JPH03280420A - Method for manufacturing semiconductor thin film - Google Patents

Abstract

PURPOSE:To make it possible to form a thin film, having large crystal diameter and excellent crystallinity, on a substrate having a large area by a method wherein a silicon thin film layer is formed on a glass substrate, and after the paste, which is formed by dispersing tin fine particles into an organic solvent, has been applied on the silicon thin film layer, the glass substrate is heated up to 232 deg.C or higher, and then the substrate is slowly cooled down. CONSTITUTION:The surface of a glass substrate having smooth surface is cleaned, and a silicon thin film layer 2 is formed thereon. Then the paste, formed by dispersing tin fine particles, is applied on the layer 2, and a tin-coated layer 3 is formed in matrix form. Then, the glass substrate 1 is heat-treated at 232 deg.C or higher, and a matrix-shaped melt layer 4 is formed. Subsequently, the above material is slowly cooled down, and a polysilicon thin film layer 5 is formed in the silicon thin film 2 in matrix form. The above-mentioned polysilicon thin film layer 5 is grown using the crystal, deposited on the surface of the melt layer 4, as a nucleus, the layer 5 has large crystal grain diameter and also has excellent crystallinity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a semiconductor thin film, and more particularly to a method for forming a polysilicon crystal thin film on a large-area glass substrate.

[Prior Art and Its Issues] There are various methods for driving display elements such as liquid crystal displays, and among them, the matrix method has been attracting attention in recent years because it allows for high image quality and large display capacity.

In this method, a semiconductor thin film is formed on a transparent glass substrate, and switching elements such as thin film diodes and thin film transistors are arranged in a matrix in this semiconductor thin film to create a substrate. It directly drives cells.

FIG. 9 shows a thin film transistor 1 as a switching element.
This figure shows an equivalent circuit of a matrix-driven liquid crystal display using 0. In FIG. 9, numerals 11, . . . are scanning lines, numerals 12, . . . are signal lines, and numerals 13, . . . are liquid crystal cells. A thin film transistor 10 as a switching element and a liquid crystal cell 13 connected thereto are respectively disposed in a portion divided by each scanning line +1 and signal line 12, thereby forming one pixel of a liquid crystal display. .

The equivalent circuit of such a liquid crystal display is formed in a semiconductor thin film formed on a transparent glass substrate. As a material for this semiconductor thin film, a hydrogenated amorphous silicon thin film produced by a plasma CVD method is mainly used. According to the plasma CVD method, this is a large substrate for a liquid crystal display with a diagonal size of about several inches, several hundred scanning lines 11 and signal lines 12, and a total number of pixels of about several dozen. This is because an amorphous silicon thin film with a large area can be formed at a low temperature below the softening point of glass.

By the way, in recent years, the demand for large-screen displays has been increasing, and it is now possible to manufacture large-screen liquid crystal displays in which the scanning line If and the signal line 12 are each more than 1,000 gi, and the total number of pixels is several hundred and five or more. To achieve this, it is necessary to manufacture a thin film transistor IO with high switching speed in a semiconductor thin film with high carrier mobility.

However, since the hydrogenated amorphous silicon thin film has a small carrier mobility of at most Icm'/vs, there is a limit to the improvement in switching speed. Therefore, it has been proposed to use a polynolycon thin film with higher carrier mobility.

This polysilicon thin film is produced using the LPCVD method (Low Pre
ssure Che+n1cal Vapor Dep
It can be formed by a laser annealing method or a laser annealing method.

The LPCVD method is a method of directly forming a polysilicon thin film on a heated glass substrate using silane gas as a raw material. However, since the thin film forming temperature cannot be made higher than the softening point of glass, the crystal grains of the polysilicon thin film cannot be sufficiently grown using this LPCVD method. Since the carrier mobility of a semiconductor thin film depends on the size of crystal grains and its crystallinity, the carrier mobility of a polysilicon thin film produced by LPCVD is also similar to that of an amorphous thin film.
The limit was about double that.

On the other hand, the laser annealing method is a method in which a thin semiconductor film formed in advance on a glass substrate is irradiated with laser light to melt and recrystallize it, so it is possible to sufficiently grow L) crystal grains with good crystallinity. . Therefore, carrier mobility is reduced to 100
cm'/vs or higher, and an element with a switching speed sufficient for driving a liquid crystal display with several million pixels can be formed. However, this laser annealing method irradiates laser light corresponding to each pixel, so even if the processing time is 1 second for each pixel, it takes a huge amount of time to process a substrate with millions of pixels. Since it takes a long time, there is a problem that it is not suitable for mass production.

