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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device using a crystalline semiconductor mainly comprising silicon. The silicon semiconductor obtained according to the method of the present invention is suitable for
use in thin film transistors, thin film diodes and the like.
2. Prior Art
Thin film transistors utilizing a semiconductor thin film (referred to simply hereinafter as "TFTs") are well known. TFTs are fabricated by forming a thin film semiconductor on a substrate and processing the thin film semiconductor thereafter.
TFTs are widely used in various types of integrated circuits, and are particularly suitable in the field of electro-optical devices such as liquid crystal displays; more specifically, as switching elements provided for each of the pixels in an active
matrix liquid crystal display device as well as the driver elements of peripheral circuits thereof.
An amorphous silicon film can be utilized most readily as a thin film semiconductor for the TFTs. However, there is a problem in that the electrical characteristics of the amorphous silicon film are poor. The use of a thin film of crystalline
silicon can solve this problem. The crystalline silicon film can be prepared by first forming an amorphous silicon film and then heat treating the film to crystallize it. Otherwise, high energy electromagnetic waves for example, a laser beam, can be
radiated onto the amorphous silicon film.
The heat treatment for crystallizing amorphous silicon film requires heating the film at a temperature of 600.degree. C. or higher for a duration of 10 hours or longer. Such a high temperature heat treatment is undesirable for a glass
substrate. For example, Corning 7059 glass, commonly used for substrates of active matrix liquid crystal display devices, has a glass distortion point of 593.degree. C. and is therefore not suitable for large area substrates that are subjected to
heating at a temperature of 600.degree. C. or higher.
According to studies carried out by the inventors of the present invention, it has been found that crystallization of an amorphous silicon film can be effected by heating the film at 600.degree. C. or less, e.g. 550.degree. C. for a duration of
about 4 hours. This can be accomplished by first disposing a trace amount of nickel or palladium, or other elements such as lead in contact with the amorphous silicon film and then applying a heat treatment at the above temperature for crystallization.
It has also been known to utilize a laser for crystallization. The silicon film obtained by this method is suitable for use in the fabrication of a TFT since it yields superior characteristics such as high field mobility, low S value, and low
threshold voltage. However, the crystallinity of the silicon film thus obtained strongly depends on the energy of the laser. It is therefore apparent that, due to the instability of laser energy, it is very difficult to stably obtain a crystalline
silicon film with a high reliability.
Further, in the case of crystallizing an amorphous silicon film with a silicon oxide in contact therewith, the resulting silicon film tends to be oriented along the (111) plane. A TFT having a channel forming region having such a crystal
orientation tends to have a threshold voltage V.sub.th shifted to a negative value due to the large positive fixed charges. Such TFTs are unfavorable in constituting a complementary circuit composed of an N-channel TFT and a P-channel TFT. Accordingly,
in order to control the shift in the threshold voltage, the silicon film must be fabricated with care that it does not orient along the crystallographic (111) plane.
SUMMARY OF THE INVENTION
In view of the above-described circumstances, it is a primary object of the present invention to form a crystalline semiconductor film having a higher electrical characteristics and a higher reliability.
It is another object of the present invention to form a crystalline semiconductor film which is not oriented along the (111) plane.
In accordance with an aspect of the present invention, crystallization of a silicon film is carried out with a silicon nitride film in contact with the silicon film at least partly. The preferred nitrogen/silicon ratio is from 1.3 to 1.5
(1.3.ltoreq..times..ltoreq.1.5). If the ratio x is lower than 1.3, the film tends to trap the charge, and is not suitable for a semiconductor device. The electrical characteristics of the film can be improved by adding hydrogen or oxygen at 0.01-2%.
In such a case, the silicon nitride film can be expressed by one of the formulas SiN.sub.x H.sub.y, SiO.sub.x N.sub.y and SiO.sub.x N.sub.y H.sub.z. The silicon nitride film mentioned in this specification includes hydrogenated silicon nitride, and a
silicon oxinitride.
