JP2010238897A - Semiconductor thin film substrate, and method of manufacturing semiconductor crystal thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent protrusions from being formed on the surface of a crystal thin film during crystallizing an amorphous thin film by melt heating. <P>SOLUTION: During crystallization by the melt heating, a buffer layer 3 buffering the volume change of the amorphous thin film 4 when crystallized by being melt at least its surface side and solidified is provided below the thin film 4. By merely adding one simple step of forming the buffer layer, the height of protrusions of the crystallized thin film can be sufficiently low in crystallization, resulting in a sufficiently thin insulating film. Furthermore, it is not necessary to introduce useless steps of such as polishing and etching when the protrusions are formed, thereby, enabling to achieve effects of such as prevention of the occurrence of contamination, improvement of the yield, and improvement of manufacturing efficiency. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a semiconductor thin film substrate for melt crystallization of an amorphous thin film and a method for producing a semiconductor crystal thin film obtained by the melt crystallization.

  A semiconductor thin film (mainly silicon thin film) deposited on a glass substrate is used as a backplane for forming a TFT (thin film transistor) which is a pixel driving circuit of a thin display panel. In a medium to large screen liquid crystal display panel such as a home TV, the semiconductor thin film is used in an amorphous structure. On the other hand, in the case of medium-to-small liquid crystal panels or organic light emitting diode (OLED) displays, particularly used in mobile phone terminals and digital cameras, the silicon thin film once deposited in an amorphous structure is subjected to a suitable heat treatment for crystallization. (So-called polysilicon TFT). One of the means industrialized as this crystallization process is ELA (melt crystallization of silicon film by excimer laser irradiation) technology (Non-patent Document 1).

When crystallizing a silicon thin film through melting and solidification by this ELA process, excimer laser annealing momentarily gives large energy to the semiconductor thin film, and therefore, the solidification termination is accompanied by a large volume expansion during solidification from the liquid phase to the solid phase. Large protrusions are formed at the portion (generally, the crystal grain boundary) (see FIG. 7). This protrusion acts extremely disadvantageously when a TFT element is formed in a later process.
That is, the protrusion on the surface of the semiconductor thin film is a serious problem because it causes a dielectric breakdown due to a high gate voltage when an element is manufactured as a TFT (however, a top gate type). For this reason, there is a case where a technique for avoiding the thickness of the gate insulating film deposited further on the semiconductor layer is performed.
Furthermore, means for removing the protrusions have been proposed. For example, Patent Document 1 proposes that the surface of a silicon crystal (polysilicon) thin film is mechanically polished and flattened. Alternatively, as a flattening means, chemical etching, electropolishing, flattening by ion implantation (Patent Document 2), or dry etching using an active gas (Patent Document 3) has been proposed.

JP-A-6-163588 JP-A-8-139334 JP-A-9-213630 JP 2000-294793 A

Mat. Res. Soc. Symp. Proc. 71, 435, (1986), T.W. Sameshima

However, increasing the thickness of the gate insulating film as in the above method is not desirable because it directly leads to deterioration of the electrical characteristics (such as switching characteristics) of the TFT.
Further, as described above, a process of mechanically and chemically shaving the surface irregularities after melt crystallization has been proposed or implemented. However, it is obvious that abrasive debris and contamination generated by polishing cause a decrease in TFT characteristics and a deterioration in yield in the photolithography process, which is also not preferable. In addition, problems such as an increase in process load and manufacturing time occur due to planarization.
In other words, if the semiconductor crystal thin film formed by the melting process has no protrusions (irregularities) from the beginning when crystallization is completed, or if the degree of protrusions (irregularities) is small, the above process of removing protrusions (irregularities) is not necessary. Therefore, it can be said that it is a more suitable process because the difficulty is solved. As a method proposed based on such an idea, there is a process of “melt crystallization while suppressing convection in a static magnetic field or inert gas” as in Patent Document 4. However, as a practical problem, it is difficult to uniformly control the distribution of the magnetic field and the flow of the inert gas, and it is difficult to say that the object can be sufficiently achieved.

