Use of plasma in ashing processes

Abstract: In an oxidation process for ashing an organic film on a supporting substrate (12), mounted in a working chamber, an oxygen-containing gas is ionized to form a plasma, comprising neutral particles and charged particles, in an ionisation chamber (16). Such particles can pass into the working chamber from the ionisation chamber (16) by way of a plasma filtering plate (24) which inhibits the passage of high-energy charged particles to the organic film. The substrate (12) is spaced from the filtering plate (24) by a distance that is preselected to ensure that some high-­ energy charged particles from the plasma impinge upon the organic film so that the ashing process is thereby speeded up without producing more than a preselected acceptable level of contamination of the substrate.

Description: The present invention relates to use of plasma in ashing processes, in a method of removing an organic coating film from a substrate material such as a semiconductor wafer. In the processing of a wafer of a semiconductor device, the basic operations of formation of a film, formation of a resist pattern, etching using the resist as a mask, and removal of the resist are repeated. As the resist-removing method in the above-mentioned cycle, a method is adopted in which the resist is ashed by using a plasma, and in this method, the resist must be removed efficiently without damage to the surface of the semiconductor wafer. An ashing treatment apparatus using a plasma is shown in Fig. 5. This ashing treatment apparatus is an oxygen down-stream type apparatus in which an oxygen plasma is applied to a semiconductor wafer . Apparatus of this type is also described in EP-A-0237078. In the Figure 5 apparatus, a heating stage 14 on which a semiconductor wafer 12 to be treated is placed is arranged in an ashing treatment chamber 10 for effecting the ashing treatment, a gas-introducing opening 18 is formed in a plasma-generating chamber 16 for generating a plasma, to introduce an oxygen-containing gas to be converted to a plasma, and a microwave waveguide 22 is disposed above a quartz plate 20. An electroconductive plate 24 having many through holes, called a "shower plate", is arranged between the plasma-generating chamber 16 and the ashing treatment chamber 10, and this shower plate 24 is earthed to capture charged particles having a high energy, such as charged active species and electrons, in the plasma generated in the plasma-generating chamber 16, and allow a transmission of neutral active seeds which are electrically neutral and have a low energy. If the plasma is directly applied, the surface of the semiconductor wafer 12 will be severely damaged by charged particles having a high energy, and the shower plate 24 is provided to eliminate this disadvantage. An exhaust opening 26 is arranged in the ashing treatment chamber 12 to discharge the gas formed by ashing. When the ashing treatment is carried out by using this ashing treatment apparatus, the semiconductor wafer 12 to be subjected to the ashing treatment, and to which an organic coating film such as a resist is adhered, is placed on the heating stage 14 and heated. The oxygen-containing gas to be converted to a plasma is introduced into the plasma-generating chamber 16 from the gas-introducing opening 18, and simultaneously, a microwave of 2.45 ± 0.1 GHz is applied to the microwave waveguide 22. The microwave is transmitted through the quartz plate 20 and acts on the gas in the plasma-generating chamber 16 to generate a plasma, and the generated plasma flows into the ashing treatment chamber 10 through the shower plate 24. The charged particles in the plasma are captured by the shower plate 24 and only the neutral active species which are not charged permeate through the shower plate 24 and impinge against the semiconductor wafer 12 on the heating stage 14. Accordingly, the shower plate 24 acts as a plasma-transmitting plate for capturing the charged particles in the plasma generated in the plasma-generating chamber 16 and allowing a transmission of the neutral active seeds. When the neutral active species impinge against the semiconductor wafer 12, mainly the neutral active seeds of oxygen oxidize and ash the organic substance coating film, for example, the resist, on the semiconductor wafer 12, and the gas formed by ashing is discharged from the exhaust opening 26. The results of the measurement of the ashing characteristics obtained by the above-described ashing treatment apparatus are shown in Fig. 6. A resist (OFPR800 supplied by Tokyo Oka) of a novolak resin having a thickness of 1.5 µm, which is formed, by coating, on a semiconductor wafer 12 having a thickness of 10.16 cm (4 inches), is ashed under the following conditions. Namely, the output of the microwave is 1.5 kW, the pressure in the ashing treatment chamber 10 is 106 Pa (0.8 Torr), the oxygen flow rate in the introduced gas is 1 SLM, and the distance between the shower plate 24 and the semiconductor wafer 12 is 35 mm. The temperature Θ(°C) of the semiconductor wafer 12 is changed in the range of from 120 to 200°C, and