Method for detecting microscopic differences in thickness of photoresist film coated on wafer

A method for detecting microscopic differences in the thickness of a photoresist film coated on a wafer through naked eyes, which is capable of accurately controlling the critical dimension of the photoresist film patterns even in devices of 256 M DRAM or more and allows the yield to be analyzed with accuracy, comprising the steps of: subjecting the photoresist film to thermal treatment at a low temperature, to make some low molecular weight compounds or solvent molecules to remain within the photoresist film; forming a special material layer over the photoresist film within a certain thickness; and executing high-temperature thermal treatment, to gush up the remaining low-molecular weight compounds or solvent molecules from the photoresist film through relative thin parts of the special material layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates, in general, to a method for detecting 
microscopic differences in the thickness of a photoresist film coated on a 
wafer. More particularly, the present invention is concerned with a method 
for detecting the microscopic differences visibly such that the topology 
of the photoresist film can be improved easily. 
2. Description of the Prior Art 
As the design rule is smaller or the relative size of a wafer is smaller, 
which is attributed to the high integration of devices, it is more 
difficult to maintain uniformly the critical dimension (hereinafter 
referred to as "CD") of the patterns which are formed on many dies all 
over the wafer by lithography. In addition, microscopic differences in the 
thickness of the photoresist film coated on the wafer causes a difference 
in standing wave effect in spite of the same exposure energy, resulting in 
an alteration of the CD of each of the dies formed over the wafer. The 
amount of standing wave effect (S) is represented as Equation I: 
EQU S=C(R.sub.1 .times.R.sub.2).sup.1/2 e.sup.-.alpha.D I! 
wherein C stands for constant; 
R.sub.1 and R.sub.2 each are the intensity of a light scattered from the 
boundary between the bottom of a photoresist film and the upper surface of 
the wafer; 
a is a light transmittivity parameter associated with the components of the 
photoresist film; and 
D is the thickness of the photoresist film. 
Because the amount of standing wave effect (S) has a proportional relation 
as represented in Equation I, the change of S value arises dependently on 
the R.sub.1 and R.sub.2 values with the same period with the thickness in 
the photoresist film. 
With reference to FIG. 1, there is shown a curve of CD with respect to 
thickness of a photoresist film coated on a wafer. As shown in this curve, 
CD values are periodically changed. The change of CD value is dependent on 
the values of R.sub.1 and R.sub.2. In the figure, reference character "t" 
is a coating thickness difference of photoresist film which corresponds to 
a period of the standing wave effect. The coating thickness difference of 
photoresist film is proportional to .lambda./4n wherein .lambda. is a 
wavelength and n is a refractive index. Thus, the coating thickness 
difference of photoresist film (t) is microscopically change din a highly 
integrated device which has been exposed to a light source with a short 
wavelength, thereby changing the CD value. For 256 M DRAM (mega dynamic 
random access memory) an excimer laser having a wavelength of 248 nm is 
used for a photoresist film with a refractive index ranging from 1.6 to 
1.7. 
Generally speaking, spin-coat processes show their own characteristic 
differences in the coating thickness of photoresist film. The differences 
according to the spin-coat processes are unable to be measured by naked 
eyes. This is possible with a special measuring apparatus. However, there 
arise problems when measuring the coating state by means of such 
apparatus. Because measurements at many locations of the coating, which 
may be necessary for the accurate examination into the coating state of 
photoresist film, cannot be carried out or recognition of the outline of 
the coating shape is not possible, it is difficult to appropriately 
control the rotational speed, the amount of rinse for the photoresist 
film, the acceleration and the annealing step prior to the exposure when 
carrying out the coating process, resulting in obstructing the high 
integration of devices. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide a method for 
detecting microscopic differences in the thickness of a photoresist film 
coated on a wafer through naked eyes which avoids the aforementioned 
problems associated with prior art techniques. 
