Coating processes – Centrifugal force utilized
Reexamination Certificate
1999-10-06
2002-04-23
Crispino, Richard (Department: 1734)
Coating processes
Centrifugal force utilized
C427S009000, C118S052000, C118S712000, C118S612000, C118S319000
Reexamination Certificate
active
06376013
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to semiconductor processing, and in particular to a system for uniformly distributing a photoresist material on a wafer.
BACKGROUND OF THE INVENTION
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as comers and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine patterns which are exposed on the photoresist material, thickness uniformity of the photoresist material is a significant factor in achieving desired critical dimensions. The photoresist material should be applied such that a uniform thickness is maintained in order to ensure uniformity and quality of the photoresist material layer. The photoresist material layer thickness typically is in the range of 0.1 to 3.0 microns. Good resist thickness control is highly desired, and typically variances in thickness should be less than ±10-20 Å across the wafer. Very slight variations in the photoresist material thickness may greatly affect the end result after the photoresist material is exposed by radiation and the exposed portions removed.
Application of the resist onto the wafer is typically accomplished by using a spin coater. The spin coater is essentially a vacuum chuck rotated by a motor. The wafer is vacuum held onto the spin chuck. Typically, a nozzle supplies a predetermined amount of resist to a center area of the wafer. The wafer is then accelerated to and rotated at a certain speed, and centrifugal forces exerted on the resist cause the resist to disperse over the whole surface of the wafer. The resist thickness obtained from a spin coating process is dependent on the viscosity of the resist material, spin speed, the temperature of the resist and temperature of the wafer. However, the resist is not always uniformly formed on the wafer because the resist may dry too early causing the resist to become more viscous and not easily flow as it disperses across the wafer. These effects become increasingly problematic for larger wafers, such as twelve inch wafers, because of the larger area the resist must cover. Larger wafers also have mechanical constraints with regards to rotation speed.
FIGS. 1
a
and
1
b
illustrate typical problems that can occur in applying resist to a wafer at a given speed. A nozzle
10
applies a resist layer
12
on a central area of a wafer
14
. The wafer
14
is vacuum held onto a rotating chuck
16
driven by a shaft
18
coupled to a motor
20
. The wafer
14
is rotated at a constant speed and the resist flows covering the entire top surface of the wafer.
FIG. 1
a.
illustrates an example where the resist employed had a viscosity that was too high at a given speed. The resulting resist layer
12
a
is concave in shape because the resist dried too quickly causing the flow of the resist to slow as it spread out over the top surface of the wafer
14
.
FIG. 1
b
illustrates an example where the resist had a viscosity that was too low at a given speed. The resulting resist layer
12
b
is convex in shape because the resist flowed too quickly from the center, and did not begin drying until much of the resist was near an outer circumference of the wafer
14
.
The resulting concave and convex resist layers
12
a
and
12
b
do not have uniform thicknesses which may lead to impaired device performance. In view of the above, a system/method is needed, for forming a uniform layer of resist across a wafer.
SUMMARY OF THE INVENTION
The present invention provides for a system and method that facilitates the application of a uniform layer of photoresist material spincoated at a constant speed onto a semiconductor substrate (e.g wafer). The present invention accomplishes this end by utilizing a measurement system that measures the thickness uniformity of the photoresist material applied on a test wafer by a nozzle, and then adjusting the viscosity of the photoresist material by varying the ratio in a solvent/resist mixture and/or adjusting the temperature of the mixture. After the viscosity adjustment, a new test wafer is spincoated and measured and the adjustment is repeated until the layer of photoresist material meets predefined tolerances of uniformity. Once the predefined tolerances are met, the values are stored for use in a mass production run. The system can also vary the overall thickness of the resist layer by using the measured data in determining the overall thickness of the resist layer, and controlling the overall volume of the solvent/resist mixture.
The present invention also provides for a system and method that employs a plurality of nozzles that disperse resist at different annular regions on a wafer to facilitate the application of a uniform layer of photoresist material spincoated at a constant speed onto the wafer. The system and method utilize a measurement system that measures the thickness and thickness uniformity of each layer of photoresist material applied at each annular region of the wafer. The measured thickness and overall thickness uniformity for each annular region is then employed to adjust the volume and viscosity of a solvent/resist mixture applied through each nozzle. The viscosity of the solvent/resist mixture of each nozzle is adjusted by varying the ratio of the solvent/resist mixture, and by varying the temperature of the mixture. After the viscosity and volume adjustment, a new test wafer is spincoated and measured and the adjustment is repeated until the layer of photoresist material meets predefined tolerances of uniformity. Once the predefined tolerances are met, the values are stored for use in a mass production run. The overall thickness can be adjusted by increasing the volume of solvent/mixture in each nozzle.
One particular aspect of the invention relates to a system for spincoating a uniformly thick layer of photoresist material on a substrate where the layer of photoresist material is spincoated onto the substrate by a rotating chuck coupled to a motor. The system includes a nozzle adapted to apply a predetermined volume of photoresist material to the center of the substrate. The system also includes a measuring system adapted to measure the thickness of the layer of photoresist material at different points along the substrate after the photoresist material is spincoated onto the substrate. A processor is operatively coupled to the measuring system and a viscosity adjustment system. The processor receives thickness data from the measuring system and uses the data to provide adjustment information to the v
Rangarajan Bharath
Singh Bhanwar
Templeton Michael K.
Yedur Sanjay K.
Amin & Turocy LLP
Crispino Richard
Tadesse Yewebdar T
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