JP3994593B2 - Thin film element transfer method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thin film element transfer method in which a thin film element such as a thin film transistor used as a display pixel of a liquid crystal display device or a constituent element of a liquid crystal driving circuit is attached to another substrate by transfer.
[0002]
[Prior art]
Semiconductor films such as polycrystalline silicon (poly-Si) are widely used for thin film transistors (hereinafter referred to as TFTs in the present specification) and solar cells. In particular, poly-Si TFTs can be made on a transparent and insulating substrate such as a glass substrate while being capable of high mobility, so that light modulation elements such as liquid crystal display devices (LCD) and liquid crystal projectors can be used. Widely used as a component of built-in drivers for liquid crystal drive, it has succeeded in creating a new market.
[0003]
As a method for producing a high-performance TFT on a glass substrate, a manufacturing method called a high-temperature process has already been put into practical use. A process using a high temperature with a maximum process temperature of about 1000 ° C. as a TFT manufacturing method is generally called a high temperature process. The characteristics of the high-temperature process are that a relatively high-quality poly-Si can be produced by solid phase growth of silicon, a high-quality gate insulating film (generally silicon dioxide) and a clean poly-Si and gate by thermal oxidation. That is, the interface of the insulating film can be formed. Due to these characteristics, a high-performance TFT having high mobility and high reliability can be stably manufactured in a high-temperature process. However, in order to use a high temperature process, it is necessary that the substrate on which the TFT is formed can withstand a high temperature heat process of 1000 ° C. or higher. The only transparent substrate that satisfies this condition is currently quartz glass. For this reason, poly-Si TFTs of recent years are all manufactured on a small and expensive quartz glass substrate, and are not suitable for enlargement due to cost problems. In addition, the solid phase growth method requires a heat treatment for a long time of ten and several hours, and there is a problem that productivity is extremely low. In addition, this method has caused a problem that the thermal deformation of the substrate becomes a big problem due to the whole substrate being heated for a long time, and it is impossible to use a substantially inexpensive large glass substrate. This also hinders cost reduction.
[0004]
On the other hand, a technique called a low-temperature process is intended to solve the above-mentioned drawbacks of a high-temperature process and to realize a poly-Si TFT with high mobility. In order to use a relatively inexpensive heat-resistant glass substrate, a poly-Si TFT manufacturing process having a maximum process temperature of approximately 600 ° C. or lower is generally called a low-temperature process. In a low temperature process, a technique for crystallizing a silicon film using a pulse laser having an extremely short oscillation time is widely used. Laser crystallization is a technology that utilizes the property of crystallizing in the process of solidifying instantaneously by irradiating an amorphous silicon film on a glass substrate with high-power pulsed laser light. Recently, a technique for forming a poly-Si film having a large area by scanning an amorphous silicon film on a glass substrate while repeatedly irradiating it with an excimer laser beam has been widely used. In addition, a relatively high quality silicon dioxide (SiO 2) film can be formed by a film forming method using plasma CVD as the gate insulating film, and the prospect for practical use is obtained. With these technologies, poly-Si TFTs can be created on a large glass substrate that is currently several tens of centimeters on a side.
[0005]
Since the TFT manufactured as described above requires a process temperature of about 400 ° C. even in a low temperature process, it is not currently possible to manufacture it on a substrate having low heat resistance such as a resin substrate or a plastic substrate. Is possible. Therefore, if the TFT is manufactured on a substrate such as glass and then these elements can be transferred to an arbitrary substrate, the thin film transistor can be formed on the arbitrary substrate. Various techniques have been reported for transferring elements using lasers. For example, Japanese Journal of Applied Physics Vol.38 (1999) pp.L217 is one of the most recently reported transfer techniques. . As shown in FIG. 3, a laser irradiation 106 is performed on the surface of gallium nitride 102, and the element is separated from the substrate by utilizing thermal excitation decomposition (generation of nitrogen gas 108) of the outermost surface that has absorbed the laser. It is. However, it is necessary to heat the substrate to 600 ° C. with the hot plate 101 in order to promote thermal decomposition, and a transfer destination substrate having low heat resistance cannot be used. In addition, since a special material called gallium nitride must be used, it can be said that it is difficult to manufacture a silicon-based device such as a TFT on the sacrificial layer due to process limitations.
[0006]
[Problems to be solved by the invention]
In view of the above-described problems, the present invention provides a thin film element transfer method for transferring a silicon thin film element such as a TFT onto an arbitrary substrate without increasing the substrate temperature.