  The present invention has been made against the background of the above circumstances, and provides a semiconductor thin film substrate and a method for manufacturing a semiconductor thin film that can suppress the height of protrusions (irregularities) generated during melt crystallization of an amorphous thin film. Objective.

That is, the semiconductor thin film substrate of the present invention is a semiconductor thin film substrate in which an amorphous thin film for melt crystallization is formed on the substrate, and at least the surface layer side melts in the lower layer of the amorphous thin film during the melt crystallization. A buffer layer is provided to relieve a volume change when the amorphous thin film is crystallized and solidified.
Further, the method for producing a semiconductor crystal thin film of the present invention includes providing a buffer layer under the amorphous thin film, heating the amorphous thin film and the buffer layer to melt crystallize the amorphous thin film, and perform the melt crystallization. When at least the surface side of the buffer layer is melted and the amorphous thin film is crystallized and solidified, the volume change is relaxed by the buffer layer.

  According to the present invention, the volume change when the amorphous thin film is solidified is alleviated by the buffer layer, and as a result, the formation of protrusions in the crystallized thin film can be significantly suppressed.

The buffer layer is preferably made of a material that expands in volume when melted and shrinks in volume when solidified. Since silicon, which is a representative material of the thin film, usually shrinks in volume when melted and expands in volume when solidified, the change in volume with the buffer layer is offset.
That is, the thin film and the buffer layer have a large effect in mitigating the volume change because the volume change tendency during melting and solidification is reversed.

The buffer layer is preferably one that coagulates simultaneously when the thin film to be melt crystallized solidifies to relieve the volume change of the thin film, and one that has a freezing point close to the thin film that crystallizes (for example, about ± 100 ° C.). . In the case of silicon, the melting point / freezing point of amorphous silicon is about 1400K, and that of crystalline silicon is about 1700K.
Furthermore, the buffer layer preferably has a thermal conductivity equal to or higher than that of the thin film to be melt crystallized. By satisfying these conditions, the amorphous thin film and the buffer layer are melted and solidified at almost the same timing, and the effect of the volume relaxation (cancellation of volume change) becomes remarkable.

The buffer layer preferably has low electrical conductivity in order not to deteriorate the electrical characteristics of the TFT element.
Furthermore, it is desirable that the buffer layer has a higher density (especially compared with the liquid phase) than the semiconductor material to be melted, which makes it difficult to mix with the semiconductor during melting.
Examples of the material of the buffer layer that satisfies the above various conditions include Nb ₂ O ₅ , Ti ₂ S, and La ₂ C ₃ .

In addition, a diffusion suppression layer can be provided below the buffer layer so that impurity components that adversely affect the semiconductor film do not diffuse from the substrate side. A known material can be used for the diffusion suppressing layer.
The buffer layer can also serve as a diffusion suppression layer. When the buffer effect is to be sufficiently obtained, it is desirable that the melt of the buffer layer does not reach the entire layer during melt crystallization of the amorphous thin film, and the lower layer side is maintained in a non-molten state and is in a solid phase. This is because the atomic diffusion rate in the liquid phase is much higher than that of the solid, and it is desirable that the solid phase be maintained in terms of suppression of diffusion.

  According to another aspect of the present invention, there is provided a method for producing a semiconductor crystal thin film, wherein the surface of the amorphous thin film is pressurized when the amorphous thin film is heated for melt crystallization.

When the amorphous thin film is melt-crystallized, the thin film is pressurized to equalize the volume expansion that occurs during solidification and suppress the formation of protrusions. This pressurization may be applied to the manufacturing method provided with the buffer layer.
The pressurization is preferably continued until the crystallized thin film is solidified. This more reliably suppresses the formation of protrusions.
Although the method of this pressurization is not specifically limited as this invention, it is possible to pressurize by pressing a pressing member against the said thin film. The pressing member may be made of a material that transmits heating energy such as a laser, or quartz or the like in the case of a laser, and the amorphous thin film may be melted and crystallized by the heating energy that transmits the pressing member. Thereby, pressurization can be executed at the irradiation position of the heating energy at which the volume change clearly appears, and the formation of protrusions can be more reliably suppressed.

