Patent Application
20060279744
The present invention relates to a method for measuring thickness of thin film deposited on a substrate at high speed. The invention also relates to a method for forming a polycrystal semiconductor thin film using the film thickness measuring method, and also to a method and an apparatus for manufacturing a semiconductor device. The invention further relates to a method for manufacturing an image display device.
In particular, the present invention is suitable as a method for reforming a semiconductor thin film, i.e. for reforming an amorphous semiconductor thin film to a polycrystal semiconductor thin film, a method for evaluation based on measurement of film thickness of amorphous semiconductor thin film before reforming, and also, the invention is suitable as a method and an apparatus for manufacturing a thin film transistor prepared from the semiconductor thin film and for manufacturing a semiconductor device including a flat panel type image display device having circuits comprising thin film transistors.
In general, as a method for measuring film thickness—in particular, for measuring film thickness of multi-layer optical thin film, ellipsometry method is known, which is described in the Non-Patented Reference 1 given below. According to this method, a light is projected to a surface to be measured by changing polarizing status of the incident light. Then, the polarizing condition of the reflection light is determined and by performing parameter fitting of the measured data, optical constant and thickness of the thin film can be determined. This ellipsometry method is widely used in the inspection device for the thin film forming procedure in the semiconductor device manufacturing process.
In particular, the size of the substrate (normally, glass substrate) to be used in a flat type image display device (the so-called “flat panel display” (FPD); hereinafter, also called “display”) represented by a liquid crystal panel is getting larger and larger in recent years. When a CVD device normally used for deposition of thin film is used to form thin film on the glass substrate, in-plane film thickness variation (amount of change in film thickness distribution on portions of glass substrate with different film deposition surface; variation of in-plane film thickness of thin film) is increased when the size of the glass substrate becomes larger. For this reason, it is necessary to have as many measuring points as possible on the surface of the glass substrate and to control the range of variation in film thickness on the surface of the glass substrate by measuring detailed distribution of the in-plane film thickness variation.
This is because, when the thin film deposited on the glass substrate is a silicon film, the in-plane film thickness variation may exert serious influence on the characteristics of semiconductor device to be formed on the silicon film. As an example, description will be given below on a manufacturing line of low temperature silicon thin film transistor (TFT) substrate (hereinafter, also called “TFT substrate”) used in FPD.
The TFT substrate as described above comprises a pixel circuit and a driving circuit incorporated in silicon semiconductor thin film (hereinafter simply called “semiconductor”) deposited on insulator substrate (hereinafter simply referred as “substrate”). Also, as driving elements to constitute the pixel circuit or the driving circuit, thin film transistor (TFT) is used in many cases. By using a polycrystal semiconductor thin film (typically called “polysilicon film or poly-Si film) instead of an amorphous semiconductor thin film (typically called “amorphous silicon film” or referred as “a-Si film”) as active layer of the thin film transistor, it is possible to achieve image display with high precision and with high image quality.
The reason for this may be that the polycrystal silicon semiconductor thin film has higher mobility of carrier (electron in n-channel; hole in p-channel) compared with the amorphous silicon semiconductor thin film. As a result, high precision can be attained by reducing cell size (pixel size). For the formation of the thin film transistor deposited on normal poly-silicon semiconductor thin film, high temperature process of 1000° C. or higher is required. On the other hand, in the technique to form poly-silicon semiconductor thin film at low temperature to have semiconductor thin film at low temperature only on silicon layer by laser annealing, the substrate with semiconductor thin film deposited on it is not turned to high temperature, and it is possible to form the thin film transistor (TFT) with high mobility in low temperature process, in which low cost glass substrate can be used.
The effect that the mobility is high is advantageous in that the size of TFT can be reduced. This advantage leads to high brightness attained by the increase of opening ratio in the area of the openings other than TFT structure, comprising pixel circuits within pixel region for FPD, and this also results in the improvement of precise image quality. In the low temperature polysilicon TFT manufacturing process, the process influenced by the variation in film thickness of the amorphous silicon film is the annealing process using laser.
For example, in case annealing is performed by a pulsed excimer laser, and when it is checked how the grain size (average grain size) of polycrystal silicon to be crystallized is changed due to laser energy density, it is found that grain size of polycrystal silicon tends to be larger when laser energy is increased. However, when energy exceeds a certain energy threshold (when it is on the excess side), it is turned to microcrystal. This will be described later by referring to
The dependency of polycrystal silicon grain size on laser energy is changed due to film thickness of the silicon thin film. If the film is thick, laser energy is turned to shortage in proportion to the increase of film thickness. If the film is thin, laser energy is turned to be in excess. Therefore, film thickness distribution causes the difference in the in-plane distribution of grain size of polycrystal silicon and also leads to the difference in average particle size for each substrate. Finally, it causes the variations of TFT characteristics. In “the Patented Reference 1”, it is described that laser annealing is performed on the amorphous silicon substrate before laser annealing through adjustment of the energy necessary for reforming of the thin film by amorphous silicon film thickness measured by ellipsometry only for the first substrate in mass production lot.
