Vacuum processing method and apparatus

Abstract: This invention relates to a vacuum processing method and apparatus. When a sample (6) is plasma-processed under a reduced pressure, a sample bed (5,7) is cooled by a cooling medium (10) kept at a predetermined temperature lower than the desired processing temperature. The sample is held on the sample bed. To control the sample temperature, a heat transfer gas is supplied between the back of the sample and the sample installation surface of the sample bed, and the pressure of the heat transfer gas is controlled so as to bring the sample to a predetermined processing temperature. In this way, a sample temperature can be regulated rapidly without increasing the size of the apparatus.

Description: This invention relates to a method and apparatus for vacuum processing. More particularly, the present invention relates to a method and apparatus for vacuum processing which is suitable for controlling a sample such as a semiconductor device substrate to different temperatures and vacuum processing the sample. As disclosed in, for example, Japanese Patent Laid-Open No. 76876/1984, in a conventional technique for vacuum-processing a sample by controlling it to different temperatures, an object (hereinafter referred to as a "substrate") to be etched is placed on an electrode disposed in a vacuum vessel, a reactive gas is introduced into the vacuum vessel, a voltage is applied to the electrode so as to generate gas discharge and the substrate is etched by this discharge gas at two or more different electrode temperatures. In the prior art technique described above, a substrate temperature (to 60 °C in this prior art technology) is raised in view of the etching selection ratio of MoSi 2 to a resist and producibility when an MoSi 2 film is etched, and the substrate temperature (to 30 °C in this prior art technology) is lowered in view of over-etching. In the apparatus used in this prior art technique, the electrode is divided into an electrode on a high temperature side and another on a low temperature side. Etching of the substrate is carried out on the higher temperature electrode side, and after this etching, the substrate is transferred to the lower temperature electrode, and over-etching of the substrate is carried out. Alternatively, only one electrode is provided and the electrode temperature can be changed to high and low temperatures while the substrate is kept placed on the same electrode. The substrate temperature is raised to a high temperature to conduct its etching during processing and is lowered to make over-etching of the substrate after the etching. In the prior art technique described above, the time necessary for raising the substrate temperature to two or more different temperatures and reduction of the scale of the apparatus have not been considered. In other words, if a plurality of electrodes are used, the apparatus becomes greater in scale because of the increase in the number of electrodes. Moreover, the time for attaining a predetermined processing condition or a predetermined temperature becomes long. A long time is necessary, too, in order to change the electrode temperature to predetermined temperatures using a single electrode because the heat capacity of this electrode is great. US-A-4,261,762 describes a method and apparatus for providing heat conduction between an article being treated in a vacuum and a support member by providing a gas at low pressure between the article and the support member. It includes a graph plotting temperature of a wafer against ion beam power which shows the effect of use of nitrogen gas at two different pressures. EP-A-260150 describes a vacuum processing apparatus wherein a heat transmitting gas is supplied in a small gap between a sample and a specimen table held at a constant temperature by water cooling. This enables the sample to be maintained substantially at the specimen table temperature under conditions of greater incident power than with other methods of cooling. The flow rate of the gas may be manipulated. The present invention provides a method as set out in claim 1. The present invention also provides an apparatus as set out in claim 6. Accordingly, the present invention seeks to provide an apparatus and method whereby the sample temperature can be set at a variety of levels with the sample bed temperature remaining constant, by varying the pressure of heat transfer gas. Embodiments of the invention are described below by way of non-limitative example. Fig. 1 is a structural view showing a microwave etching apparatus of an embodiment of a vacuum processing apparatus of the present invention; and Fig. 2 is a diagram showing the relation between the pressure of a heat transfer gas and the heat transfer coefficient of the heat transfer gas and the relation between the heat transfer gas pressure and the time constant, when a sample is held on a sample bed by electrostatic chuck. In a preferred process of the invention, a sample bed is cooled by a cooling medium kept at a predetermined temperature lower than a processing temperature. A sample is held on a sample installation surface of the cooled sample bed by electrostatic chuck, or the like. The temperature of the sample thus held is controlled to a predetermined processing temperature in the following way. A heat transfer gas having a high thermal conductivity such as helium gas (He), etc, is supplied between the sample installation surface of the sample bed and the back of the sample. When this gas is supplied, the gas pressure between the sample installation surface and the back of the sample rises. The gas pressure is regulated by regulating the quantity of the heat transfer gas supplied and is kept stably under a gas pressure at which the sample temperature is kept at the predetermined processing temperature. When the processinq temperature of the sample is to be reduced, for example, the quantity of the heat transfer gas supplied is further increased. Accordingly, the gas pressure in the space between the sample installation surface and the back of the sample becomes higher than the gas pressure that corresponds to the initial processing temperature. Thus, the number of molecules which transfer heat becomes great and the sample can be cooled more effectively. The gas pressure is regulated to the one corresponding to the subsequent processing temperature. When the processing temperature of the sample is to be raised, on the contrary, the quantity of the heat transfer gas supplied is reduced, so that the gas pressure in