GB2174108A - Method for forming a polycrystalline silicon thin film - Google Patents

Abstract

A method for forming a thin polycrystalline silicon film utilizing ions comprises the steps of heating silicon to vacuum, ionizing and accelerating at least a portion of evaporated silicon and, thereafter, bombarding said silicon onto a substrate simultaneously with the remaining neutral silicon, whereby a thin film having a high density, and a high degree of purity is formed. The method may constitute ion plating, ion beam deposition or ionized cluster beam deposition.

Description

SPECIFICATION Method for forming a polycrystalline silicon thin film Background of the invention Field of the invention The invention relates to a method for forming a polycrystalline thin film having excellent electrical characteristics at low temperatures, and more particularly, to a method for forming on an amorphous substrate a polycrystalline silicon thin film having a high density, smooth surface, and preferred orientation mainly by means of an ionised cluster beam method.

Description of the prior art A polycrystalline thin film has been conventionally used as MOS and LSI electrodes. Recently, research has been energetically made to apply it to an SOI (Silicon-On-lnsulator) device in which it is used as an active region, and to a thin film transistor (TFT) for liquid crystal displays and EL displays.

For example, in an active matrix system to be used for improving the picture quality of a liquid crystal display panel and EL thin film elements, a polycrystalline silicon as well as an amorphous sili con has been used as a thin film transistor (TFT) to switch picture elements. Polycrystalline silicon per forms as an excellent transistor material in that it provides high carrier mobility which permits fast switching speeds, an important function of transis tors. Therefore, polycrystalline silicon is not only suitable for high quality displays, but also an im portant material for integrating a drive circuit within a substrate. Further, it is a reliable material.

Thus, polycrystalline silicon performs far better than an amorphous silicon.

Conventionally, a polycrystalline silicon film has been formed mainly by an LPCVD method (Low Pressure Chemical Vapor Deposition). In the LPCVD method, a carrier gas and a material gas contain ing silicon are introduced into a reactor, and there after, a polycrystalline silicon thin film forms by a chemical reaction at reduced pressures of 0.1 to 10 TORR, with a substrate heated by means of a heat source placed outside the reactor. Although silicon vapor deposition speed is slow in the LPCVD method, it is preferable in that the resulting film displays an excellent uniformity and, moreover, many films may be processed at one time, hence, this method is widely used in LSI production proc esses.

Transparent substrates such as glass and quartz are used in a liquid crystal display because these displays use a transmitted light. A low-melting point glass is especially preferable as a substrate for liquid crystal displays because substrates made of low-melting-point glass are inexpensive, and production of large-area displays with this type of substrate is rather simple. Nevertheless, one disad vantage of this material is that it is virtually unpro cessable at temperatures above 600"C because it is not a high temperature resistant material.

It is known that when a polycrystalline silicon thin film is formed by means of the LPCVD method, a substrate must be heated preferably more than 600"C since silicon thin films obtained at low temperatures become amorphous. As a result, the LPCVD method inevitably uses quartz substrates rather than a low-melting-point glass; consequently, the cost of a semiconductor device cannot be lowered. A polycrystalline silicon thin film, formed by the LPCVD method or by an electronic beam vapor deposition method, is used as a material for a semiconductor device in the thickness of several hundreds of angstroms to several thousands of angstroms. Unfortunately, polycrystalline silicon thin films this thick are not dense, smooth on the surface and, in addition, crystal grains thereof are randomly orientated; that is, they have a slightly intense orientation of < 110 > .As will be described hereinafter, an intensity of a film's orientation and a degree of density of a film and surface smoothness strongly influence the characteristics of a semiconductor device.

The interface state density of an oxide film relative to a silicon semiconductor is very important in a MOS (Metal-Oxide-Silicon) device normally which is used as a thin film transistor for an active matrix. The lower the interface state density is, the more preferable a device's characteristics are. Generally, crystal grains of polycrystalline silicon are randomly orientated. Therefore, the interface state density is much larger than that of single crystal silicon. If polycrystalline silicon crystals can be uniformly oriented, the state density is decreased and thus approach the state density level of a single crystal silicon, in addition, the scatter of the grain boundary characteristics of a polycrystalline silicon is prevented. Thus, control of the characteristics of a semiconductor device can be easily performed.