[0007]
[Means for Solving the Problems]
The thin film element transfer method of the present invention is a thin film element transfer method in which a thin film element fabricated on a sacrificial layer on a first substrate is transferred onto a second substrate, wherein the sacrifice made of an amorphous silicon film or a germanium film. The sacrificial layer is irradiated with light in a state where a voltage is applied to the layer, and then the first substrate and the second substrate are bonded to each other through an adhesive, and then the sacrificial layer is peeled off to remove the sacrificial layer. The thin film element above is transferred onto the second substrate.
[0008]
In order to solve the above-mentioned problems, a second aspect of the invention is the thin film element transfer method according to the first aspect, wherein after the light irradiation, the first substrate and the second substrate are bonded to transfer the element. It is characterized by performing.
[0009]
In order to solve the above-mentioned problems, a third aspect of the present invention is the thin film element transfer method according to the first or second aspect, wherein the thickness of the sacrificial layer is in the range of 10 to 500 nm.
[0010]
In order to solve the above-mentioned problem, the invention according to claim 4 is the thin film element transfer method according to claim 1, 2, or 3, wherein the voltage applied to the sacrificial layer is such that the current density flowing through the sacrificial layer is 10,000 A. It is characterized in that it is set to be / cm ² or more.
[0011]
In order to solve the above problem, the invention according to claim 5 is the thin film element transfer method according to claim 1, 2, 3 or 4, wherein the sacrificial layer is peeled off in accordance with the resistance value of the sacrificial layer after light irradiation. It is characterized by monitoring the optimum condition by measuring the change and repeating the light irradiation.
[0012]
In order to solve the above-mentioned problem, the invention according to claim 6 is the thin film element transfer method according to claim 1, wherein the electrode for applying a voltage to the sacrificial layer is the first electrode. It is provided so as to be opposed to a position on the substrate across a desired transfer region.
[0013]
In order to solve the above-mentioned problems, according to a seventh aspect of the present invention, in the thin film element transfer method according to the sixth aspect, the light irradiation is performed on a region having opposite electrodes that apply a voltage to the sacrificial layer at both ends. It is characterized by.
[0015]
In order to solve the above-mentioned problems, the invention according to claim 8 is the thin film element transfer method according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the light irradiation is performed by an excimer laser. And
[0016]
In order to solve the above problems, the invention according to claim 9 is the thin film element transfer method according to claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the thin film element is a thin film transistor. And
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example of an embodiment according to the present invention will be described in detail with reference to FIG.
[0018]
As the first substrate 207, quartz glass, high melting point glass, plastic substrate, or the like that is transparent to the laser 208 irradiated at the time of transfer and can withstand the element manufacturing process is desirable. A sacrificial layer 206 is formed on the substrate 207. The sacrificial layer is preferably made of a material that efficiently absorbs the laser beam 208. For example, amorphous silicon, germanium, and other photolytic organic substances are also effective. A buffer layer 205 is formed thereon. This plays a role of preventing the influence of heat or light upon element transfer from reaching the upper element, and silicon dioxide, silicon nitride, metal oxide, etc. are effective. An element to be transferred is formed thereon. As this element, for example, a semiconductor device such as a thin film transistor, a semiconductor laser, a photodiode, or a solar cell, an arbitrary element such as an organic EL element or a liquid crystal display device may be formed. Thereafter, an electrode 204 that is electrically connected to the sacrificial layer 206 is formed. A bias voltage is applied to the sacrificial layer 206 through the electrode 204. Next, the laser beam 208 is irradiated with a bias voltage applied. Here, solid-state lasers such as high-power gas lasers such as carbon dioxide lasers and excimer lasers, YAG lasers, and semiconductor lasers can be used as laser light sources. This is effective because it does not cause thermal damage. The temperature of the sacrificial layer 206 is increased by the irradiation of the laser beam 208 and melts when the melting point is exceeded. When melted, the sacrificial layer 206 is more likely to conduct electricity, so that current flows through the sacrificial layer 206 simultaneously with melting. If the value of this current is sufficiently large, the temperature of the sacrificial layer 206 further increases due to Joule heat generation. If this current value is set appropriately, the sacrificial layer 206 will have an extremely large structural disturbance due to a rapid temperature rise, and will eventually lose its adhesion as a thin film. Therefore, the upper element portion can be easily separated from the substrate 207 with the sacrificial layer 206 as a boundary.
