Method of removing a layer of organic matter

Abstract: A method removes a first layer (31, 31a, 33) of an organic matter which is formed on a second layer (16), where the first layer is subjected to an ion implantation. The method includes the steps of generating a plasma by exciting a gas which includes H₂O using a high-frequency energy source, and removing the first layer within the plasma.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to methods of removing layers of organic matters, and more particularly to a method of removing a layer of an organic matter such as a resist which is used as a mask during an ion implantation and is degraded by the ion implantation. Ion implantation is used in production steps of a semiconductor device. When making the ion implantation, a layer of an organic matter is used as a mask, and a resist is generally used as the organic matter. Recently, some production steps make the ion implantation with a high dosage of 1 x 10¹⁴ atoms/cm². During the ion implantation, the resist is degraded and it is extremely difficult to completely remove the degraded resist by the normal O₂ plasma ashing. A large quantity of residue remains even after the O₂ plasma ashing, and a surface on which the residue exists must be slightly etched to completely remove the residue. As a result, the freedom with which the semiconductor device may be designed is limited by the need to carry out an etching after the O₂ plasma ashing. On the other hand, the present inventors have proposed a method of removing the resist by a dry process using a plasma which is obtained by exciting a gas which includes hydrogen but includes no oxygen. EP-A-0 304 068 shows such a process. According to the proposed method, however, the ashing rate is slow even when a 100 % H₂ gas is used. For this reason, this proposed method is impractical for industrial applications. For example, the ashing rate is approximately 7 x 10⁻⁸m/min (700 Å/min) for a positive resist OFPR800 manufactured by Tokyo Ooka Kohgyo Kabushiki Kaisha of Japan when a reactive ion etching (RIE) is carried out using a H₂ gas with a flow rate of 300 cc/min, pressure of 66,65 Pa (0.5 Torr) and an RF power of 300 W. The H₂ gas is flammable and it is dangerous to use H₂ gas of a high density. But when the density of the H₂ gas is set to 3 % or less which is below the explosive limits, the ashing rate with the above described conditions falls to under (100 Å/min) 1x10⁻⁸m/min. Therefore, there is a demand for a method of completely removing the degraded resist at a high speed without danger. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful method of removing a layer of an organic matter, in which the problems described above are eliminated and the above described demand is satisfied. Another and more specific object of the present invention is to provide a method of removing a layer of an organic matter which is formed on a wafer, where the layer being subjected to an ion implantation, comprising the steps of generating a plasma by exciting a gas which mainly includes H₂O using a high-frequency energy source, and removing the first layer within the plasma. According to the method of the present invention, it is possible to completely remove the first layer which is degraded by the ion implantation. The removal of the layer can be carried out at a high speed without damaging the wafer. Furthermore, the method is safe especially when compared to the method which uses a H₂ RIE, and the process can thus be carried out using an inexpensive equipment. Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a side view in cross section generally showing an RIE apparatus which is used to carry out a method of removing a layer of an organic matter according to the present invention; FIG.2 is a side view in cross section generally showing a wafer holding and cooling mechanism of the RIE apparatus shown in FIG.1; FIGS.3A and 3B respectively are plan views showing residue after an O₂ RIE with magnifications of 750 and 10,000 times; FIGS.4A and 4B respectively are plan views showing a wafer surface after a downstream ashing which follows the O₂ RIE with magnifications of 750 and 10,000 times; FIG.5 is a plan view showing a wafer surface after the downstream ashing; and FIG.6 is a cross sectional view showing residue on the wafer surface. DESCRIPTION OF THE PREFERRED EMBODIMENTS A description will be given of a first embodiment of a method of removing a layer of an organic matter according to the present invention. The positive resist OFPR800 referred above is formed on a silicon (Si) wafer to a thickness of approximately 1.1 µm and is baked for 2 minutes at 120°C to form a sample. Arsenide ions (As⁺) are injected into this sample with a dosage of 1 x 10¹⁶ atoms/cm² and an energy of 70 keV. Then, the degraded resist is removed by three different processes, namely, an O₂ RIE, a H₂ RIE and a H₂O RIE. A parallel plate type RIE apparatus shown in FIG.1 is used for the three different RIE processes. In FIG.1, the RIE apparatus generally comprises a reaction chamber 11, an opposing electrode 12, an RF power source 13 of 13.56 Hz, an insulator 14, a D.C. power source of 2 to 3 kV, a quartz (SiO₂) 17, an etching gas introducing opening 18, an introducing opening 19 for cooling water and gas, an exhaust