It is also important to make polycrystalline silicon films dense and smooth on the surfaces at low temperatures to obtain preferable characteristics for a semiconductor device; that is, a crystal having a high density is effective for reducing unfavorable density of defect levels present at polycrystalline crystal grain boundaries, and a crystal having a smooth surface is effective for reducing the interface state density described above.

A polycrystalline silicon thin film which satisfies these conditions enables a semiconductor device to function in a low-threshold voltage and attain a high carrier mobility. Therefore, formation of a po lycrystalline silicon thin film at low temperatures and the invention of a filmforming method for permitting crystal thereof to have an intense orientation is very important for improving and diversifying characteristics of a device in which a polycrystalline silicon is used.

Summary of the invention The invention has been made in view of the problems caused by conventional film forming methods, and intends to provide a method whereby, by ionizing silicon, a polycrystalline sili con thin film having a high density, smooth surface, a preferred orientation and excellent electrical characteristics can be formed at relatively low tem peratures on an amorphous substrate.

A method for forming a thin film utilizing ions according to the invention comprises the steps of heating silicon, which is an evaporation source, evaporating heated silicon to vacuum, ionizing and accelerating at least a portion of evaporated silicon and, thereafter, bombarding said silicon to a substrate simultaneously with neutral specied. This method includes an ion plating method, ion beam deposition method, and ionised cluster beam method. The description of a thin film forming method to be made hereinafter is restricted to the ionised cluster beam method whereby a thin film having a high density and a high degree of purity is formed, but the method of the invention, of course, is not restricted to this method.

Brief description of the drawings The objects and features of the invention will become apparent from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: Figure 'shows a fundamental structure of the ionised cluster beam device employed for forming a polycrystalline silicon thin film in accordance with the present invention; Figure 2 shows a cross-sectional view of the structure of an example; Figure 3 is an enlarged cross-sectional view of a surface portion of the example formed by applying an acceleration voltage of 0.5 kV; Figure 4 is an enlarged cross-sectional view of a surface portion of an example formed by applying an acceleration voltage of 4.0 kV;; Figure 5 shows a measured result by X-ray diffractometer of an example formed by applying an acceleration voltage of 0.5 kV; Figure 6 shows a measured result by X-ray diffractometer of an example formed by applying an acceleration voltage of 4.0 kV; the lateral axis and the longitudinal axis in Figures 5 and 6 showing the diffraction angle of 2 0 and the relative intensity, respectively.

Figure 7 shows a structure of a thin film transistor; and Figure 8 shows the characteristics of the thin film transistors.

Detailed description of the invention Before describing the invention, a brief description of the ionised cluster beam vapor deposition method to be used in the embodiments will be made hereinafter.

As shown in Figure 1, in the ionised cluster beam vapor deposition method, silicon 2, filled in closed type crucible 1, is heated and evaporated, and thereafter, ejected into an atmosphere where pressure is less than 1/10 of the evaporated silicon pressure. Electrons emitted from filament 4 are injected to the evaporated silicon to ionize the silicon, and thereafter, the ionized silicon is accelerated by acceleration electrode 5 and bombarded simultaneously with neutral silicon and deposited on substrate 7, resulting in formation of a silicon thin film.

In Figure 1, there provide a shutter 6, an infrared lamp 8 for heating substrate 7 and a gas introducing pipe 9 with respect to the crucible 1. According to the method described above, atomic bonds on the substrate surface are enhanced by utilizing the chemical activity of ions themselves and kinetic energy imparted by the accelerated ions. Further, the formation of a thin film which is uniform, dense, and smooth on the surface can be made by migration efficiency generated on the surface caused by the decay of the cluster.

As shown in Figure 1, in this method for forming a polycrystalline silicon thin film, an addition of impurities to the polycrystalline silicon thin film may be effected by introducing process gas into the device shown in Figure 1 from pipe 9 according to circumstances. Doping gases such as PH2, B2H6 can be introduced into the polycrystalline silicon thin film for controlling the conductivity thereof, and gases such as hydrogen can be introduced thereinto for passivating the crystal grain boundary thereof. The ionized silicon is effective for permitting these gases to be introduced thereinto.

