JPH05291163A - Substrate heat treatment method - Google Patents

Abstract

PURPOSE:To suppress thermal deformation of a substrate when a vertical type heat treatment device is used. CONSTITUTION:Substrates or wafers 1 are heat-treated at least two times in a vertical type heat treatment device wherein the wafers 1 are heat-treated by inserting a boat 3 horizontally holding the substrates or wafers 1 into a reaction tube. During the first preceding heat treatment step, the wafers 1 are heat-treated in the attitude with the heat-treated surface or the surface 1f turned upward while during the second following heat-treatment step, the wafers 1 are heat-treated in the attitude with the surface if turned downward. Through these procedures, the thermal deformation caused during both steps can be offset with each other thereby enabling the net warping amount to be decreased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a thin film element substrate used in an active matrix type liquid crystal display device, a line sensor or the like. More specifically, the present invention relates to a heat treatment method for a substrate using a vertical heat treatment apparatus during a manufacturing process.

[0002]

2. Description of the Related Art A substrate on which thin film elements such as thin film transistors are integrally formed (hereinafter referred to as a thin film element substrate) has been conventionally used as a constituent member of an active matrix type liquid crystal display device, a plasma display, an EL display or a line sensor. Widely used. The thin film element is formed in an integrated manner by applying an LSI manufacturing process using a thin film of polycrystalline silicon or amorphous silicon formed on the surface of the substrate as a semiconductor active layer. The LSI manufacturing process includes high temperature heat treatment such as an impurity diffusion process, a gate insulating film forming process or an annealing process, and the substrate must have excellent heat resistance capable of withstanding the maximum processing temperature during the manufacturing process. .. This maximum processing temperature differs depending on the thin film material and the content of the applied LSI manufacturing process. Even when a glass material is used as the substrate, an appropriate one is selected depending on the maximum processing temperature.
For example, the maximum processing temperature is generally about 500 ° C. when forming an amorphous silicon thin film transistor element.
In this case, barium borosilicate glass (for example, Corning 7059) having a strain point of about 600 ° C. is used.
The strain point is defined as the temperature at which the viscosity of the material becomes about 10 ^13.5 Pa · s and is a measure of heat resistance. When a transistor element is formed using a polycrystalline silicon thin film, when CVD is used as a method for forming a gate insulating film,
The maximum processing temperature reaches about 600 ° C. In this case,
Alumina borosilicate glass (eg Corning 1733) having a strain point of about 640 ° C. is used as the substrate material. When thermal oxidation is used instead of CVD as the method for forming the gate insulating film, the substrate processing temperature reaches about 1000 ° C. At this time, quartz glass having a strain point of about 1060 ° C. is used as a substrate material. In addition to this, various kinds of glass materials are used according to the maximum processing temperature in the LSI manufacturing process.

However, even if the temperature of the substrate is below the strain point, the viscosity decreases as the temperature rises. In particular, since the glass material is amorphous, its viscosity decreases almost monotonically as the temperature rises. Therefore, even if a heat treatment is applied at a temperature about 100 ° C. lower than the strain point, it has a property of being slightly deformed.

[0004]

Conventionally, a horizontal type heat treatment apparatus has been used for a semiconductor manufacturing process. The horizontal type has a structure in which a plurality of substrates or wafers to be heat treated are placed in a furnace while leaning against a boat, and is characterized in that there is little backflow of air during wafer transfer. However, in recent years, as the diameter of wafers has increased, and with high integration, high-quality processing capability has been demanded. Therefore, from the viewpoint of space saving and uniformity of heat treatment, a vertical type heat treatment apparatus has become the mainstream instead of the horizontal type. This heat treatment apparatus includes, for example, a diffusion / oxidation furnace and an LP-CVD furnace. A vertical heat treatment apparatus is disclosed in, for example, Japanese Patent Laid-Open No. 3-235329.