The use of a silicon nitride film prevents the silicon film from being oriented along the crystallographic (111) plane. Since crystallized silicon oxide has a diamond structure as a crystal silicon film does, a silicon film crystallized
contacting silicon oxide tends to be oriented along the (111) plane due to the boundary energy therebetween. On the other hand, crystallized silicon nitride is cubic, and the matchability with a silicon film is not so good. Thus, by contacting the
silicon nitride film, the amorphous silicon film can be crystallized without orienting along the particular plane.
On the other hand, a catalyst element which is capable of promoting the crystallization of the amorphous silicon film is disposed in contact with the silicon film to be crystallized. The catalyst element may be used either in the form of an
elemental metal or a compound thereof. Also, it may be shaped into the form of a continuous layer, or a discontinuous layer such as a number of clusters. These will be referred to simply as "a catalyst layer" in the present invention.
The use of nickel as the catalyst element is particularly effective in the process according to the present invention. However, other useful catalyst elements include palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold (Au), indium
(In), tin (Sn), phosphorus (P), arsenic (As), and antimony (Sb). Otherwise, the catalyst element may be selected from the elements belonging to Groups VIII, IIIb, IVb, and Vb of the periodic table of the former international notation.
The order of the formations of the silicon, silicon nitride, and catalyst layer may be changed arbitrarily. For example:
(1) Forming the layers in the order of silicon nitride film, amorphous silicon film, and catalyst layer;
(2) Forming the layers in the order of silicon nitride film, catalyst layer, and amorphous silicon film;
(3) Forming the layers in the order of catalyst layer, amorphous silicon film, and silicon nitride film; and
(4) Forming the layers in the order of amorphous silicon film, catalyst layer, and silicon nitride film; and
(5) Forming the catalyst layer on a part of the amorphous silicon film, and a silicon nitride film on the other part of the amorphous silicon film.
In the above examples (1) to (4), the silicon nitride film and the catalyst layer does not need to cover the entire surface of the amorphous silicon film. In the examples (2) and (4), the amorphous silicon film and the silicon nitride film seem
to be not in direct contact with each other, however, since the catalyst layer is very thin, the amorphous silicon film is substantially in contact with the silicon nitride.
The amorphous silicon film is subjected to heat treatment thereafter to crystallize the amorphous silicon film at least partly. Amorphous silicon regions may be left in the silicon film after the heating step. That is, crystallization of the
silicon film does not need to occur over the entire surface of the amorphous silicon film. Moreover, when the catalyst layer is provided only on a part of the amorphous silicon film, crystallization proceeds from the region covered by the catalyst layer
to the periphery thereof.
The crystallization of the silicon film is further progressed by irradiating it with laser beam or intense light beam equivalent thereto. By the irradiation, the amorphous regions and a part of the crystallized regions are melted. However, the
other part of the crystallized regions remains without melting. This remaining part of the crystallized regions functions as nuclei and the crystallization proceeds rapidly through the entire region of the silicon film. The degree of crystallization
(i.e. the proportion of the area of the crystallized region with respect to the entire area, as observed through a microscope) after heat treatment is in the range of from 20 to 90%. The maximum attainable amount of crystallization is about 97% by
raising the heating temperature. However, the proportion of the amorphous component can be more effectively reduced to a negligible degree by irradiation by the above laser beam or intense light beam equivalent thereto.
In a conventional crystallization step using a laser light, the laser irradiation is done onto an amorphous silicon having no nucleating site in order to crystallize through melting. In such a case, the condition for determining crystallinity is
very strict, that is, in the absence of nucleating site, it is the cooling rate that mainly determines the crystallinity. However, the cooling rate depends upon the energy density of the laser light or the surrounding temperature. Therefore, the
optimum laser energy density is naturally limited to a narrow range. If the energy density is too high, the cooling rate from the melt condition becomes too rapid and leaves an amorphous state in the obtained film. If the energy is too low, the film
cannot be wholly melted and leaves amorphous portions also.