Having a special regard to the characteristics of the film forming method using ions, research as to whether or not a polycrystalline silicon thin film can be formed at temperatures less than 600"C have been energetically made. As a result, it was discovered that a polycrystalline silicon thin film which is dense and smooth on the surface and has preferred in orientation could be produced on an amorphous substrate by controlling ions and that these factors are very effective for improving characteristic features of a semiconductor device.

It is to be noted that the structure and the characteristics of a polycrystalline silicon thin film are affected by a background vacuum of a vacuum chamber, temperature of a substrate, evaporation speed of silicon, ionization rate, and an acceleration voltage.

It is preferable to keep a base pressure less than 10-4 Pa to prevent contamination by impurities such as oxygen, carbon, and nitrogen in the film. It is also to be noted that the evaporation speed of silicon must be practicably more than 1-3 /sex.

An acceleration voltage and ion current are the most important parameters for forming a polycrystalline silicon thin film which is dense and smooth on the surface and intensely orientated. An acceleration voltage is the most important factor for permitting a film to be intensely oriented. When an acceleration voltage is in a range from several volts to several hundred volts, a substrate is purified, migration efficiency on a surface is accelerated, and thus perform-s an effective formation of a thin film, but the orientation of crystal grains of a polycrystalline silicon thin film thus formed are random. Therefore, this voltage range is inappropriate to form a polycrystalline silicon thin film in which crystal grains are intensely oriented. When a formation of a polycrystalline silicon thin film is performed with an acceleration voltage of more than 6-7 kV applied, a sputtering occurs and the film is damaged, and a formation of a thin film of high quality cannot be performed. For example, when acceleration voltage is 10 kV, silicon ions permeate into a silicon thin film to the extent of about 150 A from the surface and destroy crystal grains present to this extent, causing the crystal grains to be amorphous. It is known that the regrowth speed of a silicon crystal is several tens of an angstrom per second when film forming temperature is less than 600"C. Therefore, it is difficult for crystal grains which have been destroyed during film formation to make a complete regrowth.

When an acceleration voltage is in the range of 1-5 kV, silicon ions permeate into a silicon thin film to the extent of one to several tens of angstroms.

This means that crystal grains located typically between the top layer of atoms and the tenth layer of atoms are destroyed. But in this case the degree to which crystal grains are destroyed is not so high, thus crystal grains can recover during a film formation at the temperature range described above.

When ions bombard crystal grains parallelly to the crystal axes, a channeling occurs, and the number of ion collisions with the silicon atoms of a crystal becomes less, making it difficult to destroy crystals. A polycrystalline silicon thin film formed without using ions is randomly orientated when its thickness is several hundreds to several thousands of Fm; that is, it only has a slightly intense orientation of < 110 > . But when a polycrystalline silicon thin film is formed by an acceleration voltage of 15 kV, crystals are relatively intensely oriented; that is, the crystal grains formed randomly in their orientations on the surface of a thin film during a film formation are destroyed by silicon ions.But the degree to which crystal grain axes having relatively uniform orientation of < 110 > and perpendicular to a substrate are destroyed is not so high by virtue of the channeling described above and maintain their crystal configuration. As a result, the crystal grains having the orientation of < 110 > cause destroyed crystal grains to regrow, resulting the formation of a polycrystalline silicon thin film having the intense orientation of < 110 > . As an applicable example of this method, a change of orientation of a polycrystalline silicon thin film may be controlled by injecting ions in an orientation oblique to the substrate.

As described above ion current is an important parameter for forming a polycrystalline silicon thin film having an intense orientation, high density and smoothness on the surface. In view of a generation range of a cascade caused by an injection of ions, one percent or more of atoms of all atoms which arrive at a surface of crystals is necessary for forming a favorable polycrystalline thin film.

Migration is accelerated by the presence of atoms more than this percentage and crystal grains which have slightly deposited on a substrate are resputtered, causing resulting silicon crystals to have high density and smoothness on the surface thereof. Theoretically, there has been no means provided for forecasting the quantity of ions by which a favorable orientation of crystals is performed, but experimentally, it has been found that the quantity of ions necessary for this purpose is more than one to several percent of atoms which arrive at crystals surface.

The embodiments of the invention will be described hereinafter.

