump and a turbo molecular pump.
An isolation valve such as a slit valve provides reliable cost effective isolation between the degassing chamber and a cryo-pump. A plurality of pins may be placed under the wafer to facilitate gas flow to the wafer's backside. To further enhance wafer temperature heat-up rate and wafer temperature uniformity during the degassing process and to reduce wafer temperature drop during the chamber evacuation for wafer transfer, a frontside heating element (e.g., a heat source and/or heat reflector) is placed preferably parallel to the wafer's frontside and in sufficiently close proximity to reflect heat that radiates from the wafer back to the wafer. Thus wafer heating and degassing continues to occur even as the chamber is being pumped out for wafer transfer.
By reducing the number of particles (via elimination of the clamp ring), by reducing reabsorbed moisture (via frontside heating elements contained within the vacuum chamber), and by reducing wafer stress (via a uniform frontside/backside pressure) the present invention greatly improves the wafer degassing process. Accordingly with use of the present invention less contamination and stress induced failures occur, and product yields increase. Also, since heat-up rates and degassing rates are higher with the present invention than with conventional radiatively heated degassing systems, the throughput of the overall semiconductor fabrication system is increased. These and other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.
REFERENCES:
patent: 4816638 (1989-03-01), Ukai et al.
patent: 4854263 (1989-08-01), Chang et al.
patent: 5314541 (1994-05-01), Saito et al.
patent: 5374594 (1994-12-0
Marohl Dan
Shin Ho Seon
Applied Materials Inc.
Dugan Valerie G.
Wilson Pamela
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