JP5255739B2 - Method for manufacturing semiconductor device - Google Patents

Description

  The present invention relates to a method for annealing a semiconductor film using laser light (hereinafter referred to as laser annealing) and a laser apparatus for performing the method (laser and an optical system for guiding laser light output from the laser to an object to be processed). Apparatus).

  In recent years, thin film transistors (hereinafter referred to as TFTs) have been developed, and TFTs using a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film have attracted attention. In particular, liquid crystal display devices (liquid crystal displays) and EL (electroluminescence) display devices (EL displays) are used as elements that form elements that switch pixels and drive circuits that control the pixels.

  As a means for obtaining a polysilicon film, a technique for crystallizing an amorphous silicon film (amorphous silicon film) to form a polysilicon film is generally used. In particular, recently, a method of crystallizing an amorphous silicon film using laser light has attracted attention. In this specification, means for crystallizing an amorphous semiconductor film with laser light to obtain a crystalline semiconductor film is called laser crystallization.

  Laser crystallization is an effective technique for annealing a semiconductor film formed on a substrate having low heat resistance, such as a glass substrate or a plastic substrate, which can instantaneously heat the semiconductor film. Further, the throughput is much higher than that of a heating means using a conventional electric furnace (hereinafter referred to as furnace annealing).

  There are various types of laser light. Generally, laser crystallization using laser light (hereinafter referred to as excimer laser light) using a pulsed excimer laser as an oscillation source is used. The excimer laser has the advantage that it has a large output and can be repeatedly irradiated at a high frequency, and the excimer laser light has the advantage that the absorption coefficient for the silicon film is high.

  At present, the most noticeable problem is how to increase the crystal grain size of the crystalline semiconductor film crystallized with laser light. As a matter of course, when one crystal grain (also referred to as a grain) becomes large, the number of crystal grain boundaries that cross the channel formation region of the TFT decreases. Therefore, it is possible to improve variations in typical electrical characteristics of TFT such as field effect mobility and threshold voltage.

  In addition, the inside of each crystal grain maintains relatively clean crystallinity, and in order to improve the various characteristics of the TFT described above, the channel formation region must be completely contained within one crystal grain. It is desirable to form a TFT.

  However, it is difficult to obtain a crystalline semiconductor film having a sufficiently large crystal grain size with the current technology, and although it has been reported experimentally, it has not reached the practical level. .

Experimentally, "" High-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Method "", K. Shimizu, O. Sugiura and M. Matumura, IEEE Transactions on Electron Devices vol.40, No.1, pp112-117, 1993 "has been obtained. In this document, a three-layer structure of Si / SiO ₂ / n ⁺ Si is formed on a substrate, and excimer laser light is irradiated from both directions of the Si side and the n ⁺ Si side. It is shown that the crystal grain size can be increased by such a configuration.

Problems to be solved by the invention

  The present invention is a technique for solving the above problems, and an object thereof is to provide a laser annealing method for obtaining a crystalline semiconductor film having a large crystal grain size and a laser apparatus used in the laser annealing method.

Means for solving the problem

  The gist of the present invention is that, when crystallizing an amorphous semiconductor film, laser light is applied to the surface of the amorphous semiconductor film (the surface on which the thin film is superimposed) and the back surface (the surface opposite to the surface). Effective energy intensity of laser light (hereinafter referred to as “primary laser light”) simultaneously irradiated and irradiated on the front surface and effective energy intensity of laser light (hereinafter referred to as “secondary laser light”) irradiated on the back surface Are different from each other.

That is, when the effective energy intensity of the primary laser beam is (I ₀ ) and the effective energy intensity of the secondary laser beam is (I ₀ ′), the effective energy intensity ratio (I ₀ ′ / I ₀ ) is obtained. The laser light is irradiated so that the relationship of “0 <I ₀ ′ / I ₀ <1” or “1 <I ₀ ′ / I ₀ ” is satisfied. Of course, I ₀ · I ₀ ′ ≠ 0.

In this specification, the “effective energy intensity” is the energy intensity that the laser light has when it reaches the front or back surface of the amorphous semiconductor film, and the energy intensity that takes into account energy loss due to reflection or the like (here, The unit is defined as density: mJ / cm ² . Although it cannot be measured, if the medium existing in the laser beam path is known, it can be obtained from the calculation of reflectance and transmittance.

  For example, a specific method for calculating the effective energy intensity will be described in the case where the present invention is applied to the structure shown in FIG. In FIG. 6, 601 is a reflector made of aluminum, 602 is a Corning # 1737 substrate (thickness 0.7 mm), 603 is a 200 nm thick silicon nitride oxide film (hereinafter referred to as SiON film), and 604 is 55 nm thick amorphous. It is a silicon film. An example in which such a sample is irradiated with XeCl excimer laser light having a wavelength of 308 nm in air is taken as an example.

The energy intensity of laser light (wavelength 308 nm) immediately before reaching the amorphous silicon film 604 is defined as (I _a ). At this time, the effective energy intensity (I ₀ ) of the primary laser beam is expressed by I ₀ = I _a (1−R _Si ) in consideration of the reflection of the laser beam on the surface of the amorphous silicon film. Here, R _Si is the reflectance of the laser beam. In this case, I ₀ = 0.45 I _a is calculated.

The effective energy intensity (I ₀ ′) of the secondary laser light is expressed by I ₀ ′ = I _a T ₁₇₃₇ R _Al T ₁₇₃₇ (1-R _SiON—Si ). Where T ₁₇₃₇ is the transmittance of the # 1737 substrate, R _Al is the reflectance at the aluminum surface, and R _SiON—Si is the reflectance when entering the amorphous silicon film from the SiON film. The reflectance when entering the SiON film from the air, the transmittance of the SiON film, the reflectance when entering the # 1737 substrate from the SiON film, and the reflection when entering the SiON film from the # 1737 substrate The rate was not included in the calculation because it was found experimentally negligible. In this case, I ₀ ′ = 0.13I _a is calculated.