opening for the cooling water and gas, and an exhaust opening 21 for vacuum exhaust. A wafer 16 is positioned within the RIE apparatus as shown. The RIE apparatus shown in FIG.1 is well known, and is different from an apparatus which generates an electron cycrotron resonance (ECR) plasma (electron cycrotron resonance plasma). FIG.2 shows a wafer holding and cooling mechanism. In FIG.2, the wafer holding and cooling mechanism generally comprises an insulator 22, an electrostatic chuck electrode 23, an RF electrode (cathode) 24, a cooling water entrance 25, a cooling gas entrance 26, an exhaust opening 27 for the gas, and a waste opening 28 for the water. In the first embodiment, the O₂, H₂ and H₂O respectively have a flow rate of 300 cc/min, a pressure of 66,65 Pa (0.5 Torr) and an RF power of 300 W. A stage temperature is 5°C, and the wafer temperature is maintained to 50°C or less during each of the RIE processes. The results of the RIE processes are confirmed by studying the wafer surface. FIGS.3A and 3B respectively show the residue on the wafer surface after the O₂ RIE with magnifications of 750 and 10,000 times. FIGS.3A and 3B are based on photographs of the wafer surface. It can be seen from FIGS.3A and 3B that a large quantity of residue 31 exists on the wafer surface. On the other hand, no residue was found on the wafer surface after the H₂ RIE and the H₂O RIE, even in the photographs of the wafer surface having the magnification of 10,000 times. The time required to remove the residue on the wafer surface was 2 minutes and 40 seconds for the O₂ RIE, 15 minutes for the H₂ RIE and 3 minutes and 15 seconds for the H₂O RIE. The over-ashing is 50 to 100 % for the O₂ RIE, and 20 to 30 % for both the H₂ RIE and H₂O RIE. Hence, it was confirmed that the H₂O RIE is superior compared to the O₂ RIE and H₂ RIE when removing the residue on the wafer surface. Therefore, it was confirmed that the H₂O RIE is effective in removing by ashing the resist which is degraded by the ion implantation. When removing the resist which is used as a mask during the ion implantation, the thickness of the degraded resist layer is measured beforehand, and the H₂O RIE is carried out for a time which is required to completely remove the degraded resist layer. After removing the degraded resist layer, the remaining resist layer which is not degraded may be removed by a known wet etching so as to minimize the damage to the wafer surface. Hereunder, the resist layer which is not degraded by the ion implantation will simply be referred to as a non-degraded resist. However, when the non-degraded resist layer is subjected to the ashing using a dry etching, the resist layer can be removed within a shorter time without the need for a wet etching. Hence, the present inventors conducted the experiments outlined in a second embodiment which is described hereunder. In the second embodiment, the positive resist OFPR800 is formed on the Si wafer to a thickness of approximately 1.1 µm, and is baked for 2 minutes at 120°C to form a sample. This sample is directly subjected to an ashing by three different processes, namely, an O₂ RIE, a H₂ RIE and a H₂O RIE, so as to measure the ashing rate for each RIE. The stage temperature is 5°C and the flow rate of the gas is 300 cc/min for each RIE. The following Table 1 shows the measured results. From Table 1, it can be seen that the O₂ RIE is extremely effective in removing the resist by the ashing when the resist is not much affected by the ion implantation. Accordingly, when the degraded resist is removed by the H₂O RIE and the O₂ RIE is carried out thereafter to remove the non-degraded resist, the time required to completely remove the resist can be shortened further. On the other hand, because the H₂O RIE is much more effective than the H₂ RIE, it is practical to remove both the degraded resist and the non-degraded resist by continuously carrying out the H₂O RIE. Next, a description will be given of a third embodiment. The present inventors conducted experiments on the removal of the non-degraded resist after the removal of the degraded resist. The same sample is used as in the first embodiment, and approximately 2x10⁻⁷m (2000 Å) of the degraded resist layer is removed by the RIE. The ashing time of this RIE is 30 seconds for the O₂ RIE, 3 minutes and 41 seconds for the H₂ RIE and 1 minute for the H₂O RIE, where the pressure is 66,65 Pa (0.5 Torr) and the RF power is 300 W for each RIE. Thereafter, the remaining non-degraded resist layer is removed by a downstream of O₂+N₂ (10 % N₂) gas at a stage temperature of 150°C. As a result, no residue was found on the wafer surface after the H₂ RIE + (O₂+N₂) downstream ashing, and no residue was found on the wafer surface after the H₂O RIE + (O₂+N₂) downstream ashing, even in the photographs of the wafer surface having the magnification of 10,000 times. Therefore, the practicality of the H₂ RIE + (O₂+N₂) downstream ashing and the H₂O RIE + (O₂+N₂) downstream ashing was confirmed. In the case of the O₂ RIE + (O₂+N₂) downstream ashing, the residue 31 was found on the wafer surface as shown in FIGS.4A and 4B which respectively show the wafer surface with magnifications of 750 and 10,000 times. FIGS.4A and 4B are based on photographs of the wafer surface. It can be seen from FIG.4B that