  When the amorphous thin film is melted and crystallized by irradiating a heating energy such as a laser, the irradiation of the heating energy is usually performed relative to the amorphous thin film. When the pressing member is relatively moved along with the movement, it is possible to perform pressurization at the irradiation position of the heating energy over a wide surface. Since it is not desirable that the pressing member slide on the thin film, the pressing member may be moved away from and on the thin film to allow relative movement without sliding. The shape may be configured to allow rotational movement, and the relative movement may be performed by rotational movement.

  As described above, the semiconductor thin film substrate of the present invention is a semiconductor thin film substrate in which an amorphous thin film for melt crystallization is formed on the substrate, and at least a surface layer is formed in the lower layer of the amorphous thin film during the melt crystallization. Since the buffer layer is provided to relieve the volume change when the amorphous thin film is crystallized and solidified by melting on the side, the volume change of the thin film to be melt crystallized is melted when the amorphous thin film is melted and crystallized. This makes it possible to obtain a high-quality semiconductor crystal thin film by suppressing the formation of protrusions at crystal grain boundaries and the like, which is relaxed by the layer.

Further, the method for producing a semiconductor crystal thin film of the present invention includes providing a buffer layer under the amorphous thin film, heating the amorphous thin film and the buffer layer to melt crystallize the amorphous thin film, and perform the melt crystallization. When the amorphous thin film is crystallized and solidified by melting at least the surface layer side of the buffer layer, the buffer layer relaxes the volume change. It is possible to manufacture a simple semiconductor crystal thin film.
In another method of manufacturing a semiconductor crystal thin film according to the present invention, when the amorphous thin film is heated and melt-crystallized, pressure is applied to the surface side of the amorphous thin film. Suppresses the occurrence.

  That is, according to the present invention, only one simple process of forming a buffer layer is added, or a jig such as a pressing member that suppresses the substrate surface is added to the processing apparatus, or both means are applied simultaneously. By simple contrivance, the height of the protrusion can be made sufficiently low at the time of crystallization, and the insulating film can be made sufficiently thin (that is, a high characteristic TFT can be obtained). Alternatively, it is not necessary to introduce a wasteful process (in terms of process yield and tact time) such as polishing and etching of protrusions.

Moreover, the following secondary effects are also obtained with respect to the manufacturing method for pressurizing the thin film. In recent years, a technology called “roll-to-roll display manufacturing process” or “flexible display” is under development as a technological innovation aiming to extend the thinning of displays or improve mass production efficiency. This is a technique for producing a panel using a flexible material such as a plastic resin plate (or sheet) instead of a glass flat plate widely used as a substrate for conventional FPDs. In this manufacturing process, when it is necessary to heat-treat (e.g., crystallize) a semiconductor film to be a TFT, the flexibility of the substrate causes a mismatch with the focal height of the laser, resulting in crystallinity non-uniformity in the panel surface. . In other words, this leads to in-plane non-uniformity of the TFT characteristics, which is not preferable. Processing while suppressing the laser irradiation location with a flat plate or the like as in the present invention not only prevents the above-described protrusion, but also prevents deformation due to the flexibility of the substrate and the accompanying irradiation height deviation.
From the above, it can be said that the present invention has a great effect in simplifying the process and improving the throughput in the TFT panel manufacturing process.

It is a figure which shows before and after crystallization of the semiconductor thin film of one Embodiment of this invention. Similarly, it is a figure which shows the example of a change which provided the diffusion suppression layer. Similarly, it is a figure which shows the example which serves as a diffusion suppression layer with a buffer layer. It is a figure which shows before and after crystallization of the semiconductor thin film of other embodiment using the pressing member of this invention. Similarly, it is a figure which shows before and after crystallization of the semiconductor thin film of the example using another pressing member. Similarly, it is a figure which shows before and after crystallization of the semiconductor thin film of the example using a buffer layer and a pressing member. It is a drawing substitute photograph which shows the TEM image of the conventional crystallized film.