In contrast to the above conventional laser process, crystallization in the process according to the present invention can be more readily effected because a crystal nucleus is present, and the crystallization process is less dependent on the
cooling rate. Because a large part of the film is crystallized, the film characteristics can be assured to a certain degree even if a low energy density laser is used. Accordingly, in accordance with the present invention, an amorphous silicon film can
be uniformly crystallized with a high reliability.
Instead of using a laser beam, non-coherent intense light, particularly infrared radiation, may be used. The majority of infrared radiation is not absorbed by glass, but is readily absorbed by thin film silicon. Accordingly, thin film silicon
can be heated selectively without heating the glass substrate by the use of IR light. This process of radiating infrared light for a short period of time is called as rapid thermal annealing (RTA) or a rapid thermal process (RTP).
BRIEF
DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1F shows a manufacturing step of a semiconductor device in accordance with the first embodiment of the present invention;
FIGS. 2A to 2C are plane views corresponding to the structures shown in FIGS. 1A-1F;
FIGS. 3A to 3F are diagrams showing a manufacturing step of a semiconductor device in accordance with the second embodiment of the present invention; and
FIGS. 4 (A) to 4 F are diagrams showing a manufacturing step of a semiconductor device in accordance with the third embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in further detail below, referring to preferred embodiments. It should be understood, however, that the present invention is not to be construed as being limited to the examples below.
The catalyst element can be incorporated into the film by a deposition process, such as sputtering of the catalyst element or a compound of the element, using a vacuum deposition apparatus. In an alternative, a deposition process can be effected
in the atmosphere by coating the surface of the amorphous silicon film with a solution containing the catalyst element. The latter process is particularly advantageous in terms of the reproducibility of the process and the equipment expenses.
The solution for use in the process may be an aqueous solution or a solution based on an organic solvent or the like. The "solution" referred to herein encompasses those solutions containing the catalyst element in the form of a compound
dissolved in the solution and those containing the element in dispersed form.
The solvent containing the catalyst element may be selected from various types of polar solvents such as water, alcohol, acid, or ammonia water.
When nickel is used as the catalyst, it may be added to the polar solvent in the form of a nickel compound. More specifically, it may be selected from a group of nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride,
nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel acetylacetonate, nickel 4-cyclohexylbutyrate, nickel oxide, and nickel hydroxide.
Otherwise, a non-polar solvent can be used in the solution containing the catalyst element. For example, a solvent selected from benzene, toluene, xylene, carbon tetrachloride, chloroform, and ether. In this case again, nickel is incorporated
in the solution in the form of a nickel compound. Typical compounds to be mentioned include nickel acetylacetonate and nickel 2-ethylhexanoate.
It is also useful to add a surface active agent (surfactant) to the solution containing the catalyst element. The surfactant increases the adhesive strength of the solution and controls adsorption. The surfactant may be applied previously to
the surface of the substrate onto which the amorphous silicon is deposited.
When metallic nickel is used as the catalyst, it must be dissolved in an acid to provide a solution.
The description above is for a case where the catalyst is completely dissolved in a solution. However, the catalyst does not need to be completely dissolved in the solution, and other materials, such as an emulsion in which catalyst metal or a
catalyst compound is dispersed in the form of a powder in a dispersant. It is also possible to use a solution which is available as a material for forming an oxide film. A preferred example of the solution is OCD (Ohka Diffusion Source) manufactured by
Tokyo Ohka Kogyo Co., Ltd. A silicon oxide film can be easily obtained by applying the OCD to the desired surface, and baking it at about 200.degree. C. Since an impurity can be readily added to the solution, the OCD can be utilized in the process of
the present invention.
Although the concentration of the catalyst element in the solution depends on the kind of the solution, roughly speaking, the concentration of nickel by weight is from 0.1 to 200 ppm, and preferably from 1 to 50 ppm or lower. The concentration
is determined based on the nickel concentration in or the resistance against hydrofluoric acid of the crystallized film.
Crystal growth can be controlled by applying the solution containing the catalyst element to selected portions of the amorphous silicon film. In particular, crystals can be grown in parallel with the plane of the silicon film from the region
onto which the solution is applied to the region onto which the solution is not applied. The region in which the crystals are grown in parallel with the plane of the amorphous silicon film is referred to hereinafter as a lateral growth region.