This invention has been made to solve the above problems, and provides a method for forming a polynolycon thin film with large crystal grain size and good crystallinity in a matrix shape on a large-area glass substrate with high throughput. It is intended to.

Means for Solving 5 Problems] The method for manufacturing a semiconductor thin film according to claim 1 of the present invention includes forming a Noricon thin film layer on a glass substrate, and applying a paste made by dispersing tin fine particles in an organic solvent to the silicon thin film. The solution is to heat the glass substrate to 232° C. or higher and then slowly cool it after coating the layer. The solution was to apply a dispersed paste on a silicon thin film layer in a matrix.

1 Effect] When a paste made by dispersing tin in an organic solvent is applied onto a silicon thin film and then heated, a melt layer of a silicon-tin binary alloy is formed in the area where the paste is applied. When this is then slowly cooled, it is cooled from the melt layer side, which has a higher thermal conductivity than the glass substrate, so that Noricon crystals can be grown from the surface of the melt layer toward the glass substrate side.

The present invention will be explained in detail below.

The method for manufacturing a semiconductor thin film of the present invention includes: (1) a substrate forming step of forming a Noricon thin film on a glass substrate, (2) a coating step of applying a paste on the Noricon thin film, and (2) heating the substrate on which the paste is applied. This process consists of a heating process and (2) a cooling process in which the heated substrate is slowly cooled.

The steps will be explained below in order.

1 to 5 show the manufacturing method of the present invention in the order of steps.

■Substrate forming process First, as shown in Figure 1, a glass substrate l with a smooth surface is used.
Prepare. This glass substrate 1 is sequentially washed with detergent and acid aqueous solutions to clean the surface.

Then, as shown in FIG. 2, on this glass substrate l,
A silicon thin film layer 2 is formed to a thickness of 1 to 2 μm. This silicon thin film layer 2 may be either an amorphous silicon thin film or a polysilicon thin film. Such a silicon thin film layer 2 can be formed by a known method such as a plasma CVD method or an LPCVD method.

(2) Coating process Next, as shown in FIG. 3, a tin coating layer 3 is formed in a matrix on the silicon thin film layer 2.

In order to form such a tin coating layer 3, a paste made by dispersing fine tin particles with a particle size of 1 μm or less in an organic solvent such as polyvinyl alcohol is processed by various methods such as letterpress printing, intaglio printing, and screen printing. A coating method such as a printing method can be suitably used. The printing method and coating conditions can be appropriately selected depending on the controllability of the thickness of the paste, the ability to form patterns corresponding to each matrix, the controllability of the position of the coating area on the large-area substrate, and the like. Further, the pattern of the tin coating layer 3 and its pitch must be set so that adjacent tin coating layers 3.3 do not come into contact with each other, taking into consideration that the tin coating layer 3 will spread to the surrounding area when melted.

In the example shown in FIG. 3, the tin coating layer 3 is coated on the silicon thin film layer 2 in a matrix, but the manufacturing method of the present invention is not limited to this example. The tin coating layer 3 may be formed on the entire surface of the substrate 2.

(2) Heating process Next, the glass substrate l on which the tin coating layer 3 has been formed is subjected to a heat treatment. In this heating step, the Noricon thin film layer 2 and the tin coating layer 3 are heated, and the silicon-tin binary alloy melt layer 4 is heated.
It is intended to form a This step can be carried out in succession with the cooling step (1) to be described later, for example, in an electric furnace maintained in an inert atmosphere such as nitrogen.

An example of heating-cooling temperature conditions when an electric furnace is used is shown in FIG. The temperature is raised slowly at about +10° C./min so as not to cause thermal distortion in the glass substrate 1, and is heated to a temperature T at which the binary alloy of silicon and tin melts. This temperature T can be determined from FIG.

FIG. 7 is a phase diagram of a binary alloy of silicon (Si) and tin (Sn). As is clear from Figure 7, in the tin-rich binary alloy liquid, the solid phase of silicon, that is, crystals, precipitates at 232°C, so the lower limit of temperature increase in this heating step is 232°C or higher, and the upper limit is the softening of the glass. less than a point.