[0019]
【Example】
As an embodiment of the present invention, a method for transferring a TFT element will be described with reference to FIG. Although the substrate and the base protective film used in this embodiment conform to the above description, a 4-inch φ quartz substrate 307 is used here as an example of the substrate. First, an amorphous silicon 306 as a sacrificial layer is formed to a thickness of 100 nm on the substrate 307. In this embodiment, using a high vacuum type LPCVD apparatus, 200 SCCM of disilane (Si ₂ H ₆ ), which is a raw material gas, is flowed and deposited at a deposition temperature of 425 ° C. Next, a base protective film 305 that is an insulating material is formed. Here, a silicon oxide film having a thickness of about 200 nm is deposited by ECR-PECVD at a substrate temperature of 150.degree. Next, a semiconductor film 300 such as an intrinsic silicon film which becomes an active layer of the thin film transistor is deposited. The thickness of the semiconductor film 300 is about 50 nm. This semiconductor layer is formed by the same method as that for the amorphous silicon 306. Next, laser crystallization which becomes the channel portion of the TFT is performed. Prior to this, the amorphous silicon film is immersed in a hydrofluoric acid solution, and the natural oxide film on the semiconductor film 300 is etched. Next, laser light is irradiated. In this embodiment, an excimer laser (wavelength: 308 nm) of xenon chloride (XeCl) is irradiated. The intensity half width (half width with respect to time) of the laser pulse is 25 ns. After setting the substrate 307 in the laser crystallization chamber, vacuum evacuation is performed. Since the crystallinity of the p-Si film can be improved by irradiating the laser with the substrate 307 heated, the substrate temperature is raised to 250 ° C. after evacuation. The area of laser irradiation at one time is a 10 mm square, and the energy density on the irradiated surface is 160 mJ / cm ² . Irradiation is repeated while the laser beams are overlapped by 90% (that is, 1 mm for each irradiation) and relatively shifted (see FIG. 1). In this way, the amorphous silicon of the entire 4-inch φ substrate 307 is crystallized. A second laser irradiation is performed using the same irradiation method. The energy density for the second time is 180 mJ / cm ² . This is repeated, and finally the irradiation with the energy density of 440 mJ / cm ² is performed while increasing the irradiation energy density by about 20 mJ / cm ^{2 for} the third time and the fourth time, and the laser irradiation is finished. Here, when high energy exceeding the irradiation laser energy density of 450 mJ / cm ² was irradiated, since the grains of p-Si caused microcrystallization, further energy irradiation was avoided. Next, a gate insulating film is formed by a CVD method, a PVD method, or the like in a gate insulating film deposition chamber. In this embodiment, a 120 nm silicon oxide film is deposited at a substrate temperature of 350 ° C. by a parallel plate type rf discharge PECVD method. As the source gas, a mixed gas of TEOS (Si— (O—CH ₂ —CH ₃ ) ₄ ) and oxygen (O ₂ ) was used. Subsequently, a thin film to be the gate electrode 301 is deposited by a PVD method or a CVD method. Usually, since the gate electrode and the gate wiring are made of the same material and in the same process, it is desirable that this material has a low electric resistance and is stable to a heat process of about 350 ° C. In this embodiment, a tantalum thin film having a thickness of 600 nm is formed by sputtering. After depositing a thin film to be a gate electrode, patterning is performed, and subsequently, impurity ions are implanted into the semiconductor film 300 to form a source / drain region 302 and a channel region. At this time, since the gate electrode 301 serves as an ion implantation mask, the channel has a self-aligned structure formed only under the gate electrode 301. Next, an interlayer insulating film is formed by a CVD method or a PVD method. In this example, TEOS (Si— (O—CH ₂ —CH ₃ ) ₄ ), oxygen (O ₂ ), and water (H ₂ O) are used as source gases, and argon is used as a dilution gas at a substrate surface temperature of 300 ° C. The film is formed to a thickness of 500 nm. After the ion implantation and the formation of the interlayer insulating film, heat treatment is performed for several tens of minutes to several hours in an appropriate thermal environment of about 350 ° C. or less to activate the implanted ions and to bake the interlayer insulating film. The heat treatment temperature is preferably about 250 ° C. or higher in order to reliably activate the implanted ions. Also, a temperature of 300 ° C. or higher is preferable for effectively baking the interlayer insulating film. After the interlayer insulating film is formed, contact holes are formed on the source / drain regions 302 and the sacrificial layer 306, and source / drain extraction electrodes 303 and bias application electrodes 304 are formed by PVD, CVD, etc. The structure is complete.
