PROCESS FOR PREPARATION OF SEMICONDUCTOR DEVICE

Abstract: A process for the preparation of a photodiode or other semiconductor device by the plasma chemical vapour deposition of amorphous silicon on a substrate, which comprises generating plasma in a deposition furnace by using a high frequency wave of less than 1 MHz to deposit an amorphous silicon film, and then exposing the amorphous silicon film to an oxygen plasma.

Description: The present invention relates to the preparation of semiconductor devices by a process involving improving the electrical characteristics of amorphous silicon. The resultant devices are useful as, for instance, photodiodes. In the accompanying drawings Figure 2 shows an apparatus suitable for depositing an amorphous silicon film and Figure 3 shows a typical gap cell type photodiode. The photodiode of Figure 3 comprises a glass substrate 11, an amorphous silicon film (a-Si:H film) 12, an Al electrode 13 and reference numeral 14 represents a white light of, for example, 1000 1x. The apparatus in Figure 2 comprises a quartz jar 21 which serves as a reaction furnace, a heater 22, an electrode 23, a radio frequency (RF) generator 24, an inlet pipe 25 for gas, an exhaust opening 26 and a susceptor 28. The heater 22 heats the glass substrate 11 onto which amorphous silicon film is deposited during the process. The substrate and one end of the RF generator 24 are grounded, and when the RF generator 24 is turned on plasma 27 indicated by dots in Figure 2 is generated between the electrode 23 and the substrate 11, and Si of a silane gas (SiH₄) introduced through the gas-introducing pipe 25 is precipitated and deposited on the glass substrate 11 to form an amorphous silicon film (a-Si:H). A diluent gas such as H₂ or Ar may be added to SiH₄. It is understood that the structure of the so-prepared amorphous silicon is as follows: The plasma chemical vapour deposition process (CVD Process) using an exciting high frequency wave of 13.56 MHz has been adopted for formation of an amorphous silicon layer (e.g., Japanese Journal of Applied Physics, Vol. 21, No. 3, March, 1982, pp. 413-417). If this high frequency wave is used, the number of dangling bonds is small and amorphous silicon having excellent electrical characteristics [dark electroconductivity σd is small and bright electroconductivity (photoconductivity) σp is large] is easily obtained. However, this process is defective in that the formed amorphous silicon has poor heat durability (below 300°C) and poor adhesion to the glass substrate. High heat durability at temperatures below 300°C is necessary since otherwise the manufacturing process is difficult. Good adhesion to the substrate is necessary as otherwise the amorphous silicon film tends to peel from the substrate. Small σd and large σp are preferred for the electrical characteristics, especially when the device is to be used as a photodiode. It would therefore be desirable to be able to deposit an amorphous silicon film by a plasma CVD process using RF so as to provide an amorphous silicon layer that has satisfactory electrical properties and also has excellent heat durability and good adhesion. According to the invention a semiconductor device is made by a process comprising plasma CVD of amorphous silicon onto a substrate in a deposition furnace in which the plasma is generated using a high frequency wave, and the high frequency wave has a frequency of less than 1 MHz and the resultant amorphous silicon is exposed to O₂ plasma. In the preferred method of the invention the O₂ plasma is generated in the same deposition furnace as the furnace in which the amorphous silicon is deposited and the amorphous silicon is irradiated with the oxygen plasma, and thus the process can be conducted sequentially, with the oxygen irradiation following substantially immediately after the formation of the amorphous silicon. It is understood that when the amorphous silicon film is exposed to O₂ plasma as described above, since O₂ plasma has a high activity, O * radicals are included in dangling bonds and the dark electroconductivity is further reduced. Figure 1 is a graph illustrating the relationship between the time of irradiation with O₂ plasma and the electroconductivity in a-Si:H; Figure 2 is a sectional view of the apparatus used in carrying out the present invention; Figure 3 is a sectional view of a gap cell type photodiode; Figure 4 is a diagram illustrating the localised level in a-Si:H. The present invention will now be described in detail, by way of example, with reference to the accompanying drawings. With reference to the structure of the amorphous silicon film again, during the growth of amorphous silicon, a-Si:H is formed by bonding H to Si in SiH₄ Although the number of dangling bonds is small in the growth of amorphous silicon, dangling bonds are still present, as is seen from the above-mentioned structural formula. When the amorphous silicon film is deposited using a high frequency wave having a frequence of, for instance, 13.56 MHz, the number of SiH₂ bonds is large. This leads to poor heat durability and since the contribution of ions is small it also results in poor adhesion. However the number of dangling bonds is small and this produces excellent electrical characteristics. On the other hand, where the film is deposited using a lower frequency wave of, for instance, less than 1 MHz the number of SiH₂ bonds is small. This improves the heat durability and the contribution of ions is large and so this leads to excellent adhesion, but the number of dangling bonds is large and this leads to poor electrical characteristics. When the conduction band and valence band of amorphous silicon are examined, it is seen that, as shown in Figure 4, a localised level indicated by a broken line is present between both bands, which is due to the presence of dangling bonds in the amorphous silicon. The dark electroconductivity of amorphous silicon depends on the conduction of electrons, indicated by black dots in Figure 4, thermally excited to the conduction band from the localised level. Therefore, a further decrease of the number of dangling bonds is effective for reducing the dark electroconductivity. According to the present invention, an amorphous silicon film is grown at a high frequency on the glass substrate 11, for instance using the apparatus shown in Figure 2, and the supply of the SiH₄ gas is stopped and an O₂ gas is supplied instead into the quartz jar 21 to form O₂ plasma. The substrate 11 is irradiated with this O₂ plasma. In the present invention, the plasma CVD process may be carried out under usual conditions except for the use of a relatively low frequency wave below 1 MHz, preferably 30 to 500 KHz. Then, the O₂ plasma treatment may be carried out under the conditions of a substrate temperature of 200 to 350°C, a pressure of 26.7 to 166.6 Pa (0.2 to 2 Torr). and a power of 50 to 500 mW/cm². The frequency is not critical in the O₂ plasma treatment, and a wave of a frequency of 30 KHz to 13.56 MHz may suitably be used. The process may be carried out in the following manner. A vacuum of 133.3 Pa (1 Torr) is maintained in the quartz jar 21 and a high frequency wave having a low frequency of 200 KHz having a power of 100 mW/cm² is generated by the RF generator, and an SiH₄ gas is used for the growth of a-Si:H. After an a-Si:H film having a predetermined thickness, generally of about 1 µm, has been deposited, an O₂ gas is supplied instead of the SiH₄ gas. The time of irradiation with O₂ plasma is for at least 20 minutes, often 30 to 40 minutes. The so-obtained a-Si:H film is satisfactory in all of the heat durability, the adhesion to the substrate and the electrical characteristics. The frequency of the high frequency wave generated by RF is not limited to the above-mentioned value, and RF of the order of 30 KHz to 500 KHz can be used. We examined the relation between the time of irradiation with O₂ plasma and the electroconductivity and obtained the results shown in Figure 1. Note, the O₂ plasma irradiation time, expressed by minutes, is plotted on the abscissa and the electroconductivity (Ω-cm⁻¹), is plotted on the ordinate. In Figure 1, white circles and black circles indicate the photoconductivity and dark electroconductivity of a-Si:H formed by using a high frequency wave having a frequency of 200 KHz, respectively, and white triangles and black triangles indicate the photoconductivity and dark electroconductivity of a-Si:H formed by using a high frequency wave having a frequency of 13 MHz, respectively. As shown in Figure 1, the dark photoconductivity of a-Si:H formed by a high frequency wave having a frequency of 200 KHz is higher before irradiation with O₂ plasma than that of a-Si:H formed by using a high frequency wave having a frequency of 13 MHz, but when irradiation with O₂ plasma is carried out for 30 minutes, the dark electroconductivity becomes substantially equal to that of a-Si:H formed by using a high frequency wave having a frequency of 13 MHz. On the other hand, the photoconductivity of a-Si:H formed by using a high frequency wave having a frequency of 200 KHz is slightly higher before irradiation with O₂ plasma than that of a-Si:H formed by using a high frequency wave of 13 MHz, and the value of the photoconductivity is increased as the time of irradiation with O₂ plasma is prolonged. Thus the irradiation with O₂ plasma leads to reduction of the dark electroconductivity and an increase in the photoconductivity, and yet the resultant a-Si:H film has a high heat durability (up to 450°C according to our experiments) and a good adhesion to the substrate. Thus for the first time it is possible to combine good electrical properties with good heat durability and adhesion properties. The resultant product is of particular value as a photodiode. If the a-Si:H is used for TFT, a transistor in which the I on /I off ratio can be increased is obtained, and a solar cell in which the change with the lapse of time is small is obtained. Although the plasma treatment after the plasma CVD deposition has also been carried out using, instead of the O₂ gas, a H₂, CO₂, CO, N₂O, N₂ or NH₃ gas, no significant improvement of the electrical characteristics of the deposited amorphous silicon film was attained.

