Method and system for controlling the photolithography process

Details Method and system for controlling the photolithography process Method and system for controlling the photolithography process

2002-07-01

2003-11-04

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

C356S388000, C356S394000, C250S491100

Reexamination Certificate

active

06643017

ABSTRACT:

FIELD OF THE INVENTION
The present invention is in the field of measuring/inspecting techniques and relates to a method and a system for controlling the operation of a processing tool for processing workpieces. The invention is particularly useful in the manufacturing of semiconductor devices to control the operation of photolithography tools to optimize the entire photolithography process.
BACKGROUND OF THE INVENTION
The manufacture of semiconductor devices consists of several procedures applied to a semiconductor wafer to define active and passive elements. The wafer is prepared and one or more layers are deposited thereon. Thereafter, the process of photolithography is performed, in which the surface of a wafer with a pattern conforming to circuit elements is formed. An etching process applied to the uppermost layer follows the photolithography. By desirably repeating these processes, a multi-level semiconductor wafer is produced. Thus, photolithography is one of the main steps in the manufacture of semiconductor devices. It actually consists of the optical image transfer of a pattern from a mask to a semiconductor wafer.
It is a common goal of the semiconductor industry to minimize features on a wafer, namely to make the pattern finer and finer. Owing to the fact that optical systems used for image transfer reach their limitations, the lithography process should meet higher requirements of its operational performance. This means finer process control, as well as the development of new lithography equipment and chemicals. The major steps of the photolithography process are as follows:
(1) coating a wafer with a photoresist material (PR);
(2) exposing the PR to UV radiation through a mask in order to produce a latent image of the mask on the PR;
(3) developing the exposed PR in order to produce the image; and
(4) measuring and inspecting the wafer.
During the exposure of PR to UV light the PR becomes more or less soluble in a developing solvent, as compared to the unexposed PR, thereby producing a positive or a negative tune image, respectively.
FIG. 1
illustrates a common photolithography tools arrangement, a so-called “link arrangement”, generally designated
1
, for carrying out the photolithography process. The main idea underlying the implementation of such a link arrangement is that each tool is dedicated to serve the next one in the series, so as to minimize process/tool variations. The link arrangement
1
is composed of two main parts: a phototrack
2
and an exposure tool
3
. The phototrack
2
is formed by a coater track
4
and a developer track
5
, associated with cassette load/unload stations, designated
4
a
and
5
a,
respectively. A robot (not shown) loads the wafer from the cassette station
4
a
to the coater track
4
, and, when the coating procedure is complete, transfers it to the exposure tool
3
. Here, the pattern on a mask is aligned with a structure already on the wafer (registration) by an optical means installed inside the exposure tool
3
, and the wafer is exposed to electromagnetic radiation through the mask. After exposure, the robot transfers the wafer to the developer track
5
and then to the cassette station
5
a.
Additionally, several different baking procedures are implemented during the steps (a)-(c). The coater track
2
, exposure tool
3
and developer track
5
are tightly joined together in order to minimize process variability and any potential risk of contamination during photolithography which is a very sensitive process.
The measurement/inspection step is carried out with a metrology tool
7
, which is typically a big, stand-alone machine, that serves for the serial critical dimensions (CD) measurement. CD metrology tool
7
measures the width of representative lines, spaces and line/space pairs on the wafer. The operation of a conventional CD metrology tool is based on two main methods: scanning electron microscope (CD SEM) and atomic forced microscope (CD AFM). CD measurements typically take place after the developing step. To this end, “developed” wafers are taken out of the link arrangement
1
and transferred to the separate CD station occupied by the tool
7
. Data obtained during the CD measurements is analyzed with a processor
8
(which is typically integral with the CD metrology tool), and then a some sort of feedback is provided (e.g. an alarm in case of a width out of the permitted range) and transmitted to a relevant unit in the production line.
The quality of the entire photolithography process is defined by a combination of tolerances for all relevant parameters that can influence the final image transfer. The main parameter that should be controlled (and the easier to be adjusted and compensated) is the exposure dosage, i.e. the amount of energy reaching the PR.
According to one known technique, so-called “send ahead wafer”, a pilot wafer is sent through the arrangement
1
, namely through the coating-exposure-developing steps, applying a certain recommended exposure dose (and time), and then undergoes CD measurements. The results of the measurements will be the basis for set-up conditions of the entire lot, or for a correction signal to be applied to the tool
3
prior to the exposure of another wafer in the lot, i.e. a feedback loop. The whole sequence of such a “send ahead wafer” procedure can take many hours, during which valuable time of the production tools is not fully utilized and the wafers' flow is delayed. According to another technique, each lot is the basis for the next lot to run in this process representing a so-called “lot-to-lot control”. By considering the results of the previous lot, a small correction can be made. However, a certain increment in the risk exists, because the entire lot may be lost. Both of these techniques are time, labor and materials consuming and usually do not reveal any problematic root.
It is known that the photolithography provides sufficient results at certain levels of PR bleaching. Unfortunately, owing to the fluctuations of scan speed and light intensity, it is very difficult to reproduce each time the optimum exposure dose.
The most popular method used in production for providing a measurement directly correlated with the photoresist lithography image is a so-called “optimal exposure test”. According to this method, a wafer coated with a photoresist material is exposed through a mask using a sequence of different dosages. Following the exposure and development steps, the dose is estimated as a function of line width, utilizing the electron microscopy technique. Notwithstanding that this method considers all the relevant operations and materials of the entire photolithography process (i.e. coat, expose, develop, bakes, resist. etc.), it consumes expensive useful time of the exposure equipment.
U.S. Pat. No. 5,620,818 discloses a photolithographic dose determination technique, which utilizes diffraction of a latent image grating for constructing a calibration curve. This technique is not compatible with on-line production control, because of the following features. It requires that a special mask be designed and a special test wafer, having all the relevant stack layers, be created. A large area of a test structure is needed to provide a sufficient signal-to-noise ratio. Additionally, to consider each layer and each resist when constructing the calibration curve, a sequence of gradual exposures of the mask on the wafer should be conducted.
U.S. Pat. No. 5,635,285 discloses several methods of determining the correction for exposure. One of them is based on an exposure with a phase shift mask, which suffers from the need of an additional alignment procedure. Another method uses the known FLEX technique for exposures in several focus conditions to overcome the limits of depth of focus (DOF). This method has alignment and magnification error related problems. Yet another method is based on the use of an additional “out of focus illumination”. More specifically, additional radiation is added outside the depth of focus and the mask operates as a gray scale regim

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