This vertical type has a structure in which only the end portion of the wafer is supported by a boat and held horizontally. Therefore, the support structure is basically susceptible to thermal deformation due to the weight of the wafer. Therefore, when the vertical heat treatment apparatus is used, there is a problem or a problem that the warp deformation of the wafer occurs due to its own weight even if the heat treatment is performed at a temperature about 100 ° C. lower than the strain point. Particularly, in the thin film element manufacturing process, a plurality of heat treatments such as an impurity diffusion step, a gate insulating film forming step by thermal oxidation, and an annealing step are repeatedly performed. 1
Even if the amount of warpage generated by one heat treatment is small, the deformation is accumulated each time the thermal history is repeated, and finally the amount of warpage becomes large. When the substrate is warped, a trouble such as a wafer transfer trouble or an exposure defect due to a stage suction failure of the exposure device occurs in a subsequent process. In addition, the stress inside the thin film increases as the substrate is deformed, and the electrical characteristics of the thin film element are deteriorated. As the diameter of the wafer increases, the absolute amount of warpage increases, and various defects, failures, or defects frequently occur. At present, as the diameter of thin film element substrates is increasing, in order to avoid thermal deformation such as warpage, it is necessary to use expensive substrate materials with high strain point and excellent heat resistance, such as quartz glass, which increases manufacturing costs. Was invited. Even if quartz glass is used, thermal deformation cannot be avoided when heat treatment is performed near the strain point. Generally, the higher the heat treatment temperature, the more functionally stable thin film device can be obtained. However, in reality, there is a problem that the processing temperature cannot be set high from the viewpoint of suppressing thermal deformation.

[0006]

In view of the above-mentioned problems and problems of the conventional technique, the present invention suppresses thermal deformation of a wafer even when heat treatment is performed near the strain point of a substrate using a vertical heat treatment apparatus. It is an object of the present invention to provide a substrate heat treatment method that can be performed. Another object of the present invention is to provide a substrate heat treatment method that suppresses thermal deformation and enables high-yield production even when using an inexpensive substrate having ordinary heat resistance. In order to achieve such an object, in a substrate heat treatment method in which a boat holding a substrate horizontally at a predetermined pitch is inserted into a reaction tube to perform heat treatment, the substrate is heat-treated at least twice or more in a vertical heat treatment apparatus. Measures were taken to perform a first heat treatment step of treating the substrate with the heat treatment surface facing upward and a second heat treatment step of treating the substrate with the heat treatment surface facing downward.

[0007]

According to the present invention, when the vertical heat treatment apparatus is used to perform the heat treatment a plurality of times, the heat treatment of the substrate is set in the boat in a posture with the heat treatment surface, that is, the front side facing upward, and the heat treatment of the subsequent heat treatment is performed. Then, I set it on the boat with the back side of the board facing up. Therefore, the warp deformation direction of the substrate due to its own weight is opposite between the case of the first heat treatment and the case of the latter heat treatment, the mutual warp is offset or compensated, and the heat deformation is not accumulated. That is, in the present invention, even if the heat treatment is repeated a plurality of times, the warp deformation is not accumulated, and large heat deformation can be avoided. As a result, it is not necessary to use an expensive substrate material having a high strain point, and it is possible to effectively prevent warp deformation even when the diameter of the wafer is increased, so that it is possible to produce a high yield thin film element substrate. ..

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a process diagram for explaining the basic concept of the substrate heat treatment method according to the present invention. The substrate or wafer 1 to be heat-treated has a diameter of 150 mm, for example.
It consists of a large diameter round quartz glass plate. The wafer 1 has a heat treatment surface on which thin film elements are formed or a back surface 1r opposite to the front surface 1f. In performing the heat treatment, the individual wafers 1 are horizontally set on the boat 3 in the vertical arrangement in which the grooves 2 are formed.

In the preceding first heat treatment step, the front surface 1f of each wafer 1 is set on the boat 3 with its posture facing upward. On the other hand, in the subsequent second heat treatment step, the individual wafers 1
Is transported in the inverted state and is set on the boat 3 with the back surface 1r facing upward. Due to the thermal deformation generated in the first heat treatment step, the back surface 1r side of the wafer 1 has a convex shape. When the second heat treatment step is performed with the convex back surface facing upward, the convex shape is returned to the flat shape by the action of gravity.

FIG. 2 is an external perspective view showing an example of a boat used for implementing the present invention. The boat 3 includes a pair of discs 4 and 5 vertically spaced apart from each other with a predetermined gap, and a plurality of posts (4 in this example) 6 connected between the discs.
It consists of and. As the material, for example, quartz glass having excellent heat resistance and high purity is used. Grooves are formed on the side wall of each post 6 at a predetermined pitch. The edges of the individual wafers are engaged with and horizontally supported by the grooves of the same level. When reversing the individual wafers held in the boat having such a structure, the boat may be simply inverted.