Embodiment 1 A vacuum chamber shown in Figure 1 is evacuated less than 1 x 10-4, and thereafter, crucible 1, filled with silicon deposition source, is heated to 2,000"C to perform film formation at deposition rate of 100 A to 200 A per minute. As substrate 7, silicon single crystal substrate 10 having the orientation of < 110 > is used. This substrate is heattreated at 1,000"C to form 3,000 thick SiO2 film 11. As the substrate, quartz, CVD SiO2, CVD Si3N4 or other amorphous substrate such as glass which can withstand a temperature up to 500"C - 600"C, may be used.Substrate 7 is heated by infrared lamp 3 to keep the temperature of the substrate at 480"C. The acceleration voltages applied between crucible 1 and substrate 7 are 0.5 kV and 4.0 kV, respectively. Current density of ions which reached substrate 7 at this time is about 5 FA/cm2. The thickness of the silicon thin film thus formed is 0.2 slum.

Figure 2 shows a cross-sectional view of the structure of the sample in which 10 is a silicon (100) substrate; 11, an amorphous insulation thin film such as SiO2 or Si3N4 which has been formed on substrate 10; 12, a polycrystalline silicon thin film formed by means of the ionised cluster beam method. Figure 3 is an enlarged view of the sample formed by applying the acceleration voltage of 0.5 kV where 13 are silicon crystal grains having the orientation of < 110 > ; 14, silicon crystal grains having the orientation different from < 110 > . Figure 4 is an enlarged view of the surface of the example formed by applying the acceleration voltage of 4 kV in which 13' are silicon crystal grains having the orientation of < 110 > and 14' are silicon crystal grains having the orientation of < 110 > .

As apparent from the enlarged views of the surfaces of examples shown in Figure 3 and Figure 4, the number of crystal grains having an orientation of < 110 > is slightly more than the number of randomly oriented crystal grains 14 when the acceleration voltage of 0.5 is applied. The diameters of these crystal grains are 100 A to 300 A. The diffraction intensities of this example is measured by a diffractometer. Figure 5 shows the measured result of the diffraction intensity I relative to diffraction angle 2 0. The intense diffraction 18 seen near (2 0 = 69) is a diffraction from the (400) face of the silicon single crystal substrate. Diffractions are identified from poly-crystal silicon thin film 12, (i.e., 15, 16 and 17) diffracted from the faces (111), (220) and (311), respectively, having the peaks near (2 0 = 23, 47, 56). Diffraction intensities of X rays vary depending on the diffraction faces. The diffraction intensities of the silicon powder from the faces (220) and (311) are 0.6 and 0.35, respectively, when the diffraction intensity from the face (111) is assumed to be 1. Accordingly, the abundance ratios of the faces (111), (220), and (311) of the example are 1.0 :1.4 1.1, which means that the samples have a slightly intense orientation of < 110 > .

As shown in Figure 4, an example formed by applying the acceleration voltage of 4 kV mainly comprises crystal grains 13' which have the orientation of < 110 > . The diameters 13' of crystal grains having the orientation of < 110 > are 200 to 500 which is 1.5 as large as that of crystal grains formed by applying a lower acceleration voltage. The diffraction intensities of the example measured by a diffractometer is shown in Figure 6. As shown in Figure 6, as with the case in which crystal grains are formed by applying a low acceleration voltage, the diffractions 15', 16', and 17' of polycrystalline silicon thin film 12 from the faces (111), (220), (311) are identified. But, in this example, the diffraction 16' from the face (220) is much more intense than the diffraction 16.Conversely, the diffractions 15' and 17' from the faces (111) and (311) are not as intense as 15 and 17, respectively. The abundance ratio of the faces (111), (220), and (311) when the diffraction intensity from the face (111) is assumed to be 1 is 0.3 : 2.0 : 0.3, which means that the crystal grains of the example have an intense orientation of < 110 > .

From the foregoing, a polycrystalline silicon thin film having a highly preferred orientation can be formed on an amorphous substrate at temperatures less than 600"C by the silicon thin film forming method using accelerated silicon ions. A silicon thin film formed by using silicon ions has no cavities between grain boundaries. An observation by a transmission electron microscope showed silicon thin films thus formed are much more dense than a film formed by the vapor deposition method or the LPCVD method. Further, scanning electron microscope observation indicated that the surfaces of the silicon thin films thus formed are smooth; that is, irregularity thereof is less than t150A.