Next, description will be given on a method for measuring film thickness other than the method based on ellipsometry. In “the Non-Patented Reference 2”, a method for measuring film thickness called “R, T, t method” is described. According to this method, optical constant of thin film of single layer is determined by film thickness data measured by transmissivity and reflectivity of a vertical incident light, and this is not a method for determining film thickness. “The Patented Reference 2” describes a method for evaluating film thickness by projecting a light of multiple wavelengths and by measuring reflection spectra. This is also a method for determining film thickness by parameter fitting. “The Patented Reference 3” discloses a method called “BPR method”. This is a method to evaluate film thickness through measurement of the dependency of reflection light intensity on reflection angle by using a projection system with multiple incident angles. This is also a method to determine film thickness by parameter fitting.
[Problems to be Solved by the Invention]
As described above, the ellipsometry method is generally used for the measurement of optical multi-layer thin film. This is a method to determine complex index of refraction and thickness by performing parameter fitting on the measured data. This method is disadvantageous in that a light is projected by changing the polarizing condition in relation to time at standstill on the same measuring point and to measure the polarizing condition of the reflection light, which varies in relation to time. In this respect, a measurement mode on step-and-repeat basis is required, and it is not possible to measure while scanning at high speed. Thus, it is not suitable for short-time measurement of the objects such as glass substrate with large area used for large size image display device.
In the conventional methods as described above except the ellipsometry method, measurement can be performed while scanning. However, the process to determine the film thickness by parameter fitting is still a factor to require longer measuring time. In any way, none of the conventional methods has achieved the evaluation of the in-plane distribution of amorphous silicon film thickness over the entire surface of a large size substrate with area of one square meter at as many as 10,000 measuring points or more within the measuring time of one minute.
Next, description will be given on problems in the manufacturing line, which requires measurement of film thickness. In the laser annealing process to reform amorphous silicon film to polycrystal silicon film, it is necessary to measure film thickness in advance because there is influence from variations of film thickness of amorphous silicon film. The ellipso method as described above is used for this purpose. As a device to inspect film thickness of a large size substrate in size of 730 mm×920 mm, the device produced by Sopra group in France is known. The measuring time when this device is used is about 10 seconds per each measuring point including moving in stage scanning. The measurement throughput is considered to be about 28 hours per substrate in case the measurement is made at 10,000 points over the entire surface of a large size substrate for image display device. Because the throughput of laser annealing process is about 7 minutes per substrate, it is necessary to reduce the number of measuring points to 1/240, i.e. by about 42% or less for the purpose of performing total inspection (100% inspection). However, this number of measuring points is still too few for the evaluation of the in-plane distribution of film thickness.
It is an object of the present invention to provide a method for measuring the film thickness of thin film, a method for forming polycrystal semiconductor thin film, a method and an apparatus for manufacturing semiconductor device, and a method for manufacturing image display device, by which it is possible to perform inspection over the entire surface of the substrate at 10,000 measuring points or more within one minute.
[Means for Solving the Problems]
Now, description will be given on a method for measuring film thickness of a thin film with light absorbing property and deposited on a transparent substrate according to the present invention by referring to
A light from the laser 1 is converged through the lens 7, and the light is projected as a projecting light vertically directed to the amorphous silicon thin film 3 deposited on the glass substrate 2. Scanning is performed by moving the projecting light or the glass substrate relatively to each other, and measurement is made on transmission light intensity and reflection light intensity at many points and at the same points within the glass substrate 2 over the entire measuring area on the glass substrate 2. Transmission light intensity is monitored by the transmission light intensity monitor 4, and reflection light intensity is monitored by the reflection light intensity monitor 5. The reflection light from the amorphous silicon thin film 3 is taken by the half-mirror 6.
Next, a value A, i.e. A=1−(R+T), where R represents reflectivity and T represents transmissivity, is determined. From the relation between the value A and the film thickness, film thickness is determined and evaluated. In general, the value A represents attenuation rate by factors other than transmission and reflection of energy of the projecting light, i.e. it is a sum of absorbance and scattering ratio. In case of the amorphous silicon thin film deposited by CVD on mirror surface of the glass substrate, the surface of the amorphous silicon thin film is also a mirror surface. Thus, scattering caused by surface roughness can be generally neglected, and the value A is equal to the absorbance in case of amorphous silicon film. Further, absorbance is a function of film thickness and complex index of refraction of the film. In case the object to be measured is an amorphous silicon film, for instance, the complex index of refraction can be set as a fixed parameter, and the film thickness can be determined from the value A.
If film thickness distribution is within the control range (if it is OK), amorphous silicon thin film is reformed to polycrystal silicon film by a process using excimer laser annealing (ELA process). In this laser annealing process, the laser energy must be adjusted depending on film thickness of amorphous silicon thin film. The reasons are as follows: In general, the larger the grain size of crystal of polycrystal silicon semiconductor thin film is, the higher the mobility is increased. Thus, laser annealing must be conducted in such manner that grain size will be at the maximum value.