[0020]
After the sacrificial layer and the thin film transistor element having the above structure are formed, a process of transferring the element to the second substrate is performed. A bias of 50 V is applied to the bias application electrode 304 connected to the sacrificial layer 306. In this state, excimer laser 308 is irradiated from the substrate 307 side. Here, the laser beam is irradiated so that the gap-shaped bias application electrode 304 is positioned at both ends of the irradiation region. As a result, a uniform current can flow through the entire irradiation region, so that the yield in the peeling process can be increased. Amorphous silicon 306 as a sacrificial layer absorbs and melts all excimer laser light irradiated at an energy density of about 300 mJ / cm 2. Amorphous silicon has extremely high resistance in a solid state, but once melted, it exhibits metallic properties and becomes a very low resistance of 80 μΩcm. For this reason, a current flows through the bias application electrode 304 to the molten silicon at a very large current density. If the bias voltage is adjusted so that the current density at this time is approximately 10000 A / cm @ 2, the subsequent peeling is facilitated. Since the molten silicon film generates Joule heat due to this large current, the temperature of the molten silicon can be controlled by the bias voltage. Since the temperature of the sacrificial layer 306 cannot be controlled only by laser irradiation, it was extremely difficult to control the ease of peeling, but the ease of peeling was easily controlled by the method of the present invention in which the temperature is controlled by current control. I was able to do it. Further, by measuring the resistance value of the sacrificial layer 306 after laser irradiation, the electrical characteristics reflecting the structure of the sacrificial layer 306 can be evaluated. Therefore, the state of the sacrificial layer 306 that can be transferred is surely reproduced by adjusting the number of times of laser irradiation, energy, and bias voltage to appropriate values while monitoring the change in the resistance value of the sacrificial layer 306. A second substrate 311 serving as a transfer destination is bonded to the first substrate that is sufficiently peelable in this manner by an adhesive 310. After the adhesive is sufficiently solidified, the element is easily transferred to the second substrate 311 with the sacrificial layer 306 as a boundary by pulling the first substrate 307 and the second substrate 311 in opposite directions. .
[0021]
【The invention's effect】
As described above, in order to produce a sacrificial layer having good consistency with the TFT manufacturing process, it is important to use an amorphous silicon film which is an active layer of the TFT. As a result, the element formation process is exactly the same as in the prior art, a sacrificial layer can be formed, and there is no need to adversely affect the element due to impurities. In addition, since an excimer laser used in the TFT manufacturing process can be used as a laser for melting the sacrificial layer, the cost of the apparatus can be reduced.
[0022]
As described above, by using the thin film element transfer method of the present invention, element transfer with good controllability and reproducibility becomes possible.
[Brief description of the drawings]
FIG. 1 is a view for explaining a transfer method when the present invention is used in a TFT.
FIG. 2 is a view for explaining a thin film element transfer method of the present invention.
FIG. 3 is a view for explaining a conventional thin film transfer method.
[Explanation of symbols]
207. . . Substrate 205. . . Underlying insulating film 300. . . Semiconductor films 106, 308. . . Laser beam 306. . . Sacrificial layer 301. . . Gate electrode 302. . . Source / drain region 303. . . Source and drain electrodes 304. . . Bias application electrode

Claims (8)

In a thin film element transfer method for transferring a thin film element fabricated on a sacrificial layer on a first substrate onto a second substrate, the sacrificial layer is applied in a state where a voltage is applied to the sacrificial layer made of an amorphous silicon film or a germanium film. After the layer is irradiated with light , the first substrate and the second substrate are bonded to each other through an adhesive, and then the sacrificial layer is peeled off, thereby removing the thin film element on the sacrificial layer from the second layer. A method for transferring a thin film element, wherein the transfer is performed on a substrate.   2. The method for transferring a thin film element according to claim 1, wherein the thickness of the sacrificial layer is in the range of 10 to 500 nm. Voltage transfer process of a thin film element according to claim 1 or 2, wherein the current density flowing through the sacrificial layer is set to be 10000 A / cm ² or more to be applied to the sacrificial layer. Peeling state of the sacrificial layer, determined by the resistance value change of the sacrificial layer after light irradiation, a method of transferring a thin film element according to claim 1, 2 or 3 wherein monitoring the optimum conditions to repeat the light irradiation . Electrodes, thin-film element according to claim 1, 2, 3 or 4, wherein the composed installed to face the positions sandwiching the desired transfer regions on the first substrate for applying a voltage to the sacrificial layer Transfer method. 6. The method for transferring a thin film element according to claim 5, wherein the light irradiation is performed on a region having opposing electrodes for applying a voltage to the sacrificial layer at both ends. Method for transferring a thin film device according to claim 2, 3, 4, 5 or 6, wherein the light irradiation is characterized by being performed by an excimer laser. Method for transferring a thin film device according to claim 3, 4, 5, 6 or 7, wherein said thin film device is a thin film transistor.

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