(Embodiment 1)
Below, one Embodiment of this invention is described based on FIG.
A semiconductor thin film substrate 1 used for melt crystallization of an amorphous thin film has a buffer layer 3 formed on a substrate 2 and an amorphous silicon thin film 4 formed thereon.
The buffer layer 3 melts when the amorphous silicon thin film 4 is melted and crystallized, and solidifies in the same manner as the crystallized thin film is solidified.

The behavior when the semiconductor thin film 1 is irradiated with the laser 5 will be described below.
The semiconductor thin film 1 is irradiated with laser to melt the amorphous silicon thin film 4 and the buffer layer 3 simultaneously. The temperature of the melted amorphous silicon thin film 4 and the buffer layer 3 begins to decrease as soon as one shot of the laser pulse ends. At this time, the temperature drop does not occur uniformly at all locations of the area melted by the laser irradiation, so solidification (crystallization) is started from the place where the temperature drop is relatively fast and the time to reach the freezing point is early. The solidification interface proceeds two-dimensionally. During the solidification process, the volume of the silicon film once contracted while melting is rapidly changed to expansion. This wrinkle of volume expansion accumulates and accumulates in the crystal growth direction, and finally collides with another crystal grain growth interface.

  If the film structure of the prior art without the buffer layer 3 is present, this wrinkle escape field exists only in the upper free space, and the location where the crystal growth interfaces collide with each other (crystal grain boundary) rises violently. It solidifies as it is (protrusion occurs).

  However, with the structure of the present invention, the solidification of the silicon layer proceeds, and at the same time, the solidification of the buffer layer 3 proceeds in the same direction in the space immediately below. Then, due to the same mechanism as the solidification interface (crystallization interface) of the silicon film swells, in the portion where the solidification interfaces of the buffer layer 3 collide with each other, the dents 30 are formed in the form where atoms are simultaneously sucked into the solidification interfaces on both sides. Yes (corresponds to “shrinkage” often referred to in cast iron). It can be easily imagined that the formation of the recess 30 produces an effect of pulling the silicon film in contact with the upper portion downward (in the direction of the substrate). That is, the solidification interface between the silicon film and the buffer layer 3 collides at almost the same place in the panel surface, the interface between the silicon films rises, and the interface between the buffer layers 3 tends to be depressed, thereby canceling the overall deformation. be able to. If the volume change amount of both is precisely controlled (mainly optimizing the film thickness), the deformation amount can be made as close to zero as possible. Thereby, the crystallized silicon thin film 4a in which the formation of protrusions is suppressed is obtained.

  FIG. 2 shows a modification in which a diffusion suppression layer 6 is provided between the buffer layer 3 and the substrate 2. In this modified example, when the amorphous silicon thin film 4 is melted and crystallized, diffusion due to impurity components from the substrate 1 is suppressed by the diffusion suppression layer 6 to prevent contamination of the crystallized film. Also in this modified example, the amorphous silicon thin film 4 can be melted and crystallized by suppressing the formation of protrusions by laser irradiation or the like in the same manner as described above.

  Next, FIG. 3 shows an example in which the buffer layer 3 also serves as a diffusion suppression layer. In this example, when the amorphous silicon thin film 4 is irradiated with a laser, the entire layer of the amorphous silicon thin film 4 and the surface layer 3a of the buffer layer 3 are melted simultaneously, and the lower layer 3b of the buffer layer is maintained in an unmelted state. The lower layer 3b suppresses diffusion from the substrate 2 side in a solid state. On the other hand, the surface layer 3a of the buffer layer 3 obtains a crystallized silicon thin film by relaxing the volume change of the silicon film and suppressing the formation of protrusions on the surface of the silicon film in the same manner as described above. As in this example, the non-molten state of the lower layer of the buffer layer can be obtained by appropriately adjusting the thickness of the buffer layer or appropriately adjusting the energy density of the laser.