It is confirmed that the lateral growth region contains the catalyst element at a lower concentration. Although it is useful to utilize a crystalline silicon film as an active layer region for a semiconductor device, in general, the
concentration of the impurity in the active region as low as possible. Accordingly, it is advantageous to employ the lateral growth region as an active region of a semiconductor device.
EXAMPLE 1
The present example refers to a process for fabricating a crystalline silicon film on the surface of a glass substrate. Referring to FIGS. 1A-1F, a Corning 7059 glass substrate 100 mm.times.100 mm in size and 1.1 mm in thickness was prepared. A
silicon nitride film from 1,000 to 5,000 .ANG., for example, 2000 .ANG. thickness was deposited as a base film 102 by sputtering or plasma CVD on the substrate 101. The stoichiometric ratio of nitrogen and silicon (nitrogen/silicon) was set to 1.3 to
1.35, preferably 1.32 to 1.34.
The silicon nitride film is formed, for example, by a plasma CVD with the following conditions:
RF power: 13.56 MHz, 300 W
Substrate Temperature: 300 .degree. C.
SiH.sub.4 /NH.sub.3 =30 sccm/210 sccm
Pressure: 0.3 Torr
Generally, when the temperature is higher, the concentration of hydrogen in the obtained film is reduced.
Then, an amorphous silicon film 103 was deposited to a thickness of 100 to 1,500 .ANG., for example, 800 .ANG. by plasma CVD or LPCVD.
A silicon oxide film 104 was formed to a thickness of 500 to 3,000 .ANG., for example, 1,000 .ANG.. Holes were perforated selectively in the resulting silicon oxide film to expose the underlying amorphous silicon film as shown in FIG. 1A.
The resulting amorphous silicon film was immersed in an aqueous hydrogen peroxide solution to form an extremely thin silicon oxide film (not shown in the figure) on the exposed portions of the amorphous silicon film. The thickness of the silicon
oxide film is 10 to 100 .ANG.. However, due to its extreme thinness, the accurate thickness of the film was unknown. Alternatively, the thin oxide film may be formed through an ultraviolet (UV) light irradiation in an oxygen atmosphere. In this case,
the surface may be exposed to a UV light in an oxygen atmosphere for 1 to 15 minutes. It is also possible to employ thermal oxidation.
The oxide film was provided with the purpose of spreading the acetate solution containing nickel, which is to be applied in a later step, over the entire surface of the amorphous silicon film uniformly. That is, the oxide film was provided for
improving the wettability of the amorphous silicon film. If the aqueous acetate solution were to be applied directly onto the surface of the amorphous silicon film, the solution tends to be repelled so that the nickel can not be uniformly formed on the
silicon film, resulting in that the uniformity of the crystallinity would be hindered. This problem can be solved by the provision of the thin oxide film.
Next, an aqueous acetate solution containing nickel at a concentration of 100 ppm was prepared. Two milliliters of the acetate solution was dropped onto the surface of the oxide film on the amorphous silicon film and maintained as it is for a
duration of 5 minutes. Spin drying at 2,000 rpm was effected for 60 seconds thereafter.
The amorphous silicon film coated with the solution above was kept as it is for a duration of 1 to 10 minutes thereafter. Although the final concentration of nickel in the silicon film can be controlled by changing this duration, the most
influential factor in controlling the nickel concentration was the concentration of the solution.
This step of applying the nickel solution was repeated one to several times, whereby a nickel-containing layer (catalyst layer) 105 having a thickness of from several .ANG. to several hundred .ANG. was obtained on the exposed surface of the
amorphous silicon film 103 after spin drying. The nickel in the layer will diffuse into the amorphous silicon film during a heat treatment and will functions as a catalyst for promoting crystallization.
The silicon film coated with nickel-containing solution thus obtained was subjected to a heat treatment at a temperature of 550.degree. to 600.degree. C., e.g. 550 .degree. C. for a duration of 1 to 12 hours, e.g. 8 hours in a nitrogen
atmosphere using a furnace. As a result, crystallization proceeded from the opening in the silicon oxide film 104 to form the crystallized regions 106 and 107 as shown in FIG. 1B. The other regions 108 and 109 remained in an amorphous state.