Therefore, in the structure of Figure 6, the effective energy intensity of the primary laser beam (I ₀₎ is 0.45I _a, the effective energy intensity of the second laser beam (I ₀ ') is determined to 0.13I _a . That is, the effective energy intensity ratio (I ₀ ′ / I ₀ ) is 0.29. One of the features of the present invention is that the effective energy intensity ratio obtained as described above satisfies 0 <I ₀ ′ / I ₀ <1.

Further, the present invention can be realized even when the intensity of the primary laser beam is smaller than the intensity of the secondary laser beam. That is, the present invention can be realized even when the effective energy intensity ratio satisfies 1 <I ₀ ′ / I ₀ .

In order to make the effective energy intensities of the primary laser beam and the secondary laser beam different, the following methods can be mentioned.
1) Effective energy intensity of the secondary laser light by adjusting the reflectance of the reflector when irradiating the surface and back surface of the amorphous semiconductor film with the reflector provided under the substrate Is attenuated so that it is relatively smaller than the effective energy intensity of the primary laser beam.
2) Divide the primary laser beam halfway to form the secondary laser beam, and attenuate the effective energy intensity of the primary laser beam or the effective energy intensity of the secondary laser beam with a filter (variable attenuator, etc.) And relatively different the effective energy intensity of the two.
3) A method in which the effective energy intensity of the secondary laser light is attenuated and made relatively smaller than the effective energy intensity of the primary laser light depending on the material of the substrate on which the amorphous semiconductor film is formed.
4) An insulating film is sandwiched between the substrate and the amorphous semiconductor film, and the effective energy intensity of the secondary laser light is attenuated by the insulating film so as to be relatively smaller than the effective energy intensity of the primary laser light. Method.
5) By covering the surface of the amorphous semiconductor film with an insulating film and reducing the reflectivity of the primary laser beam on the surface of the amorphous semiconductor film, the effective energy intensity of the primary laser beam is made relatively To make it larger than the effective energy intensity of the secondary laser beam.
6) A method in which the amorphous semiconductor film is covered with an insulating film, and the effective energy of the primary laser light is attenuated by the insulating film so as to be relatively smaller than the effective energy of the secondary laser light. .
7) A method in which the primary laser beam and the secondary laser beam are formed using different lasers as oscillation sources, and the effective energy intensities of the laser beams differ from each other.

  The present invention does not depend on the type of laser, but is generally known excimer laser (typically KrF laser or XeCl laser), solid-state laser (typically Nd: YAG laser or ruby laser), gas laser. (Typically, an argon laser or a helium-neon laser), a metal vapor laser (typically a copper vapor laser or a helium-cadmium laser), or a semiconductor laser can be used.

  In addition, when using a laser beam having a long fundamental wave (first harmonic: wavelength 1064 nm) like an Nd: YAG laser, it is preferable to use the second harmonic, the third harmonic, or the fourth harmonic. . These harmonics can be obtained using a nonlinear crystal (nonlinear element). Further, a known Q switch method may be used.

[Background to Invention]
Here, the background that the present applicant has conceived of the present invention will be described based on experimental results. A SEM (Scanning Electron Microscopy) photograph shown in FIG. 7 is a photograph after a seco-etching is performed on a polysilicon film formed by laser crystallization. For more information on Seco etching technology, see “F. Secco d 'Aragona:“ Dislocation Etch for (100) Planes in Silicon ”. J. Electrochem. Soc. Vol. 119. No. 7, pp. 948-950 (1972). Can be referred to.

In both cases, an amorphous silicon film (thickness: 55 nm) is formed on a # 1737 substrate (thickness: 0.7 mm) manufactured by Corning, Inc. via a silicon oxide film (thickness: 200 nm) and irradiated with excimer laser light. Yes. The excimer laser beam used in this experiment was a pulse laser beam having a wavelength of 308 nm using XeCl gas as an excitation gas, the pulse width was 30 ns, the number of shots was 20 shots, and the energy density was 370 mJ / cm ² .

  FIG. 7A shows a polysilicon film (average crystal grain size is about 0.3 μm) obtained by irradiating only the surface of the amorphous silicon film with laser light, and FIG. This is a polysilicon film (average crystal grain size is about 1.5 μm) obtained by irradiating the back surface with laser light. According to this, it is confirmed that the polysilicon film obtained by irradiating the front and back surfaces of the amorphous silicon film has a crystal grain size about 5 times larger, and irradiation from both sides is very effective. It was done.

  In this specification, the definition of the average crystal grain size conforms to the “definition of the average diameter of crystal grain region” in the specification of Japanese Patent Application No. 10-020666.

As described above, it was confirmed that the crystal grain size can be increased by irradiating the front and back surfaces of the amorphous semiconductor film with laser light. The experiment in the literature shown in the conventional example is aimed at the heat storage effect by using the residual heat of n ⁺ Si without directly irradiating the back surface of the semiconductor film to be crystallized with the laser beam. The configuration is completely different from human experiments.

Next, the applicant conducted a similar experiment using a quartz substrate instead of the glass substrate (however, the energy density of the laser beam was 200 mJ / cm ² ). As a result, a result (SEM photograph after Seco-etching) as shown in FIG. 8 was obtained.

  FIG. 8A is a polysilicon film obtained by irradiating only the surface of the amorphous silicon film with laser light, and FIG. 8B is obtained by irradiating the front and back surfaces of the amorphous silicon film with laser light. A polysilicon film. According to this, when a quartz substrate is used as the substrate, the average crystal grain size is at most about 0.4 to 0.5 μm, and an increase in the grain size as shown in FIG. Further, there was no difference in the crystal grain size when irradiated from one side or both sides of the substrate. That is, as described above, the effect of increasing the average crystal grain size was not confirmed despite the irradiation of laser light on the front and back surfaces of the amorphous semiconductor film.

  Therefore, the present applicant considers the above experimental results, and the difference between the experiments shown in FIGS. 7 and 8 is the difference between the transmittance of the glass substrate (about 50%) and the transmittance of the quartz substrate (about 93%). That is, it was expected to be a difference in effective energy intensity of the laser light irradiated on the back surface of the amorphous semiconductor film. And the following experiment was done for confirmation.