(Embodiment 2)
Next, another embodiment will be described with reference to FIG. In addition, about the structure similar to the said embodiment, the same code | symbol is attached | subjected and demonstrated.
An amorphous silicon thin film 4 is formed on the substrate 2, and a flat plate-like pressing member 10 made of quartz and having extremely high bottom flatness is placed in contact with the amorphous silicon thin film 4 and pressed against the amorphous silicon thin film 4. Press and apply pressure. Note that the pressing may be performed by further applying pressure through the pressing member 10 or may use only the weight of the pressing member 10.

  The laser 5 is irradiated from above the pressing member 10 and the amorphous silicon thin film 4 is irradiated with the laser 5 transmitted through the pressing member 10. As a result, the amorphous silicon thin film 4 melts in a pressurized state, and during solidification, the generation of surface protrusions is suppressed by this pressurization, and the crystallized silicon thin film 4a is obtained. In other words, since the collision between the solidification interfaces of the semiconductor film is a cause of the generation of the protrusion, the generation of the protrusion is suppressed by filling in advance the space where the protrusion should rise. Since the pressing member 10 is in direct contact with the melted portion, it is a necessary condition that the pressing member 10 is a material that sufficiently transmits energy (laser or the like) input for melting. Since sufficient effects cannot be obtained if there are gas molecules, dust, or the like between the pressing member and the semiconductor film, at least the vicinity of the laser irradiation portion is ultra-clean and has a vacuum (for example, inside a vacuum chamber). Is desirable. The technique of joining flat surfaces in contact with each other in a vacuum is called “vacuum bonding”, “vacuum bonding”, etc. According to this technique, the adhesion between the pressing member of the present invention and the semiconductor film is vacuum. Therefore, it can be understood that the surface flatness of the semiconductor film cannot be improved in an environment where the degree of vacuum is not good (pressure is high).

FIG. 5 shows another modified example, in which the pressing member is made of quartz, has a cylindrical shape, and is changed to the pressing member 11 having high surface smoothness. The laser 5 is incident from the radial direction of the pressing member 11, passes through the pressing member 11, and is irradiated onto the amorphous silicon thin film 4.
In this example, since the laser 5 is moved while rotating the pressing member 11 on the amorphous silicon thin film 4, it is possible to perform melting and heating by laser irradiation while pressing the thin film in a wide area on the film. . Also in this example, the crystallized silicon thin film 4a in which the surface protrusion is suppressed is obtained. Note that the pressing member 10 can be moved relative to the thin film together with the laser by repeatedly separating and contacting the silicon film.

  6A and 6B, the pressing members 10 and 11 are used, and the buffer layer 3 is provided below the amorphous silicon thin film 4 in the semiconductor thin film substrate. In these examples, the buffer layer 3 relaxes the volume at the time of crystallization, and is pressed by the pressing members 10 and 11 when the thin film melts and solidifies. Is more effectively suppressed.

Examples of the present invention will be described below.
After depositing a silicon oxide layer as a diffusion suppressing layer on the glass substrate for display, niobium oxide (Nb ₂ O ₅ ) was formed to a thickness of 50 nm as a buffer layer for suppressing unevenness. A silicon thin film having a thickness of 50 nm was further formed thereon.
The “diffusion suppression layer” and the “concave / convex suppression buffer layer” are formed with different compositions. However, the gist of the present invention may be a form in which one type of film has the functions of both layers.
After the film formation, laser irradiation was performed so that the silicon film and the unevenness suppressing buffer layer were melted simultaneously. When the surface roughness of the silicon film after crystallization was measured and compared with the case where laser melt crystallization was performed by an ordinary method, Ra was remarkably improved from about 50 nm to 5 nm or less.

