Coating processes – Centrifugal force utilized
Reexamination Certificate
1997-12-10
2001-01-23
Beck, Shrive (Department: 1762)
Coating processes
Centrifugal force utilized
C427S385500
Reexamination Certificate
active
06177133
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved system and process for coating semiconductor wafers with photoresist polymer solution. In particular, this invention relates to a process control apparatus and method for improving coating thickness control of photoresist coatings on semiconductor wafers which improves line width control of device features.
BACKGROUND OF THE INVENTION
The manufacture of integrated circuits involves the transfer of geometric shapes on a mask to the surface of a semiconductor wafer. Thereafter the semiconductor wafer corresponding to the geometric shapes or corresponding to the areas between the geometric shapes is etched away. The transfer of the shapes from the mask to the semiconductor wafer typically involves a lithographic process. This includes applying a photosensitive pre-polymer solution to the semiconductor wafer. The solvent in the pre-polymer solution is removed by evaporation, and the resulting polymer film is then baked. The film is exposed to radiation, for example ultraviolet light, through a photomask supporting the desired geometric patterns. The images in the photosensitive material are then developed by soaking the wafer in a developing solution. The exposed or unexposed areas are removed in the developing process, depending on the nature of the photosensitive material. Thereafter the wafer may undergo a wet etch process in an etching solution, or a dry plasma etch process. Both the wet and dry etch processes etch away the areas not protected by the photosensitive material. Due to their resistance to the etching process, the photosensitive materials are also known as photoresists. These may for instance be sensitive to ultraviolet light, electron beams, x-rays, or ion beams.
The high cost of the photoresist pre-polymer solutions makes it desirable to devise methods of improving the efficiency of the coating process so as to minimize the polymer solution's consumption. Furthermore, thickness control and uniformity of the photoresist layer is an important criterion in the manufacture of integrated circuits. It ensures satisfactory reproduction of the geometric patterns on the semiconductor wafer. With the requirement for smaller dimensions, the photolithography processes have shifted to using light with shorter wavelengths and a resulting smaller depth of focus. Variations in thickness over the surface of the wafer and variations in thickness from one wafer to the next introduce non-reproducible variations in photoresist exposure and consequent non-reproducible variations in the ultimate details of the product which limit the possible size reductions.
Critical dimension (CD) control becomes more difficult with smaller feature sizes in which the line width is to be maintained, for example, within ten percent of the feature size. This problem will increase in importance in future devices with smaller feature sizes. For example, the speed of microprocessor and memory devices is strongly determined by the line width control of the critical dimensions. A chip with improved line width control can operate at higher frequencies due to smaller capacitive and resistive losses. One method of achieving improved CD control, i.e. line width control, is by means of thickness control of the photoresist.
Convective diffusion and evaporation are two strongly coupled mass transfer mechanisms that determine film thickness uniformity profiles of spin coated photoresist films. Convective diffusion is the dominant thinning mechanism during the first few seconds of spin coating. Even though the evaporation mechanism starts out two orders of magnitude smaller during the initial moments of dispense, its nonzero and constant value causes viscosity of the resist material to increase dynamically, thus decreasing the convective diffusion of the photoresist. Subsequently, evaporation becomes the dominant mechanism which eventually determines the dry film thickness profile. The thinning rate due to evaporation starts to decrease eventually due to lowered diffusivity of the remaining solvents. This strong dependence of the spin coating process on the evaporation mechanism requires today's modem wafer tracks to tightly control evaporation related physical parameters. Tightening of the electrical and/or mechanical control specifications on equipment contributes to the spiraling cost of processing wafers and may well limit the mean thickness control capability required by future generation devices since mechanical tolerances on equipment will not alleviate required process variances. Thus, an alternative approach is needed to achieve consistently more stringent process latitudes.
We have discovered that changes in barometric pressure and relative humidity, as well as variations in photoresist solution temperature, wafer temperature and ambient temperature introduce coating variations in the spin-coating process which account for much of the variations experienced in spin coating wafers. While temperatures can be controlled by traditional sensors and feed-back systems, and relative humidity can be controlled by providing an enclosed system, barometric pressure fluctuations are beyond control unless one places the entire processor in a pressure chamber, an impractical approach.
SUMMARY OF THE INVENTION
This invention is based on a revolutionary and dynamic approach to process control by adapting the process to the changes in environmental variables which provides a remarkable increase in coating thickness control and coating uniformity which, in turn, improves the feature size control. In this invention, barometric pressure and relative humidity are measured and the measured values are input to a model which calculates a drying spin speed corresponding to the measured environmental values. This new spin speed is then communicated to the spindle controller to adjust the spin speed from a nominal value in order to control the mean resist film thickness.
The invention pertains to a spin coating process for coating a wafer surface with a solution, the surface having a central axis normal thereto. The process comprises the steps of applying the solution to the wafer surface, and spinning the wafer about the central axis at a spindle speed until the solution has dried. The mean thickness of the solution is a function of the relative humidity, barometric pressure, and spindle speed during drying. The physics of spin coating predicts that the mean thickness is proportional to the inverse square root of the spin speed, and linearly dependent on relative humidity and barometric pressure, and can be modeled according to the following equation:
MT=A+B×RH+C×BP+D/SS
1/2
wherein:
MT is mean thickness in Å;
RH is relative humidity in percent;
BP is barometric pressure in mm of Hg;
SS is spindle speed in rpm; and
A, B, C and D are constant coefficients.
Therefore, the desired spindle speed, at a known relative humidity and a known barometric pressure, can be determined for a desired mean thickness by solving the above equation for SS.
SS=
(
D/
(
MT−A−B×RH−C×BP
))
2
Barometric pressure is not controllable, and varies at a slow rate. The time constant for the barometric pressure to change significantly to have an impact on thickness may be days or weeks. Therefore, the methodology of the above model can be used without including barometric pressure in instances where it is known that the change in barometric pressure will not be significant. The effect on the above equation would be to combine the constant product C×BP into the constant A coefficient.
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Gurer Emir
Savage Richard
Beck Shrive
Calcagni Jennifer
Silicon Valley Group Inc.
Wilson Sonsini Goodrich & Rosati
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