  First, in this experiment, a sample having the structure shown in FIG. 6 was manufactured using a quartz substrate for the substrate 602 and a tantalum nitride film for the reflector 601. Then, this sample was irradiated with XeCl excimer laser light under the same conditions as those for obtaining the photograph of FIG. 7B, and the average crystal grain size of the obtained polysilicon film was converted into an SEM photograph after the secco-etching. Confirmed. The result is shown in FIG.

  As can be seen from FIG. 9, it was confirmed that the crystal grains of the obtained polysilicon film were distributed in a state almost the same as that of the polysilicon film of FIG. In the case of the sample obtained from the photograph of FIG. 7B, it has already been described that the effective energy intensity ratio between the primary laser beam and the secondary laser beam is 0.29. This is a result of the secondary laser light being attenuated substantially on the glass substrate. Similarly, as a result of calculating the effective energy intensity ratio for the sample of this experiment, a value of 0.33 was obtained. This is a result of the secondary laser light being substantially attenuated by the reflector.

  The sample in FIG. 8B (combination of reflectors made of quartz and aluminum) and the sample in FIG. 9 (combination of reflectors made of quartz and tantalum nitride) have the same structure except for the material of the reflector surface. 9 is different from the sample of FIG. 9 in that the reflectance of the reflector surface is smaller than that of the sample of FIG. 8B.

  Considering the above results, when crystallization is performed by irradiating the surface and back surface of the amorphous semiconductor film with laser light, the effective energy intensity of the laser light on the front surface side (primary laser light) is larger than that on the back surface side. It was found that when the effective energy intensity of the laser beam (secondary laser beam) is small, an increase in the average crystal grain size is confirmed.

Embodiment 1
One embodiment of the present invention will be described. FIG. 1A is a diagram showing a configuration of a laser apparatus of the present invention. The laser apparatus includes a laser 101, an optical system 201 that linearly transforms laser light using the laser 101 as an oscillation source, and a stage 102 that fixes a translucent substrate. The stage 102 includes a heater 103 and a heater controller 104. To hold the substrate at a temperature ranging from room temperature to 550 ° C. A reflector 105 is provided on the stage 102, and a substrate 106 on which an amorphous semiconductor film is formed is provided thereon.

  Next, a method for holding the substrate 106 in the laser device having the structure shown in FIG. 1A will be described with reference to FIG. The substrate 106 held on the stage 102 is set in a reaction chamber 107 and irradiated with linear laser light using the laser 101 as an oscillation source. The reaction chamber can be in a reduced pressure state or an inert gas atmosphere by an exhaust system or a gas system not shown, and can be heated to 100 to 450 ° C. without contaminating the semiconductor film.

  Further, the stage 102 can move along the guide rail 108 in the reaction chamber, and can irradiate the entire surface of the substrate with linear laser light. The laser light is incident from a quartz window (not shown) provided on the upper surface of the substrate 106. In FIG. 1B, a transfer chamber 109, an intermediate chamber 110, and a load / unload chamber 111 are connected to the reaction chamber 107, and the chambers are separated by gate valves 112 and 113.

  A cassette 114 capable of holding a plurality of substrates is installed in the load / unload chamber 111, and the substrates are transferred by a transfer robot 115 provided in the transfer chamber 109. Substrate 106 ′ represents the substrate being transferred. With such a configuration, laser annealing can be continuously processed under reduced pressure or in an inert gas atmosphere.

  Next, the configuration of the optical system 201 that linearizes laser light will be described with reference to FIG. 2A is a view of the optical system 201 as viewed from the side, and FIG. 2B is a view of the optical system 201 as viewed from the top.

  Laser light using the laser 101 as an oscillation source is divided in the vertical direction by a cylindrical lens array 202. The divided laser light is further divided in the horizontal direction by the cylindrical lens 203. That is, the laser light is finally divided into a matrix by the cylindrical lens arrays 202 and 203.

  The laser light is once condensed by the cylindrical lens 204. At that time, it passes through the cylindrical lens 205 immediately after the cylindrical lens 204. Thereafter, the light is reflected by the mirror 206, passes through the cylindrical lens 207, and reaches the irradiation surface 208.

  At this time, the laser light projected on the irradiation surface 208 shows a linear irradiation surface. That is, it means that the cross-sectional shape of the laser light transmitted through the cylindrical lens 207 is linear. The homogenization of the linearly deformed laser light in the width direction (short direction) is performed by the cylindrical lens array 202, the cylindrical lens 204, and the cylindrical lens 207. Further, the homogenization of the laser light in the longitudinal direction (long direction) is performed by the cylindrical lens array 203 and the cylindrical lens 205.

  Next, a structure for irradiating laser light from the front and back surfaces of the film to be processed formed on the substrate will be described with reference to FIG. FIG. 3 shows the positional relationship between the substrate 106 and the reflector 105 in FIG.

  In FIG. 3, reference numeral 301 denotes a light-transmitting substrate, and an insulating film 302 and an amorphous semiconductor film (or microcrystalline semiconductor film) 303 are formed on the surface (the surface on which a thin film or an element is formed). Yes. A reflector 304 for reflecting the laser light is disposed under the translucent substrate 301.

  As the light-transmitting substrate 301, a glass substrate, a quartz substrate, a crystallized glass substrate, or a plastic substrate is used. The translucent substrate 301 itself can adjust the effective energy intensity of the secondary laser light. The insulating film 302 may be an insulating film containing silicon such as a silicon oxide film or a silicon nitride oxide film (SiOxNy), and the effective energy intensity of the secondary laser light may be adjusted by the insulating film 302. The amorphous semiconductor film 303 includes a compound semiconductor film such as an amorphous silicon germanium film in addition to the amorphous silicon film.

  Further, the reflector 304 may be a substrate on which a metal film is formed on the surface (laser light reflecting surface) or a substrate made of a metal element. In this case, any material may be used for the metal film. Typically, a metal film containing any element of silicon (Si), aluminum (Al), silver (Ag), tungsten (W), titanium (Ti), and tantalum (Ta) is used. For example, tungsten nitride (WN), titanium nitride (TiN), or tantalum nitride (TaN) may be used.