It is another object of the present invention to provide a method for 
detecting microscopic differences in thickness of a photoresist film 
coated on a wafer, which is capable of accurately controlling the critical 
dimension of the photoresist film patterns even in devices of 256 M DRAM 
or more. 
It is a further object of the present invention to provide a method for 
detecting microscopic differences in the thickness of a photoresist film 
coated on a wafer, which allows the yield to be analyzed with accuracy. 
In accordance with the present invention, the above objects could be 
accomplished by providing a method for detecting microscopic differences 
in the thickness of a photoresist film coated on a wafer, comprising the 
steps of: subjecting the photoresist film to thermal treatment at a low 
temperature, to make some low-molecular weight compounds or solvent 
molecules to remain within the photoresist film; forming a special 
material layer over the photoresist film within a certain thickness; and 
executing high-temperature thermal treatment, to gush up the remaining 
low-molecular weight compounds or solvent molecules from the photoresist 
film through relative thin parts of the special material layer.

DETAILED DESCRIPTION OF THE INVENTION 
The application of the preferred embodiments of the present invention is 
best understood with reference to the accompanying drawings. 
With reference to FIG. 2, there is illustrated a process for detecting 
microscopic differences in the thickness of a photoresist film coated on a 
wafer. As shown in this figure, a photoresist film 3 is formed on a wafer 
4 and then subjected to thermal treatment at low temperatures to make some 
of low-molecular weight compounds or solvent molecules 2 remain within the 
photoresist film 3. Thereafter, a special material layer 1, which is 
different from the photoresist film 3 in component and property, is formed 
thinly on the treated photoresist film 3. Finally, a thermal treatment at 
high temperatures allows the low-molecular weight compounds of solvent 
molecules 2 to gush up in the relative thin parts of the special material 
layer 1 formed on the photoresist film 3. 
The special material layer 1 is about 500 Angstrom or less thick and is 
formed with a spin-on-glass (SOG) coating or an oxide layer. The latter is 
formed by a plasma enhanced chemical vapor deposition (PECVD) process. 
While the low temperature thermal treatment is carried out at a 
temperature of 150 to 300.degree. C., the high-temperature thermal 
treatment is at a temperature of 200 to 500.degree. C. 
Upon executing the high-temperature thermal process, the low molecular 
weight compounds or solvent molecules 2 remaining within the photoresist 
film 3 escape from the photoresist film 3 through the thin parts of the 
special layer 1 or burst them. In FIG. 1, reference character "t" is a 
thickness difference of the photoresist film 3, meaning the step level 
thereof. Accordingly, the special material layer on a low step level is 
thicker than on a high step level and thus, little defects are shown after 
the high-temperature thermal treatment. 
FIG. 3 is a top view showing defects which are gushed up owing to the 
difference in thickness of the photoresist film 3 coated on the wafer 4, 
wherein more densely dotted parts indicate more defects which are 
generated upon the high-temperature thermal treatment, traceable to 
relative thin special material layer formed on relative high parts of the 
photoresist film 3. 
As described hereinbefore, the defects generated on coating of a 
photoresist film can be detected by forming a special material layer 
subsequent to a low-temperature treatment of the photoresist film and 
gushing up the defects through thin parts of the photoresist film, or by 
carrying out the coating step of the special material layer and the 
low-temperature treatment for 2 to 3 seconds simultaneously. 
In accordance with the present invention suggested, coating defectives 
attributable to microscopic differences in the thickness of a photoresist 
film can be readily detected, which allows CD to be accurately controlled 
in devices of 256 M DRAM or more. So, the detecting and monitoring of the 
process defectives can be achieved, leading to accurate and easy yield 
analysis. 
Other features, advantages and embodiments of the invention disclosed 
herein will be readily apparent to those exercising ordinary skill after 
reading the foregoing disclosures. In this regard, while specific 
embodiments of the invention have been described in considerable detail, 
variations and modifications of these embodiments can be effected without 
departing from the spirit and scope of the invention as described and 
claimed.