A plane view of the structure of FIG. 1B is shown in FIG. 2A. It can be seen that the crystallized region extends from the opening region 130 to an oval region.
Then, the silicon oxide film 104 was etched. And also, the silicon film 103 is patterned into island regions 110 and 111. The etching of the silicon film was performed by means of an RIE (reactive ion etching) method which is perpendicularly
anisotropic. The island regions 110 and 111 will be an active region of TFTs. A very thin silicon oxide film 112 was then formed to a thickness of 100.ANG. or less on the surface of the silicon islands 110 and 111 by a heat treatment in an oxygen
atmosphere at 550.degree. C. as shown in FIG. 1C.
Next, the crystallinity of the island-like regions 110 and 111 was further improved by laser beam irradiation using a KrF excimer laser operated at a wavelength of 248 nm and with a pulse width of 30 nsec. The laser was operated to provide
several shots per site with an energy density of from 200 to 400 mJ/cm.sup.2, for example, 300 mJ/cm.sup.2, in a nitrogen atmosphere or in the air. Instead of using a KrF excimer laser, other types of excimer lasers such as an XeCl laser (wavelength of
308 nm), an ArF laser (193 nm), or an XeF laser (353 nm) may be used. Also, a rapid thermal annealing (RTA) process may be employed.
Referring to FIG. 1D, a 1,000 .ANG. thick silicon oxide film 113 was deposited after the laser irradiation as a gate insulating film by sputtering or by plasma CVD. When sputtering is employed, silicon oxide is used as a target, the substrate
temperature is in the range of from 200.degree. to 400.degree. C., for example at 350.degree. C., and the sputtering gas comprises a mixture of oxygen and argon at an argon to oxygen ratio (Ar/O.sub.2) of 0 to 0.5, preferably, 0.1 or less.
Subsequently, a silicon layer containing phosphorus at 0.1-2% was deposited to a thickness of 3,000 to 8,000 .ANG., for example, 6000 .ANG., through reduced pressure CVD. It is desirable to form the silicon film successively following the
formation of the silicon dioxide film 113. The silicon film was then patterned to provide gate electrodes 114 to 116 as shown in FIG. 1E. A planar view corresponding to FIG. 1E is shown in FIG. 2B. The oval area defined by the broken lines corresponds
to the regions 106 and 107 shown in FIG. 2A.
Then, by means of ion doping, phosphorus and boron were implanted as impurities into the active layer using the gate electrodes 114 to 116 as masks. In the present example, phosphine (PH.sub.3) was used as the doping gas to implant phosphorus,
and diborane (B.sub.2 H.sub.6) was used for the implantation of boron. In implanting phosphorus, an acceleration voltage of 60 to 90 kV, for example 80 kV, was applied, and in implanting boron, an accelerating voltage of 40 to 80 kV, e.g. 65 kV, was
applied. The dose amount was in the range of from 1.times.10.sup.14 to 8.times.10.sup.15 cm.sup.-2, for example, phosphorus was implanted at a dose of 1.times.10.sup.15 cm.sup.-2, and boron was implanted at a dose of 2.times.10.sup.15 cm.sup.-2. The
elements were each selectively doped by covering the unnecessary portions with a photoresist. Thus were obtained N-type impurity regions 118 and 119, and P-type impurity regions 117.
The ion-implanted impurities were activated thereafter by laser annealing. Annealing was performed by a laser beam, for example, using a KrF excimer laser (wavelength of 248 nm and a pulse width of 20 nsec). The condition of the laser
irradiation was 2 to 10 shots per site, for example, 2 shots per site, at an energy density of 200 to 400 mJ/cm.sup.2, for example 250 mJ/cm.sup.2. Furthermore, it was possible to improve the uniformity in resistance if the substrate was heated at
200.degree.-450.degree. C. during the laser irradiation. Because nickel was diffused among the previously crystallized region, the recrystallization is easily proceeded by the laser irradiation and the P-type regions 117 doped with a P-type impurity
and N-type regions 118 and 119 doped with an N-type impurity were effectively activated. It is possible to use other light sources as mentioned before.