  Further, the reflector 304 may be provided in contact with the translucent substrate 301 or may be provided separately. Further, instead of disposing the reflector 304, it is also possible to form the metal film as described above directly on the back surface (surface opposite to the front surface) of the substrate 301 and reflect the laser beam there. In any case, the effective energy intensity of the secondary laser light can be adjusted by the reflectance of the reflector 304. In the case where the reflector 304 is placed away from the light-transmitting substrate 301, the energy intensity of the secondary laser light can be controlled by a gas (gas) filling the gap.

  Then, the amorphous semiconductor film 303 is irradiated with linearly deformed laser light via the optical system 201 described in FIG. 2 (only the cylindrical lens 207 is shown in the drawing). Irradiation of the laser beam transformed into a linear shape is performed by scanning the laser beam.

In any case, the primary laser beam 305 that passes through the cylindrical lens 207 and is irradiated on the surface of the amorphous semiconductor film 303, and is once reflected by the reflector 304 and irradiated on the back surface of the amorphous semiconductor film 303. It is important that the effective energy intensity ratio (I ₀ ′ / I ₀ ) with the secondary laser beam 306 to satisfy the relationship 0 <I ₀ ′ / I ₀ <1 or 1 <I ₀ ′ / I ₀ It is. For this purpose, the reflectance of the reflector 304 to the laser light is preferably 20 to 80%. At this time, a plurality of means for attenuating the effective energy intensity of the secondary laser light described in the present embodiment may be combined to obtain a desired intensity ratio.

  The laser beam that has passed through the cylindrical lens 207 has an incident angle of 45 to 90 ° with respect to the substrate surface in the process of being condensed. Therefore, the secondary laser light 306 irradiates around the back surface side of the amorphous semiconductor film 303. Moreover, the secondary laser beam 306 can be obtained more efficiently by providing the undulating portion on the reflecting surface of the reflector 304 to diffusely reflect the laser beam.

[Embodiment 2]
In the present embodiment, an embodiment different from the first embodiment will be described. In this embodiment, an example is shown in which a two-system laser beam dispersed in the middle of an optical system is irradiated from the front and back surfaces of an amorphous semiconductor film without using a reflector as in the first embodiment.

  FIG. 4A is a diagram showing the configuration of the laser apparatus of this embodiment. Since the basic configuration is the same as that of the laser apparatus of FIG. 1 described in the first embodiment, different reference numerals are used for the description.

  The laser apparatus includes a laser 101, an optical system 401 that linearly transforms laser light using the laser 101 as an oscillation source and splits the laser light into two lines, and a translucent stage 402 that fixes a translucent substrate. . A substrate 403a is provided over the stage 402, and an amorphous semiconductor film 403b is formed thereon.

  In the case of this embodiment, the stage 402 must have a light-transmitting property in order to irradiate the amorphous semiconductor film 403b with the laser light transmitted through the stage 402. Further, since the laser light (secondary laser light) irradiated from the stage 402 side passes through the stage 402, the effective energy intensity must consider the attenuation when passing through the stage 402.

  FIG. 4B illustrates a method for holding the substrate 403a in the laser apparatus illustrated in FIG. 4A. FIG. 4B illustrates the method except that the light-transmitting stage 402 is used. Since the configuration is the same, the description thereof is omitted.

  Next, the structure of the optical system 401 illustrated in FIG. 4A will be described with reference to FIG. FIG. 5 is a side view of the optical system 401. Laser light using the laser 501 as an oscillation source is divided in the vertical direction by a cylindrical lens array 502. This divided laser beam is further divided in the lateral direction by a cylindrical lens 503. In this way, the laser light is divided into a matrix by the cylindrical lens arrays 502 and 503.

  The laser light is once condensed by the cylindrical lens 504. At that time, it passes through the cylindrical lens 505 immediately after the cylindrical lens 504. Up to this point, the optical system is the same as that shown in FIG.

  Thereafter, the laser light enters the half mirror 506, where the laser light is split into a primary laser light 507 and a secondary laser light 508. The primary laser beam 507 is reflected by the mirrors 509 and 510, passes through the cylindrical lens 511, and then reaches the surface of the amorphous semiconductor film 403b.

  The secondary laser light 508 dispersed by the half mirror 506 is reflected by the mirrors 512, 513, and 514, passes through the cylindrical lens 515, passes through the substrate 403a, and reaches the back surface of the amorphous semiconductor film 403b. .

  At this time, similarly to the first embodiment, the laser light projected on the irradiation surface of the substrate shows a linear irradiation surface. Further, the homogenization in the width direction (short direction) of the laser beam deformed linearly is performed by the cylindrical lens array 502, the cylindrical lens 504, and the cylindrical lens 515. The laser beam is homogenized in the longitudinal direction (long direction) by the cylindrical lens array 503, the cylindrical lens 505, and the cylindrical lens 511.

In any case, the primary laser light that is transmitted through the cylindrical lens 511 and applied to the surface of the amorphous semiconductor film 403b, and the back surface of the amorphous semiconductor film 403b that is transmitted through the cylindrical lens 515 and applied to the back surface of the amorphous semiconductor film 403b. It is important that the effective energy intensity ratio (I ₀ ′ / I ₀ ) with the secondary laser light satisfies the relationship 0 <I ₀ ′ / I ₀ <1 or 1 <I ₀ ′ / I ₀ .

  In the present embodiment, a glass substrate (made of a material having a laser beam transmittance of about 50% used here) is used as the substrate 403a so that the above relational expression is satisfied. Of course, besides the substrate, the transmittance of the insulating film (not shown) provided on the substrate 403a, the stage (not shown) on which the substrate 403a is installed, and the reflectance of the interface are adjusted to adjust the secondary laser light. The effective energy intensity may be attenuated.

  In addition, a neutral density filter may be provided at an arbitrary position in the optical path of the secondary laser light of the optical system 401 to attenuate the effective energy intensity of the secondary laser light. It is also possible to attenuate the effective energy intensity of the primary laser light by providing a neutral density filter at an arbitrary place in the optical path of the primary laser light.

  Further, a plurality of means for attenuating the effective energy intensity of the primary laser light or the secondary laser light described in some embodiments may be combined to obtain a desired intensity ratio.