the ashing rate (Å/min) is measured. Figure 6 is an Arrhenius plot in which the ashing rate V (Å/min) is plotted on the ordinate and (1/T) x 10³ (°K⁻¹), in which T represents the absolute temperature (°K) calculated by adding 273 to the temperature Θ (°C), is plotted on the abscissa. From the gradient of this graph, it is seen that the activation energy at the ashing treatment is about 0.5 eV. The activation energy value of 0.5 eV is in full agreement with the value measured when the ashing treatment is effected mainly by a chemical reaction between active species of oxygen and the resist (J.M. Cook and B.W. Benson, J.Electrochem. Soc., Vol. 13, No. 12, P.2459 (1983); J.E. Spencer, R.A. Borel and A. Hoff, Electrochemical Society Extended Abstracts, Vol. 86-2, Oct. 19 - 24 (1986), Abstract No. 285). As apparent from the graph of Fig. 6, in the ashing treatment concerned, the ashing rate greatly depends on the temperature of the semiconductor wafer 12. More specifically, when the temperature of the semiconductor wafer 12 is 200°C, the ashing rate is 20 x 10³ (Å/min), but as the temperature is lowered, the ashing rate is drastically reduced and when the temperature is 140°C, the ashing rate is about 3.8 x 10³ (Å/min), i.e., lower than 1/4 of the ashing rate attained at 200°C. It is difficult to keep the temperature of the semiconductor wafer 12 constant, and the ashing rate is greatly changed by a slight change of the temperature, and thus it is difficult to obtain a stable ashing rate. This temperature dependency however, is peculiar to the chemical reaction of the ashing treatment, and therefore, it has been considered that, to moderate this temperature dependency, it is necessary to change the reaction per se of the ashing treatment by changing the resist or the introduced gas, but it is difficult to obtain an improvement by this technique. To reduce contamination, preferably the ashing treatment is carried out at a lower temperature, but with such an ashing treatment, especially at low temperature, the ashing rate may be undesirably low requiring an undesirably long time for completion of the ashing treatment, and therefore, such an ashing treatment can suffer from a problem of a low productivity. According to the present invention there is provided an oxidation process for ashing an organic film on a supporting substrate, mounted in a working chamber, in which process an oxygen-containing gas is ionised to form a plasma, comprising neutral particles and charged particles, in an ionisation chamber from which such particles can pass into the said working chamber by way of a plasma filtering plate for inhibiting the passage of high-energy charged particles to the said film; characterised in that for an initial part of the process the said substrate is spaced from the said filtering plate by a distance preselected to ensure that during that initial part of the process some high-energy charged particles from the plasma impinge upon the said organic film, whereafter the spacing between the said substrate and the said filtering plate is increased gradually, during the course of the said process, to reduce the danger of contamination of the said substrate by such high-energy charged particles from the plasma impinging upon the said organic film. In such a process the ashing rate is less susceptible to changes in temperature, and the ashing treatment can be carried out at a desirably high ashing rate. In an embodiment of the present invention, since charged particles of high energy temperature which have been transmitted through the plasma filtering plate without being captured thereby are applied to the film to be treated together with neutral active species which have been transmitted through the plasma filtering plate, the ashing speed is elevated as a whole and the temperature dependency of the process is moderated. Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a sectional view illustrating an ashing treatment apparatus for putting into effect an embodiment of the present invention; Fig. 2 is a graph illustrating characteristics of ashing treatment processes; Fig. 3 illustrates graphically the distance dependency of electron temperature; Fig. 4 is a graph showing contamination characteristics in ashing treatment processes; Fig. 5 is a sectional view illustrating an alternative ashing treatment apparatus; and Fig. 6 is a graph illustrating ashing treatment characteristics of a process described with reference to Fig. 5. Elements having the same structure as in the ashing treatment apparatus shown in Fig. 5 are represented in Fig. 1 by the same reference numerals as used in Fig.5, and an explanation of these elements is omitted. The ashing treatment apparatus of Figure 1 includes a vertical driving device (distance-adjusting means) 28 for vertically driving the heating stage 14, to adjust the distance between the semiconductor wafer 12 to be subjected to the ashing treatment and the shower plate (plasma filtering plate) 24 and change the mounting position of the semiconductor wafer 12. A pole 14a of the heating stage 14 pierces the wall of the ashing treatment