When the glass substrate 1 is held at a temperature higher than this temperature T for several minutes, the silicon thin film 2 and the tin coating layer 3 formed thereon melt, forming a matrix-like melt as shown in FIG. Layer 4 is formed.

Note that the melt layer 4 is the layer on which the tin coating layer 3 is formed];/7
'l M O/7) Xi I+ 1'/2
Cover * Q /7)2. 7 Knee J”F
Since it is formed by simultaneously melting the silicon thin film 2 around 1i3, its area is larger than that of the tin coating layer 3.

■Cooling process Next, the glass substrate 1 on which the melt layer 4 has been formed is slowly cooled.

The temperature is lowered slowly at a rate of about -10° C./min, similar to the temperature increase. Since the thermal conductivity of the Noricon-tin alloy is higher than that of glass, a temperature distribution occurs from the surface of the melt layer 4 toward the glass substrate 1, and silicon crystals first precipitate from the surface of the melt layer 4. . As it is cooled, this silicon crystal grows toward the glass substrate l side, and as shown in FIG.
A polynolycon thin film layer 5 is formed therein in a matrix shape.

The polynolycon thin film layer 5 formed in this manner is grown using the crystals precipitated on the surface of the melt layer 4 as nuclei, and therefore has a large crystal grain size of about 10 μm and has good crystallinity. Become. Therefore, it is possible to obtain a polygonal thin film having carrier mobility comparable to or higher than that of a polygonal thin film formed by laser annealing.

In the manufacturing method of the present invention, silicon crystals are precipitated from the melt layer 4 of the Noricon-tin alloy to form the polynolycono thin film layer 5, but since the silicon crystal grains grow from the surface side of the melt layer 4, The amount of tin mixed on the surface of the thin film layer 5 is less than a few ppm, and the concentration of tin is approximately 10.
It is 0%. Furthermore, since tin is a group IVb element like silicon, it is electrically inactive even if it is mixed into silicon. It does not affect the semiconductor characteristics at all.

At the interface between the polysilicon thin film layer 5 and the glass substrate 1, the tin concentration increases dramatically by 9%, while the noricon concentration increases by several percent.
The following is true. Therefore, if a coplanar type thin film transistor is constructed using this polynolylic thin film layer 5, for example,
Since the surface of the polynolycon thin film layer 5 becomes a channel layer through which carriers travel, a thin film transistor with an ideal structure can be obtained.

Furthermore, in the manufacturing method of the present invention, since the tin coating layer 3 is formed all at once on the silicon thin film layer 2 by the printing method, even if the glass substrate l has a large area, the glass substrate - The time required for printing per sheet can be reduced to just a few minutes, and throughput can be improved. Further, in the heating step and the cooling step, it is possible to simultaneously process a large number of glass substrates l, so that throughput, that is, mass productivity can be further improved.

In particular, in the manufacturing method according to claim 2 of the present invention, since the tin coating layer 3 is formed in a matrix-like fine area, each crystal grain is It is possible to reduce the contact between the crystal grains, and it is possible to grow the crystal grain size to approximately the same size as that of the matrix-like fine regions, and it is possible to significantly improve the switching speed.

[Example] A rectangular glass substrate measuring 00 mm x 1000 mm was prepared and its surface was cleaned by sequentially washing with detergent and acid aqueous solutions. As shown in FIG. 2, an amorphous Noricon thin film was formed on one side of this glass substrate by plasma CVD to a thickness of 1 to 2 μm. At this time, Noranokasu was used as the raw material, and the glass substrate was heated to 250°C. Next, on the amorphous Noricon thin film, a paste made by dispersing fine tin particles with a particle size of 1 μm or less in polyhinyl alcohol is applied by an intaglio printing method to form a tin coating layer with a thickness of 2 to 3 μm. As shown in FIG. 3, it was formed into a matrix shape. The pattern of the tin coating layer is 10 μm x 10 μ + n17) angle t, the pitch is 150 μm in the horizontal direction and 450 μm in the vertical direction, and the number is 6000 in the horizontal direction and 1000 in the vertical direction.
The total number was 6 million. This printing took 3 minutes.