[Example 1]
In this example, an example in which the amorphous silicon film is crystallized with the structure shown in Embodiment Mode 1 is shown. FIG. 3 is used for the description.

  In this embodiment, a 1.1 mm thick quartz substrate is used as the substrate 301, a 200 nm thick silicon nitride oxide film (SiON film) is used as the insulating film 302, and an amorphous silicon film is used as the amorphous semiconductor film 303. At this time, the SiON film 302 and the amorphous silicon film 303 were formed using a plasma CVD method.

In this embodiment, SiH ₄ is introduced into the reaction chamber as ₄ SCCM and N ₂ O as 400 SCCM, and the SiON film 302 is formed with a film forming temperature of 400 ° C., a reaction pressure of 30 Pa, a discharge power density of 0.41 W / cm ² and a discharge frequency of 60 MHz. Formed. Next, SiH ₄ was introduced into the reaction chamber at 100 SCCM, and an amorphous silicon film 303 was formed with a film forming temperature of 300 ° C., a reaction pressure of 45 Pa, a discharge power density of 0.037 W / cm ² , and a discharge frequency of 13.56 MHz. In practice, the amorphous silicon film was patterned into an island pattern.

  Next, laser crystallization of the amorphous silicon film 303 was performed using an excimer laser apparatus as shown in FIG. At this time, the reflector 304 shown in FIG. 3 was formed by forming a tungsten nitride film on a silicon substrate. A 150 μm gap was left between the reflector 304 and the quartz substrate 301.

In this state, as shown in FIG. 3, excimer laser light (primary laser light 305 and secondary laser light 306) was irradiated to the amorphous silicon film 303 at room temperature in the atmosphere. The excimer laser light was scanned from one end of the substrate to the other end by deforming the cross-sectional shape into a linear shape (0.4 mm × 160 mm) by the optical system shown in FIG. The scanning speed was 1 mm / s, the energy density (energy intensity conceived of I _a in FIG. 6) was 336 mJ / cm ² , the pulse width was 30 ns, the repetition frequency was 30 Hz, and the overlay rate was 90%. As a result, 20 shots of laser light could be irradiated to one place.

In the case of performing laser crystallization in the configuration of this embodiment, the effective energy intensity of the primary laser beam (I ₀₎ is 151.2mJ / cm ^2, the effective energy intensity of the second laser beam (I ₀ ') Was 77.3 mJ / cm ² . Therefore, the effective energy intensity ratio (I ₀ ′ / I ₀ ) was 0.51.

  Here, an SEM photograph of the polysilicon film crystallized according to the present embodiment is shown in FIG. FIG. 10 shows a state after Secco etching. In this secco etching, a room temperature etchant to which 50 cc of hydrofluoric acid solution, 25 cc of water, and 1.14 g of potassium chromate (divalent) were added was used.

  As a result, as shown in FIG. 10, relatively large crystal grains having an average crystal grain size of about 0.5 to 0.6 μm were confirmed near the center of the island pattern. There are crystal grains having a small crystal grain size at the end of the island pattern, but the position formed by changing the laser energy density changes. When the polysilicon film actually formed according to the present embodiment is used as the active layer of the TFT, it may be designed so that such a portion having a small crystal grain size does not hit the channel formation region.

[Example 2]
In this example, an example in which the amorphous silicon film is crystallized with the structure shown in Embodiment Mode 1 is shown. Note that the laser crystallization performed in this example is merely changing the film formed on the surface of the reflector 304 in Example 1 to a tungsten film and the laser energy density to 369 mJ / cm ² . The detailed description may be referred to the first embodiment.

  FIG. 11 shows a SEM photograph of the polysilicon film crystallized according to this example. As in the first embodiment, FIG. 11 shows a state after secco-etching. Example 1 may be referred to for the conditions for the secco etching.

In the case of performing laser crystallization in the configuration of this embodiment, the effective energy intensity of the primary laser beam (I ₀₎ is 166.1mJ / cm ^2, the effective energy intensity of the second laser beam (I ₀ ') Was 88.6 mJ / cm ² . Therefore, the effective energy intensity ratio (I ₀ ′ / I ₀ ) was 0.53.

  As a result, as shown in FIG. 11, relatively large crystal grains having an average crystal grain size of about 0.6 to 0.7 μm were confirmed in the entire island pattern. In FIG. 11, small crystal grains at the end of the island pattern as seen in FIG. 10 were not noticeable. However, there are some conditions that are noticeable by changing the laser energy density, so it is necessary to optimize the laser energy density. Further, even if there is a portion with a small crystal grain size, there is no problem if it is designed so that it does not hit the channel formation region of the TFT as in the first embodiment.

Example 3
In this example, an example in which the amorphous silicon film is crystallized with the structure shown in Embodiment Mode 1 is shown. Note that the laser crystallization performed in this example is merely changing the film formed on the surface of the reflector 304 in Example 1 to a titanium nitride film and the laser energy density to 384 mJ / cm ^2. The detailed description may be made with reference to the first embodiment.

  FIG. 12 shows an SEM photograph of the polysilicon film crystallized according to this example. As in the first embodiment, FIG. 12 shows a state after secco-etching. Example 1 may be referred to for the conditions for the secco etching.

In the case of performing laser crystallization in the configuration of this embodiment, the effective energy intensity of the primary laser beam (I ₀₎ is 172.8mJ / cm ^2, the effective energy intensity of the second laser beam (I ₀ ') Was 57.6 mJ / cm ² . Therefore, the effective energy intensity ratio (I ₀ ′ / I ₀ ) was 0.33.

  As a result, as shown in FIG. 12, large crystal grains having an average crystal grain size of about 0.8 to 1.0 μm were confirmed in the entire island pattern. These crystal grains have a shape that is long in the horizontal direction toward the paper surface, and it seems that crystallization may have progressed from the horizontal ends of the island pattern. This tendency is slightly confirmed also in FIG.

  In addition, since there are conditions that are noticeable by changing the laser energy density, it is necessary to optimize the laser energy density. Further, even if there is a portion with a small crystal grain size, there is no problem if it is designed so that it does not hit the channel formation region of the TFT as in the first embodiment.