Excimer laser is irradiated to a structure for a general TFT composed of three layers of glass substrate / SiO ₂ diffusion suppression layer / silicon thin film to melt and crystallize the outermost silicon film. At this time, laser irradiation was performed in advance while bringing a polished quartz plate into contact with the surface of the molten semiconductor portion. Since the laser beam shape is a line beam shape with a width of 0.4 mm and a length of 500 mm, the size of the smooth surface of the quartz plate in contact with the silicon film was set to 0.5 mm × 520 mm. Since most of the energy of the laser is transmitted through the quartz plate, the attenuation of the energy is very small, and there is no need to make a significant change in the laser irradiation conditions.

  By bringing the quartz plate into contact with and detaching from the semiconductor surface in accordance with the laser irradiation period (300 Hz), the problem of rubbing and scratching the semiconductor film surface could be avoided. Further, instead of a flat plate, a cylindrical quartz roller (50 mmφ × 520 mm) was rotated and moved along the semiconductor surface while being transmitted through the cylinder and irradiated with laser. In this case, it is necessary to narrow the line beam width to some extent. In this embodiment, the width is set to 15 μm (thus, throughput is slightly reduced). When the surface roughness of the produced semiconductor crystal film was measured, Ra was remarkably improved from about 50 nm to 8 nm or less.

DESCRIPTION OF SYMBOLS 1 Semiconductor thin film substrate 2 Substrate 3 Buffer layer 4 Amorphous silicon thin film 4a Crystallized silicon thin film 5 Laser 6 Diffusion suppression layer 10 Pushing member 11 Pushing member

Claims (13)

  A semiconductor thin film substrate in which an amorphous thin film to be used for melt crystallization is formed on a substrate, and at the time of the melt crystallization, at least a surface layer side melts and the amorphous thin film is crystallized and solidified in the lower layer of the amorphous thin film A semiconductor thin film substrate, characterized in that a buffer layer is provided to alleviate the volume change.   2. The semiconductor thin film substrate according to claim 1, wherein the buffer layer is made of a material that expands in volume when melted and contracts in volume when solidified.   The semiconductor thin film substrate according to claim 1, wherein a diffusion suppression layer is provided below the buffer layer and above the substrate.   The semiconductor thin film substrate according to claim 1, wherein the buffer layer also serves as a diffusion suppression layer. 5. The semiconductor thin film substrate according to claim 1, wherein the buffer layer is any one of Nb ₂ O ₅ , Ti ₂ S, and La ₂ C ₃ .   A buffer layer is provided below the amorphous thin film, and the amorphous thin film and the buffer layer are heated to melt and crystallize the amorphous thin film, and at the time of the melt crystallization, at least a surface layer side of the buffer layer is melted to form the amorphous When the thin film is crystallized and solidified, the volume change is relaxed by the buffer layer.   7. The method of manufacturing a semiconductor crystal thin film according to claim 6, wherein when the amorphous thin film is crystallized and solidified, the buffer layer is solidified and the volume shrinks, thereby relaxing the volume change due to the crystallization. .   8. The method for producing a semiconductor crystal thin film according to claim 6, wherein, during the crystallization, the lower layer side of the buffer layer maintains a non-molten state to suppress diffusion of impurities from the lower layer of the buffer layer. 9.   The method for producing a semiconductor crystal thin film according to any one of claims 6 to 8, wherein when the amorphous thin film is heated for melt crystallization, pressure is applied to a surface side of the amorphous thin film.   A method for producing a semiconductor crystal thin film, wherein the surface side of the amorphous thin film is pressurized when the amorphous thin film is heated for melt crystallization.   11. The method of manufacturing a semiconductor crystal thin film according to claim 9, wherein the pressurization is continued until the amorphous thin film is crystallized and solidified.   The pressurization is performed by pressing a pressing member that transmits heating energy against the amorphous thin film, and the amorphous thin film is irradiated with heating energy through the pressing member to cause the melt crystallization. The manufacturing method of the semiconductor crystal thin film in any one of Claims 9-11.   The semiconductor crystal thin film according to claim 12, wherein the pressing member is moved relative to the amorphous thin film as the irradiation of the heating energy is moved relative to the amorphous thin film. Production method.