Claims: A process for manufacturing a semiconductor device comprising plasma CVD of amorphous silicon onto a substrate in a deposition furnace in which the plasma is generated using a high frequency wave, characterised in that the high frequency wave has a frequency of less than 1 MHz and the resultant amorphous silicon film is exposed to O₂ plasma. A process according to claim 1 in which the O₂ plasma is generated in the same deposition furnace and the amorphous silicon film is irradiated by the plasma in that furnace. A process according to claim 1 or claim 2, in which the plasma chemical vapour deposition is carried out using a wave of a frequency of 30 to 500 KHz, preferably about 200 KHz. A process according to claim 3, wherein the plasma chemical vapour deposition is carried out with a power of about 100 mW/cm² and preferably under a pressure of about 133 Pa (1 Torr). A process according to any preceding claim, wherein the oxygen plasma treatment is carried out with a wave of a frequency of 30 KHz to 13.56 MHz. A process according to any preceding claim, wherein the oxygen plasma treatment is carried out at a substrate temperature of 200 to 350°C. A process according to any preceding claim, wherein the oxygen plasma treatment is carried out under a pressure of 26.7 to 166.6 Pa (0.2 to 2 Torr). A process according to any preceding claim, wherein the oxygen plasma treatment is carried out with a power of 50 to 500 mW/cm². A process according to any preceding claim, in which the exposure to oxygen is conducted for at least 20 minutes. A process according to any preceding claim, in which the semiconductor is a photodiode.

Inventors: TAKASAKI, KANETAKE
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: H01L31
IPC codes: C23C16, H01L21, H01L31