Example 4
In this example, an example in which a polysilicon film serving as an active layer of a TFT is formed by the method of Embodiment 1 or Embodiment 2 will be described. FIG. 13 is used for the description.

  First, a silicon nitride oxide film (not shown) having a thickness of 200 nm is formed on a glass substrate, and an amorphous silicon film (not shown) having a thickness of 50 nm is formed thereon. Next, the amorphous silicon film is patterned to form island patterns 701a and 701b made of an amorphous silicon film. (FIG. 13 (A))

  Next, the island-shaped patterns 701a and 701b are laser crystallized by the method of the first embodiment or the second embodiment. The island-like patterns 702a and 702b made of a polysilicon film obtained by laser crystallization may have regions 703a and 703b with small crystal grains at the ends. Further, the end portions of the island-like patterns 702a and 702b are regions containing a lot of crystal defects and lattice distortions. (Fig. 13B)

  Note that dotted lines indicated by 704a and 704b are traces of the island-shaped patterns 701a and 701b made of an amorphous silicon film, which means that the island-shaped pattern is reduced by about 1 to 15% by laser crystallization. This reduction is thought to occur due to the densification and vaporization of the silicon film, but the details are not clear.

  Next, the island-like patterns 702a and 702b made of a polysilicon film are patterned again to form active layers 705a and 705b. Note that dotted lines indicated by 706a and 706b are traces of regions 703a and 703b having small crystal grains. (Fig. 13 (C))

  Next, a gate insulating film made of a silicon nitride oxide film having a thickness of 80 nm is formed so as to cover the active layers 705a and 705b, and a gate electrode 707 is formed thereon. The gate electrode 707 is formed with a stacked structure of a tungsten nitride film and a tungsten film, and has a thickness of 300 nm. (Fig. 13D)

  When the gate electrode 707 is formed, an impurity element imparting n-type conductivity is added to form a source region 708a, a drain region 709a, and an LDD region 710. Further, an impurity element imparting p-type conductivity is selectively performed, so that a source region 708b and a drain region 709b are formed. At the same time, channel formation regions 711a and 711b (regions in which no impurity element is added in the active layer) are formed.

  Next, an interlayer insulating film (not shown) made of a silicon oxide film is formed to a thickness of 1 μm, and then contact holes are opened to form source wirings 712a and 712b and a drain wiring 713. These wirings may be formed of a low resistance conductive film mainly composed of an aluminum film. (Fig. 13 (E))

  Through the above steps, a CMOS circuit 716 in which an n-channel TFT 714 and a p-channel TFT 715 having a structure as shown in FIG.

  Note that this embodiment is an embodiment in which the present invention is implemented when forming an active layer of a TFT, and is not necessarily limited to this manufacturing process. The present invention can be used in any known TFT manufacturing process. However, the case where a light shielding film or the like is provided under the active layer, that is, the case where it is impossible to perform laser annealing simultaneously on the front surface and the back surface of the amorphous semiconductor film is excluded.

  In this embodiment, an example in which a CMOS circuit is formed is shown, but a pixel TFT provided in a pixel portion of an active matrix image display device can be easily manufactured by using a known technique.

Example 5
In the fourth embodiment, an example in which the present invention is implemented for forming an active layer of a TFT has been described. However, the present invention can be implemented for all semiconductor devices using a TFT. That is, an active matrix liquid crystal display, an active matrix EL (electroluminescence) display, and an active matrix EC (electrochromic) display may be used.

  Furthermore, the present invention can be implemented when forming an SRAM load transistor used in an IC or LSI, and the present invention is also effective when forming a TFT with a three-dimensional structure on an IC or LSI. .

Example 6
In this embodiment, the case where the structure shown in FIGS. 14A and 14B is irradiated with laser light under the conditions shown in Embodiment 1 will be described.

  In the structure of FIG. 14A, reference numeral 801 denotes a 1.1 mm thick quartz substrate, 802 denotes a 200 nm thick silicon nitride oxide film, and 803 denotes a 55 nm thick amorphous silicon film. That is, normal laser crystallization was performed in the structure of FIG.

  In the structure of FIG. 14B, 804 is a reflector whose surface (reflection surface) is a tantalum nitride film, 805 is a 1.1 mm thick quartz substrate, 806 is a 200 nm thick silicon nitride oxide film, and 807 is 55 nm. It is a thick amorphous silicon film. That is, in the structure of FIG. 14B, laser crystallization was performed by implementing the present invention.

  15A and 15B show TEM (Transmission Electron Microscopy) photographs of the resulting polysilicon film. 15A is a TEM photograph of a polysilicon film obtained by crystallizing the amorphous silicon film 803 with the structure of FIG. 14A, and FIG. 15B is an amorphous structure with the structure of FIG. 4 is a TEM photograph of a polysilicon film obtained by crystallizing a silicon film 807.

  Comparing FIG. 15A and FIG. 15B, it can be confirmed that the polysilicon film of FIG. 15B embodying the present invention clearly has a larger crystal grain size. As described above, it was confirmed from a TEM photograph that the average crystal grain size of the crystalline semiconductor film can be increased by implementing the present invention.

Example 7
According to the experiments conducted by the present applicant, the average energy intensity ratio (I ₀ ′ / I ₀ ) is particularly average when the relationship 0 <I ₀ ′ / I ₀ <1 or 1 <I ₀ ′ / I ₀ is satisfied. There was a condition where the crystal grain size was significantly increased.

  In this embodiment, an experiment will be described in which the substrate (all 1.1 mm thick) or the reflector (strictly speaking, the reflecting surface of the reflector) is changed in various ways in the structure shown in FIG. First, Table 1 shows the substrate and reflector in the samples (A) to (B) used in the experiment and the effective energy intensity ratio at that time.

  In Table 1, # 1737 is a product name of a glass substrate manufactured by Corning, and AN100 is a product name of a glass substrate manufactured by Asahi Glass.

  Thus, the sample obtained in the effective energy intensity ratio range of 0.07 to 1.0 is irradiated with XeCl excimer laser light under the same conditions as in Examples 1 to 3, and the resulting polysilicon is obtained. The film was observed with a SEM photograph.