FIG. 3 shows a plan view of the boat shown in FIG. The plurality of posts 6 are arranged along the circumferential direction of the disc 4 with a predetermined interval. However, the interval between the posts that are adjacent to each other on one side is enlarged, so that the wafer 1 can be mounted. The ends of the wafer 1 are point-supported by the grooves 2 formed in the individual posts 6.

FIG. 4 shows an example of a vertical heat treatment apparatus used for carrying out the present invention. A gas supply pipe 8 is provided in a bottomed cylindrical quartz reaction tube 7 that opens at the lower end and extends upward.
The individual wafers 1 are horizontally held on the boat 3 in a state of being separated from each other at a predetermined pitch, and are conveyed through the heat retaining cylinder 9. When the boat 3 is completely inserted inside, the opening of the reaction tube 7 is
It is adapted to be sealed by a cap 10 fixed to the heat retaining cylinder 9. A heater 11 is provided around the reaction tube 7 to maintain the ambient temperature at a high temperature. For example, when performing thermal oxidation treatment using this heat treatment apparatus, the process gas is introduced from the gas supply pipe 8 while maintaining the inside of the reaction tube 7 at a high temperature atmosphere of, for example, 1000 ° C. and maintained for a predetermined reaction time. ..

Referring back to FIG. 1, a specific example of the substrate heat treatment method according to the present invention will be described. In this example, a transistor was formed on a polycrystalline silicon thin film formed on the surface of a round quartz glass wafer having a diameter of 150 mm. In particular, an ONO laminated structure of silicon dioxide / silicon nitride / silicon dioxide having excellent pressure resistance was formed as a gate insulating film of a transistor. In this step, the first heat treatment and the second heat treatment were performed. The first heat treatment was performed with the surface 1f of the wafer 1 facing upward, and the surface of the polycrystalline silicon thin film was thermally oxidized to form the first layer of the three-layer structure gate insulating film. This thermal oxidation treatment was performed at a temperature of 1000 ° C. for 60 minutes. Thereafter, silicon nitride was deposited and laminated by LP-CVD to form a second layer of the gate insulating film.

Subsequently, a second heat treatment was performed to thermally oxidize the surface of silicon nitride to form a third layer of the gate insulating film. Second
The heat treatment was performed with the back surface 1r of the wafer 1 facing upward, and the wafer 1 was heated at 1000 ° C. for 60 minutes. All other processes such as impurity doping and gate electrode formation are 1000
A thin film transistor of polycrystalline silicon was formed on the substrate by performing the treatment at a processing temperature of ℃ or less.

In order to evaluate the present invention, the amount of warpage of the thin film element substrate manufactured by the above method was measured. Figure 5
As shown in, the measurement was performed by the stylus method, and the difference in height when the surface of the wafer 1 was scanned over a span of 130 mm was detected as the amount of warpage. According to this measurement result, the wafer warp amount was about 12 μm before the first heat treatment step. After the first heat treatment step, the warp amount increased to about 40 μm. Further, after the second heat treatment step, the amount of warp was reduced to about 15 μm, and the wafer returned to the initial state. In this case, there was no transportation trouble during the process, and the Vth characteristics of the thin film transistor thus produced were within a range of about 1 V within the same wafer surface, and no defective electrical characteristics of the element occurred.

On the other hand, as a comparative example, a thin film element substrate was prepared by performing the first heat treatment step and the second heat treatment step without inverting the wafer. That is, in this comparative example, in both the first heat treatment step and the second heat treatment step, the wafer 1 is set on the boat 3 with the surface 1f facing upward. Other processing conditions and processing conditions are the same as those of the specific example of the present invention described above. When the degree of thermal deformation of the wafer was measured, the amount of warpage was about 12 μm before the first heat treatment process, the amount of warpage after the first heat treatment process was increased to about 40 μm, and it was further increased after the second heat treatment process. 8
It increased to 0 μm and the variation was significant depending on the individual wafer. After the second heat treatment step, due to the variation in the amount of warpage of the quartz glass wafer, a transport trouble occurred at a rate of about 1 in 10, and the Vt of the formed thin film transistor was reduced.
As for the h characteristic, a variation of about 3 V occurred within the same wafer surface.

Another specific example of the substrate heat treatment method according to the present invention will be described. In this specific example, the surface of the polycrystalline silicon thin film was thermally oxidized in the first heat treatment step to form a single-layer gate insulating film. This thermal oxidation treatment was performed at 1000 ° C. for 60 minutes. In the first heat treatment step, the front surface 1f of the wafer 1 is set on the boat with its posture facing upward. After that, impurities were injected into the polycrystalline silicon thin film through the formed gate insulating film.