Next, the glass substrate on which the tin coating layer was formed was heated in an electric furnace maintained in a nitrogen atmosphere. The temperature was raised to 300°C over 30 minutes, held at 300°C for several minutes, and then heated for another 3 minutes.
The mixture was cooled to room temperature over 0 minutes. During this heating, the amorphous Noricon and tin melt, the area of the tin coating layer formed in a matrix increases, and the pattern becomes two.
The size was 0 μm×20 μm, approximately 4 times larger than when applied. Note that this heat treatment can be performed in batches on a large number of sheets.
Fifty glass substrates were processed together to improve throughput.

In order to investigate the crystal structure of the polynolycon thin film thus formed, the surface of the polynolycon thin film layer was etched with a dilute hydrofluoric acid aqueous solution and then observed using a differential interference microscope. Normally, the crystal grains of the polynolycono thin film formed by the LPCVD method are small, 1 μm or less, but the crystal grains of the polynolycono thin film obtained by the manufacturing method of the present invention are large, and the IO
J1m or more. In other words, the number of grain boundaries in a matrix pattern of 20 μm x 20 μm is one, which is less than a few. In addition, the composition of this polynolycon thin film was measured along the thickness direction using an ion microanalyzer (IMS).
I looked it up. As a result, the surface of the thin film was almost 100% silicon, and the amount of tin mixed was less than a few pp. Furthermore, when we investigated the variation in the number of grain boundaries between matrices on a glass substrate, it was less than double even between matrices separated by about 1000 mm, indicating that even on a large-area substrate, a uniform thin film was obtained. It could be confirmed.

Next, on each of the polysilicon thin films formed in a matrix in this way, a Cobranar type field effect thin film transistor as shown in FIG. 8 was formed. This was fabricated using a normal thin film transistor manufacturing process. In the 851 cl, reference numeral 6 indicates a source electrode, 7 indicates a drain electrode, 8 indicates a gate electrode, and 9 indicates a gate insulating film.

The channel length and channel width of this thin film transistor are
The diameters were 5 μm and 10 μm, respectively. By making the size of the thin film transistors much smaller than the pattern size of the matrix of polysilicon thin film layers, it was possible to form thin film transistors on each polysilicon thin film layer over the entire surface of the glass substrate.

When the carrier mobility of the polysilicon thin film layer was determined from the current-voltage characteristics of the thin film transistor manufactured in this way, a high value of approximately 120 cva"/vs was obtained. This value is higher than that of the thin film produced by the laser annealing method. As a result, it was possible to realize a high-quality liquid crystal display with a large display capacity and a total of 6 million thin film transistors using the equivalent circuit shown in FIG.

[Effects of the Invention] As explained above, according to the method for producing a semiconductor thin film of the present invention, silicon crystals are grown from a silicon-tin alloy melt, so the crystal grain size is large and the crystallinity is low. A good polysilicon thin film can be formed.

Therefore, a semiconductor thin film having sufficient carrier mobility to drive a large area liquid crystal display can be obtained.

Further, according to the manufacturing method of the present invention, since the substrates are formed all at once by a printing method, a large area of the substrate can be processed in a short time. Furthermore, since a large number of glass substrates can be treated simultaneously in the heat treatment, throughput can be improved and mass productivity can also be improved.

[Brief explanation of drawings]

! 1 to 5 are schematic cross-sectional views showing the glass substrate in each step of the manufacturing method of the present invention, and Figure 6 is a graph showing the temperature conditions of the heating and cooling steps of the manufacturing method of the present invention. FIG. 7 is a state diagram of a binary alloy of Noricon tin, FIG. 8 is a schematic cross-sectional view of a field-effect thin film transistor according to an embodiment of the present invention, and FIG. 9 is an equivalent circuit diagram of a liquid crystal display. l...Glass substrate, 2...Silicon thin film, 3.
...Tin coating layer, 4...Melt layer, 5...Polysilicon thin film layer.

Claims (2)

[Claims] (1) After forming a silicon thin film layer on a glass substrate and applying a paste made by dispersing tin particles in an organic solvent onto the silicon thin film layer, heating the glass substrate to 232°C or higher, and then slowly cooling it. A method for manufacturing a semiconductor thin film characterized by (2) The method for manufacturing a semiconductor thin film according to claim 1, characterized in that a paste made by dispersing tin fine particles in an organic solvent is applied in a matrix on the silicon thin film layer.