As a result, when the effective energy intensity ratio is 0.29, 0.33, 0.53 or 0.67, it is confirmed that the average crystal grain size is about 1 μm, and the effective energy intensity ratio is 1.0, 0.16. , 0.11, 0.07, it was confirmed that the average crystal grain size was about 0.3 μm. That is, it is considered that the average crystal grain size significantly increases under the condition that the effective energy intensity differs between the primary laser beam and the secondary laser beam by 20% or more. Therefore, the above results show that there is an optimum crystallization condition when the effective energy intensity ratio is I ₀ ′ / I ₀ = 0.2 to 0.9 (preferably 0.3 to 0.7). It seems to suggest.

Example 8
In this example, an example in which an optical system having a structure different from that in Embodiment 2 is used will be described with reference to FIG. Specifically, a configuration example in which the length of the linear laser beam in the longitudinal direction or the width direction is made variable is shown.

  When the optical system 10 shown in this embodiment is used, a semiconductor film that requires a higher energy for crystallization, a semiconductor that can be crystallized with a relatively low energy by reducing the length of the linear laser beam in the longitudinal direction. The length of the linear laser beam in the longitudinal direction can be set long for the film. This maximizes energy efficiency at any time. Further, by making the length of the linear laser light in the width direction variable, the length in the width direction most suitable for crystallization of the semiconductor film can be determined.

  The optical system shown in FIG. 16 differs from the optical system shown in FIG. 5 in that the optical system shown in FIG. 16 is similar to the cylindrical array lens 502 that plays the role of dividing the laser light in the width direction. The cylindrical array lens 11 that plays a role, and the cylindrical array lens 12 that plays a similar role in addition to the cylindrical array lens 503 that plays a role of dividing in the longitudinal direction are used.

  In the present embodiment, the method for changing the cross-sectional shape of the linear laser beam is the same in both the longitudinal direction and the width direction, and therefore only two cylindrical array lenses that play a role in the longitudinal direction will be described here. .

  The individual laser beams divided in the longitudinal direction of the laser beam by the cylindrical array lens 503 are incident on the corresponding individual cylindrical lenses forming the cylindrical array lens 12. Specifically, if the cylindrical array lens 503 is divided into seven, the cylindrical array lens 12 is also divided into seven. The cylindrical array lens 503 and the cylindrical array lens 12 may have the same shape, or only the curvature radius of the cylindrical lens may be different.

  At this time, the variable region of the length of the laser beam can be determined by the combination of the focal lengths. That is, the length of the linear laser beam in the longitudinal direction can be controlled by changing the distance between the cylindrical array lens 503 and the cylindrical array lens 12.

  In addition, the distance between the cylindrical array lens 503 and the cylindrical array lens 12 is preferably shorter than twice the focal length of the cylindrical array lens 503. As a result, the laser light divided by the cylindrical array lens 503 can be incident on the individual cylindrical lenses forming the cylindrical array lens 12 in a one-to-one correspondence.

  In this embodiment, a variable transmittance half mirror is used as the half mirror 13. Here, the configuration of the variable transmittance half mirror will be described with reference to FIG. First, an example of the variable transmittance half mirror shown in FIG.

  The laser beam 902 incident from the left side of the paper is separated into a laser beam 903 and a laser beam 904 by a variable transmittance half mirror 901. Further, the variable transmittance half mirror 901 has a structure in which regions 905 to 908 having different transmittances are provided.

  By moving the variable transmittance half mirror 901 in the direction of the arrow 909 parallel to the variable transmittance half mirror 901 in the figure, the energy intensity of the transmitted laser beam 903 and the reflected laser beam 904 is different. can do. In FIG. 17A, four regions 905 to 908 having different transmittances are shown, but any number of these regions may be used as long as there are two or more.

  Next, another example is shown in FIG. The laser beam 912 incident from the left side of the paper is separated into a laser beam 913 and a laser beam 914 by a transmittance variable half mirror 911. The variable transmittance half mirror 911 is further divided into a plurality of regions as compared with the variable transmittance half mirror 901 of FIG. 17A, and has a structure in which the transmittance changes finely and stepwise in each region. ing.

  Although such a variable transmittance half mirror is commercially available, even if the regions having different transmittances are provided in steps, the laser beam 913 transmitted through is moved by moving in the direction of the arrow 915. The energy intensity of the reflected laser beam 914 can be different.

  By using the optical system described above, it is possible to adjust the energy intensity of the laser light finally irradiated to the semiconductor film. This configuration can also be used for the first embodiment.

Example 9
In this embodiment, an example in which the effective energy intensity ratio is obtained in consideration of the influence of multiple reflection on the reflecting surface of the reflector in the seventh embodiment will be described. The samples (A) to (H) used in the experiment are the same as those in Example 7. In the case of this example, the effective energy intensity (I ₀ ′) of the secondary laser beam is I ₀ ′ = I _a T _sub R _mirror T _sub (1-R _SiON-Si ) / 1-R _{SiON- Si} T _sub R _mirror T _sub

Where T _sub is the transmittance of the substrate, R _mirror is the reflectance at the surface of the reflector, and R _SiON-Si is the reflectance when entering the amorphous silicon film from the SiON film. The reflectivity when entering the SiON film from the air, the transmittance of the SiON film, the reflectivity when entering the substrate from the SiON film, and the reflectivity when entering the SiON film from the substrate are experimental values. Was not included in the calculation.

  Table 2 shows data calculated from the above formula. The data shown in Table 2 is obtained by modifying the data in Table 1 in consideration of the influence of multiple reflection.

Based on the data shown in Table 2, the optimum crystallization conditions described in Example 7, that is, the effective energy intensity ratio is I ₀ '/ I ₀ = 0.2 to 0.9 (preferably 0.3 to The condition satisfying 0.7) was not changed.

Example 10
In this embodiment, the effect of the present invention will be described based on experimental results. In this example, the crystallinity was evaluated in five stages. In the present specification, the crystal state is distinguished and evaluated as follows.