Grain size tends to be larger when laser energy is increased. When energy exceeds a certain energy threshold, it is turned to microcrystal. However, the microcrystal threshold on high energy side is varied according to the thickness of amorphous silicon thin film before laser irradiation. When the film is thin, it is shifted to low energy side. Because film thickness is not even over the entire substrate (in-plane), it is necessary to know the in-plane film thickness distribution of the substrate before laser annealing and to adjust so that grain size will be higher than the lower limit (control range) or more over the entire substrate.
Returning to
On the other hand, in case the energy is on the excess side as shown in
In the procedure as described above, high speed measuring method must be performed on grain size similarly to the high speed measuring method of film thickness of the present invention. For this purpose, the method described in the Patented Reference 4 as filed by the present applicant should be carried out as the method for grain size inspection. However, the grain size inspecting method is not limited to this.
In the manufacturing line to carry out the method for manufacturing semiconductor device to the transparent substrate as described above, for instance, in the manufacturing line to manufacture display device on glass substrate, as many film thickness measuring devices as the amorphous semiconductor thin film depositing devices must be used and at least as many polysilicon particle size inspecting devices as the laser annealing devices must be used. By performing 100% inspection on the film thickness in-plane distribution and 100% inspection on the grain size in-plane distribution, feedback for each substrate is carried out in the laser annealing process.
[Effects of the Invention]
According to the present invention, it is possible to perform measurement on film thickness with higher throughput compared with the conventional methods. Further, by applying this evaluation method to the manufacturing line of the substrates for semiconductor device or image display device, it is possible to carry out total (100%) inspection at many measuring points as possible sufficient to obtain in-plane distribution information on film thickness of the amorphous semiconductor thin film, and the manufacturing method with high efficiency can be achieved.
Now, description will be given on Embodiment 1 of the invention by referring to
A light reflected from the sample substrate 2 is branched off from the projection optical axis by a half-mirror, and reflection light intensity is measured by a reflection light intensity monitor 5. Transmission light intensity is measured by a transmission intensity monitor 4. By this measurement, a value of “1−(T+R)” is determined at each measuring point.
Referring to
Further, in the process where the amorphous silicon thin film is reformed to polycrystal silicon thin film by laser annealing, it is necessary to regulate laser energy by adjusting the film thickness of the amorphous silicon thin film. The reasons are as follows: In general, the larger the grain size of the crystal of polycrystal silicon thin film is, the higher the mobility is increased. When grain size is small, the mobility is low. Therefore, in order to maintain the characteristics of TFT on a constant level, the laser annealing must be carried out so that grain size is kept to a certain fixed value or more.
However, as shown in
As described above, particle size is decreased in both cases, i.e. when laser energy is in shortage or it is in excess. There is difficulty in that the condition of energy shortage or excess is changed due to the variation in film thickness even when laser is very stable and there is no change in energy. To overcome this difficulty, it is necessary to perform total inspection on in-plane distribution of grain size. For the inspection of grain size, the method described in the Patented Reference 4 is used as the total inspection for grain size. The method to inspect grain size is not limited to this, and it will suffice if total inspection can be carried out on the entire surface of the substrate. This inspection is the process to inspect the grain size as shown in
Next, referring to
On the surface of the glass substrate 11, a primer layer (a laminated film of silicon nitride and silicon oxide) 12 is formed. On it, an amorphous silicon semiconductor layer is deposited, and it is reformed to a layer of polycrystal silicon thin film by laser annealing in the present invention. The thin film transistor 25 is incorporated in the layer of polycrystal silicon thin film prepared by laser annealing. Namely, by doping impurities on both sides of the semiconductor layer 13, which comprises polycrystal silicon semiconductor thin film, a source side semiconductor layer 14a made of polycrystal silicon semiconductor thin film and a drain side semiconductor layer 14b made of polycrystal silicon are formed. On it, a gate electrode 16 is deposited via a gate oxide film (a gate insulating layer) 15.
A source/drain electrode 18 is connected to the source side semiconductor layer 14a and the drain side semiconductor layer 14b respectively via contact hole formed on an interlayer insulator 17, and a protective film 19 is deposited on it. A color filter 20 and a pixel electrode 21 are formed on the protective film 19.
The thin film transistor 25 constitutes a pixel circuit of the liquid crystal display device. It is selected according to a selection signal from a scan driver circuit (not shown), and a pixel electrode 511 is driven by an image signal from a signal line driving circuit (not shown). An electric field is formed between the pixel electrode 511 thus driven and a counter electrode 513 with a counter glass substrate 514 on inner surface. By this electric field, molecular orientation direction of a liquid crystal 512 is controlled and the display is performed. In the liquid crystal display device as shown in
The thin film transistors to constitute the scan driving circuit and the signal line driving circuit may be prepared by the polycrystal silicon semiconductor thin film—similarly to the pixel circuit as described above. The present invention is not limited to the liquid crystal display device and may be applied to any type of image display device such as organic EL display device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-124830 | Apr 2005 | JP | national |