Referring to FIG. 1F, a 6,000 .ANG. thick silicon oxide film 120 was deposited by plasma CVD as an interlayer dielectric. Further, a 500 .ANG. thick ITO film was deposited by sputtering, and patterned to provide a pixel electrode 121. Contact
holes were formed as shown in FIG. 2C, and electrodes with interconnections 122 to 126 for the TFT were formed using a metallic material, for example, a multilayered film of titanium nitride and aluminum. Finally, the resulting structure was annealed in
a hydrogen atmosphere at 1 atom and 350.degree. C. for 30 minutes. Alternatively, instead of effecting hydrogen annealing, hydrogen ions may be accelerated with an acceleration voltage of 10 to 100 keV and implanted into the active layer, followed by
annealing. This step may be done during the steps shown in FIG. 1C or FIG. 1D.
A circuit comprising TFTs was thus obtained. For example, a so-called monolithic active matrix circuit, i.e. an integrated circuit having an active matrix circuit and the logic circuit for driving the active matrix circuit on the same substrate
can be formed, more specifically, the N-channel TFT and the P-channel TFT associated with the island-like region 110 are arranged in a complementary form to serve mainly as the logic circuit, and the TFT formed in the silicon island 111 is used as a
switching transistor in an active matrix circuit.
As is apparent from FIG. 2B, the channel region of the TFT according to the present example was provided in the lateral growth region of the regions 106 and 107. The lateral growth region yields excellent crystallinity, and hence, the electrical
characteristics such as threshold voltage, field-effect mobility and the like could be improved. On the other hand, the region into which nickel was directly incorporated contained a higher concentration of nickel, and also the regions 108 and 109 have
a poorer crystallinity. Hence, it is not preferable to form a channel region in these regions. However, the source, drain, etc. may be formed therein.
EXAMPLE 2
The process for fabricating a semiconductor device according to the present example will be described below with reference to FIGS. 3A to 3F. A silicon oxide film 202 was deposited by a known method such as plasma CVD to a thickness of 1,000 to
5,000 .ANG., for example, 2000 .ANG. as a base film on a Corning 7059 glass substrate 201 (10 cm.times.10 cm in size). An amorphous silicon film 203 was deposited by plasma CVD or reduced pressure CVD to a thickness of 1,000 .ANG..
After forming an ultra-thin silicon oxide film (not shown) on the surface of the amorphous silicon film using aqueous hydrogen peroxide, 5 ml of an acetate solution containing 25 ppm of nickel was dropped onto the oxidized surface of the
amorphous silicon film in the same manner as in Example 1. Further, at this time, spin-coating was effected at 50 rpm for a duration of 10 seconds to form a uniform aqueous film over the entire surface of the substrate. After further retaining this
state for a duration of 5 minutes, spin-drying was effected for a duration of 60 seconds at 2,000 rpm using a spinner. The substrate may be retained with being rotated by the spinner at 150 rpm or less. A catalyst layer 204 containing nickel was formed
in this manner as shown in FIG. 3A.
Referring to FIG. 3B, a silicon nitride film 205 was deposited thereafter to a thickness of 500 to 3,000 .ANG., e.g. 1000 .ANG., through plasma CVD in the same manner as in Example 1. Then, the amorphous silicon film 203 was crystallized by a
heat treatment in a nitrogen atmosphere at a temperature of 550.degree. C. for a duration of 4 hours. Since the nickel is formed between the amorphous silicon film 203 and the silicon nitride film 204, crystallization proceeded downwardly through the
amorphous silicon film from the upper portion thereof.
After the above crystallization step was performed by the heat treatment, the crystallinity of the silicon film 12 was further improved by the use of an XeCl laser (a wavelength of 308 nm) through the silicon nitride film 205 as shown in FIG. 3C. It was effective to heat the su | | |