Crystalline state (0): State in which the film has disappeared due to ablation.
Crystalline state (1): A microcrystalline state in which fine crystal grains are observed as shown in FIG.
Crystal state (2): As shown in FIG. 18B, a crystal state in which crystal grains having an average crystal grain size of about 300 to 450 nm are observed.
Crystal state (3): As shown in FIG. 19A, a crystal state in which relatively large crystal grains having an average crystal grain size of about 600 to 800 nm are observed.
Crystal state (4): As shown in FIG. 19B, a crystal state in which very large crystal grains having a major axis exceeding about 3 μm are observed. In this embodiment, the crystal grains in this state are called crystal grains formed by SLG (Super Lateral Growth).

Based on the above evaluation, the relationship between the laser crystallization conditions and the crystal state was examined. The data shown in FIG. 20 is a result of comparison with the irradiation energy (corresponding to the energy intensity I _a of the laser beam just before reaching the amorphous silicon film) and a single irradiation and dual irradiating relationship crystalline state. In addition, single irradiation is a case where the laser beam is irradiated only on the front surface, and dual irradiation indicates a case where the laser beam is irradiated on the front surface and the back surface.

As can be seen from FIG. 20, a film with a good crystal state can be obtained with a lower irradiation energy in the dual irradiation. That is, in the case of single irradiation, an irradiation energy of about 510 mJ / cm ² is required to cause SLG, but in the case of dual irradiation, an irradiation energy of about 440 to 460 mJ / cm ² is sufficient. This indicates that a semiconductor film having higher crystallinity can be obtained with lower irradiation energy in the dual irradiation used in the present invention than in the conventional single irradiation.

  Experimentally, it has been found that the higher the irradiation energy, the higher the effective energy of the primary laser light, and the surface roughness of the formed crystalline semiconductor film increases. This suggests that the dual irradiation can reduce the damage to the film surface in obtaining a crystal formed by SLG.

  Next, an experimental result in which the effective energy intensity ratio is changed by changing the reflectance of the reflector in the case of dual irradiation will be shown. FIG. 21A shows the relationship between irradiation energy and crystal state, and FIG. 21B shows the relationship between effective incident energy and crystal state.

  As shown in FIG. 21A, the higher the reflectivity of the reflector (the higher the effective energy intensity of the secondary laser light), the better the crystal state with the same irradiation energy. This is presumably because, in the case of the same irradiation energy, the effective irradiation energy is higher in dual irradiation. The effective incident energy is the total effective energy incident on the amorphous semiconductor film, and corresponds to the sum of the effective energy intensity and the secondary effective energy intensity of the primary laser beam.

  Therefore, the relationship between the effective incident energy and the crystal state was investigated with the same irradiation energy fixed. Then, as shown in FIG. 21B, the higher the reflectivity, the more the effective incident energy necessary to obtain a crystal formed by SLG (crystalline state 4) is shifted to a higher energy side. In other words, the lower the reflectance of the reflector, the easier it is to obtain crystal grains formed by SLG with less effective incident energy, that is, crystallization with less energy loss is possible.

  Further, as shown in FIG. 21B, it is confirmed that when the reflectance of the reflector is lowered, the effective incident energy reaching the SLG is also lowered, but no SLG is generated when the reflectance is zero. Yes. Therefore, it is considered that there is an optimum value for the reflectance of the reflector when SLG is generated.

Effect of the invention

  As shown in the present invention, when laser-crystallizing an amorphous semiconductor film, laser light is irradiated simultaneously on the front surface and the back surface of the amorphous semiconductor film, and the effective energy intensity irradiated on the back surface side By making the effective energy intensity irradiated to the surface side different, it becomes possible to obtain a crystalline semiconductor film having a larger average crystal grain size than the conventional one.

  By obtaining a crystalline semiconductor film having a large crystal grain size, the performance of a semiconductor device typified by a TFT or an active matrix display device formed of TFTs can be greatly improved.

    The figure which shows the structure of a laser apparatus.     The figure which shows the structure of the optical system of a laser apparatus.     The figure which shows the method of laser annealing.     The figure which shows the structure of a laser apparatus.     The figure which shows the structure of the optical system of a laser apparatus.     The figure for demonstrating a primary laser beam and a secondary laser beam.     The SEM photograph which shows the mode of the crystal grain of a polysilicon film.     The SEM photograph which shows the mode of the crystal grain of a polysilicon film.     The SEM photograph which shows the mode of the crystal grain of a polysilicon film.   The SEM photograph which shows the mode of the crystal grain of a polysilicon film.   The SEM photograph which shows the mode of the crystal grain of a polysilicon film.   The SEM photograph which shows the mode of the crystal grain of a polysilicon film.   10A and 10B illustrate a manufacturing process of a CMOS circuit using a TFT.   The figure which shows a sample structure.   TEM photograph showing the appearance of crystal grains in the polysilicon film   The figure which shows the structure of the optical system of a laser apparatus.   The figure explaining a transmissivity half mirror.   The SEM photograph which shows the crystalline state of a polysilicon film.   The SEM photograph which shows the crystalline state of a polysilicon film.   The figure which shows the relationship between irradiation energy and a crystal state.   The figure which shows the relationship between irradiation energy or effective incident energy, and a crystal state.

Claims (1)

Forming an amorphous or microcrystalline semiconductor film on a light-transmitting substrate;
A method for manufacturing a semiconductor device, wherein the semiconductor film is irradiated with laser light to crystallize the semiconductor film,
The crystallization is performed by simultaneously irradiating a first laser beam irradiated on the surface of the semiconductor film and a second laser beam irradiated on the back surface of the semiconductor film,
The second laser light is reflected by a reflector, and after the effective energy intensity is attenuated by passing a gas provided between the reflector and the translucent substrate, the second laser light is irradiated on the back surface. And
There is a relationship of 0 <I ₀ ′ / I ₀ <1 between the effective energy intensity (I ₀ ) of the first laser beam and the effective energy intensity (I ₀ ′) of the second laser beam. Yes,
The first laser beam is a linear laser beam, and the length in the longitudinal direction and the length in the width direction are variable in accordance with the crystallization conditions of the semiconductor film. Method.

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