chamber (working chamber) 10 and is connected to the vertical driving device 28 disposed outside thereof. A worm gear mechanism 28b is rotated by a motor 28a to vertically drive the pole 14a. The influence of the adjustment of the distance between the shower plate 24 and the semiconductor wafer 12 by driving the heating stage 14 by the vertical driving device 28 during the ashing treatment will now be described. The plasma generated in the plasma-generating chamber (ionisation chamber) 16 contains not only uncharged neutral active species but also charged particles such as charged active species and electrons. The shower plate 24 is disposed to capture the charged particles and allow the transmission of only the neutral active species which are electrically neutral and have a low energy. In practice, however, the charged particles are not completely captured by the shower plate 24, and parts thereof flow into the ashing treatment chamber 10. In the charged particles which have flowed into the ashing treatment chamber 10 through the shower plate 24, the electron temperature is lowered as the distance from the shower plate 24 increases. Accordingly, the adjustment of the distance from the shower plate 24 results in the adjustment of the electron temperature of the charged particles acting on the semiconductor wafer 12. In the ashing treatment apparatus of Figure 1 the electron temperature of the charged particles acting on the semiconductor wafer 12 can be adjusted by adjusting the distance between the shower plate 24 and the semiconductor wafer 12 by the vertical driving device 28. As apparent from the foregoing description, in the ashing treatment apparatus of Figure 1 the semiconductor wafer is mounted at a position apart by an optional distance from the shower plate, to adjust the electron temperature of the acting charged particles, and the ashing treatment can be carried out in this state. An ashing treatment process using the ashing treatment apparatus of Figure 1 will now be described. The results of the measurement of the ashing treatment characteristics, conducted by changing the distance between the shower plate 24 and the heating stage 14 (defined as the distance between the lower face of the shower plate 24 and the upper face of the heating stage 14) in such a process using the Figure 1 apparatus, are shown in Fig. 2. Conditions such as the output of the microwave, the pressure in the ashing treatment apparatus 10, the flow rate of oxygen in the introduced gas, and the resist to be treated are the same as those adopted for obtaining the data shown in Fig. 6. In Fig. 2, P shows the characteristics of the Fig. 5 ashing treatment, where the distance between the shower plate 24 and the heating stage 14 (hereinafter referred to as "distance from the shower plate 24") is 35 mm, R shows the ashing characteristics obtained when the distance from the shower plate 24 is 10 mm, and S shows the ashing characteristics obtained when the distance from the shower plate 24 is 5 mm. From these ashing characteristics P, R, and S it is seen that, as the distance from the shower plate 24 becomes short, the ashing rate V is increased as a whole and the activation energy is lowered. Namely, in the ashing characteristics P obtained when the distance from the shower plate 24 is 35 mm, the activation energy is about 0.5 eV, but in the ashing characteristics R and S obtained when the distances from the shower plate 24 are 10 mm and 5 mm, the activation energy is about 0.16 eV. The lowering of the activation energy results in a moderation of the temperature dependency of the reaction speed V. Accordingly, if the distance from the shower plate 24 is made shorter than 35 mm and is adjusted to less than 20 mm, the ashing speed V is not greatly changed even when the temperature of the semiconductor wafer 12 is changed, and the ashing treatment can be completed in a shorter time than in the conventional process, whereby the productivity can be improved. The mechanism of the ashing treatment will now be considered. As pointed out hereinbefore, the activation energy is peculiar to the chemical reaction, and the value of the activation energy is not changed unless the reaction per se, i.e., the introduced gas or resist, is changed. Nevertheless, in the process of Figure 1, even though the reaction per se is not changed, the activation energy is lowered only by shortening the distance from the shower plate 24. It is considered that in a process embodying the present invention, ashing is advanced by not only a pure chemical action but also a physical action. The plasma making a contribution to the ashing contains not only uncharged neutral active species but also charged particles such as charged active species and electrons. It is known that, since the neutral active species are not charged, the neutral active species exert only a chemical action on an organic substance coating film to be ashed, such as a resist. In contrast, it is considered that the charged particles such as charged active species or electrons promote ashing by a chemical action and physical action singly or in