Embodiment 2 In a manner similar to Embodiment 1, polycrystalline silicon whose film thickness is 2,000 is deposited on a substrate (thermal oxide film 22 whose thickness is 1,000 A is formed on silicon 21) by applying acceleration voltages of O kV and 4 kV, and thin film transistors where deposited silicon acts as an active layer are formed to compare the characteristics of the two. As shown in Figure 7, these thin film transistors comprise active layer 23 consisting of the above described polycrystalline silicon, gate insulation film 24, and polycrystalline silicon 25.More specifically, formed on active layer 23 is gate insulation film 24 as thick as 1,000 formed by normal pressure CVD method. 4,000 thick polycrystalline silicon 25 for a gate electrode is formed on gate insulation film 24 by means of the ionised cluster beam method or the reduced pressure CVD method. Boron ions in the amount of 1 - 3 x 10'5/cm are injected to a gate electrode 25, a source, and a drain region (26 and 27) by applying 70 KeV to reduce the resistances of the thin film transistors.

AlSi films 5,000 A thick (film 26' and 27') are formed as sources and drain electrodes. After forming semiconductor devices, a hydrogen plasma treatment is made for one hour at 350"C by applying 100W of plasma electric power.

Figure 8, A and B, show a characteristic relationship between ISD and VG of semiconductor devices formed by applying the acceleration voltage of 4 KV and 0 KV in which the threshold of B is 17.0, and the threshold of A is as low as 10.7 V.

The carrier mobility of A is three times to four times as high as that of B; that is, a preferable value ( > = 20 to 30 cm2NS) is obtained from A.

As described above, according to the silicon thin film forming method, a polycrystalline silicon thin film having a density, smooth surface, and an intense orientation can be formed at low temperatures ( < 600 C). Using this method allows the use of non-heat resistant glass as a substrate. As a result, liquid crystal displays become inexpensive, and further, a production of large liquid crystal displays can be performed. A polycrystalline silicon thin film formed according to this method is not only dense and smooth on the surface, but it also has an intense orientation.Therefore, in a MOS transistor formed using the polycrystalline silicon thin film, the state density at the interface between a polycrystalline silicon and an oxide film is reduced, and the threshold voltage is also reduced. in addition, carrier mobility is improved because a crystal has a favorable orientation, and dispersion of the characteristic feature in a semiconductor device is also reduced.

A polycrystalline silicon thin film formed according to this method not only permits its application in wider ranges, but also improves the characteristics of a device in which the above described thin film is used. It is to be noted that an active matrix having a better performance can be constituted at leass than 600"C. A polycrystalline silicon thin film of this quality cannot be formed at this temperature range according to conventionai methods.

Thus, the polycrystalline silicon thin film formed according to the invention contributes greatly to a production of high quality displays such as a liquid crystal. The advantage obtained by the method for forming a thin film according to the invention is great.

Claims (6)

1. A method for forming a polycrystalline silicon thin film on an amorphous substrate comprising the step of: ionizing partially and accelerating evaporated silicon, bombarding the ionized silicon to said amorphous substrate simultaneously with neutral silicon to form a polycrystalline silicon thin film having a dense, smooth surface and an intense orientation on the substrate.

2. A method for forming a polycrystalline silicon thin film as claimed in Claim 1 comprising the steps of: heating and evaporating silicon material, which is an evaporation source, filled in a closed type of crucible having at least one nozzle; forming clusters consisting of atom groups by ejecting said evaporated silicon from said nozzle to an evaporation chamber utilizing condensation caused by an adiabatic expansion; ionizing at least a portion of evaporated silicon consisting of said cluster.

3. A method for forming a polycrystalline silicon thin film as claimed in Claim 1 or 2 comprising the step of: keeping the temperature of said amorphous substrate below 600"C, to which said evaporated silicon is bombarded by partially ionizing and accelerating.

4. A method for forming a polycrystalline silicon thin film as claimed in Claim 1 or 2 comprising the step of: applying an acceleration voltage to said silicon ion in the range of 1 - 5 kV.

5. A method for forming a polycrystalline silicon thin film as claimed in Claim 1 or 4 comprising the step of: introducing a gas to said vapor deposition chamber for controlling the characteristics of grain boundary or of doping.

6. A method for forming a polycrystalline silicon thin film as claimed in Claim 5 comprising the step of: using any one of H2 gas, PH3 gas, B2HG gas as said gas.