Subsequently, in the second heat treatment step, 1000 ° C.
The implanted impurities were activated by annealing for 30 minutes. In the second heat treatment step, the back surface 1r of the quartz glass wafer 1 was set on the boat 3 with its posture facing upward. 10 except for the gate insulating film forming step and the impurity activating step
Processing was performed at a processing temperature of 00 ° C. or lower to form a polycrystalline silicon thin film transistor having a gate oxide silicon oxide film single layer.

When the amount of thermal deformation of the wafer was measured in the same manner as above, the warpage was about 12 μm before the first heat treatment step. The warp amount after the first heat treatment step increased to about 40 μm. After the second heat treatment step, the warp amount is about 15 μ
The wafer was returned to a substantially initial state after the reduction to m. In this case, there was no transportation trouble during the process, and the Vth characteristics of the thin film transistor thus produced were within 1 V within the same wafer surface, and the device characteristic failure did not occur.

Further, as a comparative example, a polycrystalline silicon thin film transistor was produced by performing the above-mentioned first heat treatment step and second heat treatment step without performing wafer inversion.
Other processing conditions are the same as those of the above-described specific example. When the degree of thermal deformation of the quartz glass wafer was measured, it was about 12 μm before the first heat treatment step. First
After the heat treatment step, the amount of warp increased to about 40 μm, and after the second heat treatment step, it increased to about 80 μm, and the variation was remarkable among individual wafers. After the second heat treatment step, a transport trouble occurs at a rate of about 1 in 10 due to variations in the amount of warpage of the quartz glass wafer, and the Vth characteristics of the created thin film transistors also vary by about 3 V within the same wafer surface. It was

In each of the specific examples described above, when the wafer is turned over between the first heat treatment step and the second heat treatment step, the plane orientation of the substrate may be simultaneously changed by rotation. Thereby, it becomes possible to cancel or compensate the thermal warp deformation depending on the plane posture.

Further, although the quartz glass material is used as the wafer in the above-mentioned specific examples, the present invention is not limited to this, and various kinds of glass plate materials can be used depending on the heat treatment temperature. Furthermore, it is clear that not only the glass material but also the processing of other insulating substrates such as ceramics and metal substrates is effective in suppressing the warp due to the weight of the substrate when the vertical heat treatment apparatus is used. is there.

Further, although the thin film transistor element made of polycrystalline silicon is formed in the above-mentioned embodiment, the present invention is not limited to this, and amorphous silicon or CdS is used.
It is also useful when forming a semiconductor thin film element such as e, an MIM element, or a thin film element used in a plasma display, an EL display, a line sensor, or the like.

[0024]

As described above, according to the present invention, when a vertical heat treatment apparatus for supporting a substrate and holding it horizontally is used, the front side of the substrate is placed upward. Both the heat treatment step in which the substrate is oriented and the heat treatment step in which the backside of the substrate is oriented upward are performed. For this reason, the warp generation direction due to the weight of the substrate is reversed between the both steps, and the mutual thermal deformations are canceled or compensated, so that the accumulation of the warp can be prevented. According to the present invention, the warp is not accumulated even if the heat treatment is repeatedly performed in each step, and the substrate can be maintained close to the initial flat shape. With this method, it is not necessary to use an expensive substrate material having a high strain point, and since warpage can be suppressed, it is possible to produce a thin film element substrate with a high yield.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the basic concept of a substrate heat treatment method according to the present invention.

FIG. 2 is a perspective view showing a boat in a vertical arrangement used for implementing the present invention.

FIG. 3 is also a plan view of the boat.

FIG. 4 is a schematic view showing an example of a vertical heat treatment apparatus used for carrying out the present invention.

FIG. 5 is a principle diagram of measuring the amount of warpage of a substrate.

[Explanation of symbols]

 1 Wafer 1f Wafer front surface 1r Wafer back surface 2 Groove 3 Boat 6 Post 7 Reaction tube 8 Gas introduction tube 11 Heater

Claims (1)

[Claims] 1. A substrate heat treatment method for heat treating a substrate at least twice in a vertical heat treatment apparatus in which a boat holding a substrate horizontally at a predetermined pitch is inserted into a reaction tube for heat treatment. A substrate heat treatment method comprising: a first heat treatment step of treating the heat treatment surface facing upward; and a second heat treatment step of treating the heat treatment surface facing downward.