co-operation with the neutral active species. This mechanism has not been sufficiently elucidated, but by the action of the charged articles on the coating film of the organic substance together with the action of the neutral active species, ashing is accelerated and the activation energy is lowered. As indicated above, in the ashing treatment process of Figure 1 the charged particles as well as the neutral active species in the plasma make a contribution to the advance of the ashing reaction. In this regard, the relationship between the distance from the shower plate and the electron temperature of charged particles and the recovery time is examined. The results are shown in Fig. 3 and 4, respectively. The electron temperatures shown in Fig. 3 are those measured by the Langmuir probe method. From Fig. 3 it is seen that, as the distance from the shower plate becomes shorter, the electron temperature becomes higher. Accordingly, it is considered that the ashing rate is high in Fig. 2 when the distance from the shower plate is 5 mm or 10 mm, because charged particles (electrons) having a high electron temperature contribute to the ashing reaction. The contamination of the semiconductor wafer during ashing treatment is caused by an intrusion of contaminant molecules contained in the resist, such as iron, to the semiconductor wafer. Where neutral active seeds in the plasma act on the resist, since only the chemical reaction occurs, contaminant molecules contained in the resist do not intrude to the semiconductor wafer. However, where charged particles in the plasma act on the resist, since the energy of the charged particles is high, the charged particles are violently implanted to the resist and the implanted charged particles impinge against the contaminant molecules in the resist, whereby the contaminant molecules are implanted and intruded to the semiconductor wafer. Accordingly, the intensity of the action of the charged particles of the plasma can be determined. Figure 4 shows the results obtained by preparing an MOS diode and measuring the degree of contamination by the CT method. The recovery time (sec) of the MOS diode indicating the contamination degree is plotted on the ordinate. A shorter recovery time indicates a higher contamination degree. The points for the measurement of the contamination degree are shown on the abscissa, and points A through E of the semiconductor wafer are the measurement points. The contamination characteristics P are those obtained in the Fig. 5 process where the distance from the shower plate 24 is 35 mm, the contamination characteristics Q are those obtained when the distance from the shower plate 24 is 15 mm, and the contamination characteristics R are those obtained when the distance from the shower plate 24 is 10 mm. Furthermore, the contamination characteristics S are those obtained when the distance from the shower plate 24 is 5 mm, and the contamination characteristics 0 are those obtained when the resist processing such as the resist coating or resist ashing is not carried out. Moreover, the contamination characteristics T are those obtained when the treatment is carried out in the plasma. From the contamination characteristics P through S it is seen that, as the distance from the shower plate 24 becomes shorter, the contamination is advanced as a whole. Accordingly, it is seen that, as the distance from the shower plate 24 becomes shorter, the intensity of the action of charged particles in the plasma is increased. Furthermore, because the contamination characteristics P obtained when the distance from the shower plate 24 is 35 mm are equal to the contamination characteristics 0 obtained when the resist processing is not carried out, it can be seen that, when the distance from the shower plate 24 is as long as about 35 mm, the charged particles do not act at all. From the foregoing results of the measurement of the contamination characteristics it is seen that, according to the ashing treatment process of Figure 1, wherein ashing treatment is carried out while the distance from the shower plate 24 is less than 20 mm, not only neutral active species but also charged particles in the plasma make a contribution to the ashing. Note, from the ashing characteristics shown in Fig. 2 and the contamination characteristics shown in Fig. 4 it is seen that, if the distance from the shower plate 24 is made shorter, the ashing characteristics are improved but the degree of contamination is increased. The allowable degree of contamination of a semiconductor wafer differs according to the kind of fabricated semiconductor device or the preparation steps. As illustrated in Figs. 2 and 4, the ashing characteristics and the contamination characteristics can be controlled only by adjusting the distance from the shower plate. Therefore, the distance from the shower plate can be controlled so that good ashing characteristics can be obtained while adjusting the contamination characteristics within the allowable range. More specifically, by vertically moving the heating stage 14 by the vertically driving device 28 shown in Fig. 1, the distance