a large quantity of the residue 31 exists on the wafer surface, and rough portions 32 are formed on the wafer surface. The rough portions 32 are formed when the wafer surface is scraped. Therefore, although the ashing time of the O₂ RIE is shorter than that of the H₂ RIE, the residue 31 remains and the rough portions 32 are formed on the wafer surface even when the O₂ RIE is changed to the downstream ashing at a suitable time. On the other hand, the ashing time of the H₂O RIE is considerably shorter than that of the H₂ RIE, and it was confirmed that no residue remains and there is no damage to the wafer surface after the H₂O RIE. For the sake of comparison, the present inventors also carried out an experiment for the O₂+N₂ downstream ashing only. But in this case, the residue 31 remains on the wafer surface as shown in FIG.5 which is based on a photograph of the wafer surface with a magnification of 750 times. In addition, it was confirmed that a non-degraded resist 33 remains within some residue portions 31a. For this reason, it was found that this method of O₂+N₂ downstream ashing only is impractical. It is assumed that the residue (degraded resist) 31, the residue portions (degraded resist) 31a and the non-degraded resist 33 adhere on the surface of the wafer 16 as shown in FIG.6. Accordingly, when removing the resist, the residue remains on the wafer surface when the O₂ RIE is used. No residue remains on the wafer surface when the H₂ RIE or the H₂O RIE is used. In addition, no residue remains on the wafer surface when the H₂ RIE or the H₂O RIE and the (O₂+N₂) downstream ashing are used. But when the O₂ RIE and the (O₂+N₂) downstream ashing are used, rough portions are formed on the wafer surface. When the H₂ RIE and the H₂O RIE are compared, the ashing time required in the case of the H₂O RIE is approximately 1/4 or less than the ashing time required in the case of the H₂ RIE. Furthermore, with respect to the non-degraded resist, the ashing rate of the H₂O RIE is several times faster than that of the H₂ RIE. Therefore, it was confirmed that the H₂O RIE is a practical method of removing the resist. A Japanese Laid-Open Patent Application No.63-216346 proposes a method of removing a layer of an organic matter by adding O₂ to introduce a gas which is added with H₂O into the electron cycrotron resonance (ECR) plasma and decomposing the gas within the ECR plasma. But in the embodiments of the present invention, the energy source for generating the plasma is a high-frequency (RF) energy source and the ECR plasma is not used. In addition, the embodiments use only the H₂O plasma and does not use the O₂+H₂O plasma. In the described embodiment, the (O₂+N₂) downstream ashing is used. However, it is possible to use a (O₂+H₂O) downstream ashing in place of the (O₂+N₂) downstream ashing. In this case, it is possible to reduce the degradation of the resist caused by Na.

Claims: A method of removing a layer of an organic matter which is formed on a wafer (16), said layer being subjected to an ion implantation, characterized in that said method comprises the steps of: generating a plasma by exciting a gas which mainly includes H₂O using a high-frequency energy source; and removing the layer within said plasma. The method as claimed in claim 1, characterized in that said step of removing the layer removes a first portion of the layer, and said method further comprises the step of removing a remaining portion of the layer by a downstream ashing using O₂. The method as claimed in claim 2, characterized in that the first portion of the layer is degraded by the ion implantation and the remaining portion of the layer is virtually unaffected by the ion implantation. The method as claimed in claim 3, characterized in that the layer is made of a resist. The method as claimed in claim 4, characterized in that the step of removing the remaining portion uses a downstream ashing using O₂+N₂. The method as claimed in claim 4, characterized in that the step of removing the remaining portion uses a downstream ashing using O₂ + H₂O. The method as claimed in claim 1, characterized in that said step of removing the layer removes a first portion of the layer, and said method further comprises the step of removing a remaining portion of the layer by a wet etching. The method as claimed in claim 7, characterized in that the first portion of the layer is degraded by the ion implantation and the remaining portion of the layer is virtually unaffected by the ion implantation. The method as claimed in any of claims 1 to 8, characterized in that said step of generating the plasma uses a parallel plate type ractive ion etching apparatus to excite the gas which mainly includes H₂O. The method as claimed in claim 1, characterized in that said step of removing the layer removes a first portion of the layer, and said method further comprises the step of removing a remaining portion of the layer by a downstream ashing using a gas mixture which includes one of O₂, H₂O and N₂. The method as claimed in any of claims 1 to 10, characterized in that the layer is subjected to an ion implantation at a dosage of at least 1 x 10¹⁴ atoms/cm².

Inventors: FUJIMURA SHUZO, HIKAZUTANI KENICHI, FUJIMURA, SHUZO, HIKAZUTANI, KENICHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, G03F7