from the shower plate 24 is adjusted so that the ashing treatment can be carried out in a desirably efficient manner. The present invention is not limited to the foregoing embodiments but various modifications can be made thereto. In the above-described Figure 1 ashing process the material to be treated is subjected to ashing while located at a position less than 20 mn from the shower plate. In general it is sufficient if the material to be treated is located at a position at which charged particles of a high electron temperature which are not captured by the shower plate but are transmitted therethrough, impinge thereon together with neutral active seeds of the plasma which are transmitted through the shower plate. In the apparatus of Figure 1, the plasma is generated by the microwave of 2.45 ± 0.1 GHz, but the plasma can be generated by other methods, for example, application of a microwave of 13.56 MHz, application of a direct current voltage, and an irradiation with light. In the apparatus of Figure 1, a resist is subjected to ashing treatment, but any coating film of an organic substance such as a polyimide may be ashed. Moreover, other gases containing nitrogen, steam, nitrogen oxide, hydrogen or halogen in addition to oxygen can be used as the introduced gas. In the ashing treatment apparatus of Figure 1, the heating stage on which the semiconductor wafer as the material to be treated is placed is vertically moved by the vertically driving device, but the distance between the shower plate and the material to be treated can be adjusted by other means. A further embodiment of the present invention will now be described. In this embodiment, a microwave output of 1.5 kW, a plasma-generating chamber pressure of 106 Pa (0.8 Torr) and an oxygen gas flow rate of 1 SLM are adopted as the ashing conditions as in the foregoing Fig. 1 embodiment. The stage temperature is 160°C and the ashing time is 60 seconds. The movable heating stage is driven so that the distance from the shower plate at the start of the ashing treatment is 5 mm and the stage is separated from the shower plate at a speed 1 mm/sec after the start of the ashing treatment. When 30 seconds have passed from the start of the ashing treatment, the distance from the shower plate is 35mm. At this point, where the distance from the shower plate is 35 mm, the movement of the stage is stopped, and for the remaining 30 seconds, the ashing treatment is carried out at this point. The ashing rate is 1 µm/min. The contamination characteristics U obtained in this embodiment are shown in Fig. 4. The contamination characteristics U are substantially the same as the contamination characteristics O and P, and it was determined that no substantial contamination occurred during the ashing treatment. The ashing rate is about 2 times the ashing rate attained in the Fig. 5 process, in which the distance from the shower plate is 35 mm. According to the above-mentioned further embodiment of the present invention, the problem in the Fig. 5 ashing process of a trade-off relationship between the increase of the ashing rate and decrease of the degree of contamination, can be reduced, so that the ashing treatment can be performed at a higher ashing rate for a particular acceptable level of contamination. As apparent from the foregoing description, in an embodiment of the present invention, the ashing rate is not greatly changed even if the temperature is changed, and the ashing treatment can be performed at a desirably high ashing rate.

Claims: An oxidation process for ashing an organic film on a supporting substrate (12), mounted in a working chamber (10), in which process an oxygen-containing gas is ionised to form a plasma, comprising neutral particles and charged particles, in an ionisation chamber (16) from which such particles can pass into the said working chamber by way of a plasma filtering plate (24) for inhibiting the passage of high-energy charged particles to the said film; characterised in that for an initial part of the process the said substrate (12) is spaced from the said filtering plate (24) by a distance preselected to ensure that during that initial part of the process some high-energy charged particles from the plasma impinge upon the said organic film, whereafter the spacing between the said substrate and the said filtering plate is increased gradually, during the course of the said process, to reduce the danger of contamination of the said substrate by such high-energy charged particles from the plasma impinging upon the said organic film. A process as claimed in claim 1, wherein during the said initial part of the process the substrate is spaced by less than about 20 mm from the said filtering plate. A process as claimed in claim 1 or 2, wherein during the said initial part of the process the said spacing is such that the electron temperature of the said high-energy charged particles impinging on the film is higher than 2.8 eV.

Inventors: FUJIMURA SHUZO, SHINAGAWA KEISUKE, FUJIMURA, SHUZO, SHINAGAWA, KEISUKE
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H01J37
IPC codes: H01L21, G03F7, H01J37