the space between the sample installation surface and the back of the sample falls below the one that corresponds to the initial processing temperature. In this way, the number of gas molecules which transfer heat becomes small and drop of temperature of the sample can be restrained. This gas pressure is regulated to the one corresponding to the subsequent processing temperature. In this case, in order to improve the follow-up property of the sample temperature, the heat transfer gas between the sample installation surface of the sample bed and the back of the sample is exhausted and the supply of the heat transfer gas is stopped or the quantity of the supplied gas is reduced. In this way, the gas pressure in the space between the sample installation surface and the back of the sample drops more rapidly. Thereafter, the exhaust of the heat transfer gas between the sample installation surface and the back of the sample is stopped. As described above, while the sample is held on the sample installation surface of the sample bed, that is, while the gap between the sample installation surface and the back of the sample is kept small (or in other words, in a state where the gap is kept smaller than the mean free path of the heat transfer gas), the coefficient of heat transfer path between the sample and the sample bed can be changed by changing the gas pressure of the heat transfer gas in this gap, and only the sample temperature can be rapidly varied to a different temperature while the temperature of the sample bed is kept as such. Hereinafter, an embodiment of the present invention will be described with reference to Figs. 1 and 2. Fig. 1 shows a parallel flat plate type dry etching apparatus in this case. A sample bed 5 for placing a sample 6 thereon is disposed at a lower part of a vacuum processing chamber 1. The sample bed 5 is electrically insulated from the vacuum processing chamber 1 by a dielectric member 9. A dielectric film 7 is formed on the upper surface of the sample bed 5. The sample 6 is placed on or removed from the sample bed 5 by a conveyor device not shown in the drawing. The sample 6 is supported on the sample bed 5 through the dielectric film 7. An insulating ring 8 is disposed on the part surrounding the sample 6 and on the side surface of the sample bed 5 in such a manner as to cover these portions. An upper electrode 2 is disposed inside the vacuum processing chamber 1 so as to face the sample bed 5. A gas introduction passage for introducing a processing gas into the vacuum chamber 1 through a gas introduction pipe 3 is formed in the upper electrode 2. An evacuator (in this case, this evacuator comprises a turbo molecular pump 26 and a mechanical booster pump 27) for decompressing and exhausting the inside of the vacuum processing chamber 1, is connected to the vacuum processing chamber 1 through an exhaust port 4 disposed on the side surface of the vacuum processing chamber 1. A high frequency power supply 12 is connected to the sample bed 5 through a matching circuit 11 and a D.C. power supply 14 is likewise connected to the sample bed 5 through a high frequency cutoff circuit 13. A flow passage for circulating a cooling medium is defined inside the sample bed 5, and a pipe 10 for transferring the cooling medium between the sample bed and a temperature regulator not shown in the drawing is connected to this flow passage. This circulated heat medium is controlled to a predetermined temperature and is sent to the sample bed 5. A flow passage 15a for a heat transfer gas is formed in this sample bed 5 in such a manner as to pass through the sample bed 5 and the dielectric film 7, and a heat transfer gas supply line 15 is connected to this flow passage. The heat transfer gas such as He gas is supplied under the back of the sample 6 on the dielectric film 7 through a flow rate regulator 16 and a supply valve 17 that are disposed in this heat transfer gas supply line 15. A portion of the dielectric film 7, on which the sample 6 is placed, is provided with a groove that makes the passage of the heat transfer gas easy. This groove is disposed in an area which does not reach the periphery of the sample 6. Accordingly, the pressure of the heat transfer gas under the back of the sample 6 is substantially equal to the pressure of the heat transfer line 15. It is preferable that the heat transfer gas pressure on the back of the sample be higher than the pressure in the vacuum processing chamber 1 so as to provide a pressure difference. The heat transfer gas supplied through the heat transfer gas line 15 passes through the gap between the sample 6 and the sample installation surface of the sample bed 5, in other words, through the gap between the back of the sample 6 and the dielectric film 7 formed on the sample bed 5, and finally flows into the vacuum processing chamber 1 and is exhausted. An absolute pressure vacuum gauge 18 is provided in the heat transfer gas line 15 between the sample 6 and the supply valve 17, and a heat transfer gas exhaust line 19 is also connected between them. The heat transfer gas line 19 is provided with an exhaust valve 20, and is connected, in this case, to the evacuator for decompressing and exhausting the inside of the vacuum processing chamber 1. A controller 25 is connected to the absolute pressure vacuum gauge 18 so that the pressure of the vacuum gauge 18 can be monitored, and is connected also to the flow rate regulation valve 16, the supply valve 17 and the exhaust valve 20 so that it can control them, respectively. The controller 25 stores the relation between the heat transfer gas pressure and the sample temperature and can control the flow rate regulator 16, the supply valve 17 and the exhaust valve 20 by the pressure of the heat transfer gas supply line 15 measured by the absolute pressure vacuum gauge 18. The relation between the heat transfer gas pressure and the sample temperature that is stored in the controller 25 is the values which is obtained by making examination in advance and, in this case, which is obtained by cooling the sample bed 5 by a cooling medium at a predetermined temperature, and holding the sample 6 on the cooled sample bed 5 and supplying the heat transfer gas under the back of the sample 6. Reference numeral 21 represents a bypass line for the bypasses of the flow rate regulator 16; 22 is a valve disposed in the bypass line 21; 23 is an exhaust line connected between the supply valve 17 of the heat transfer gas supply line 15 and the flow rate regulator 16; and 24 is a valve disposed in the exhaust line 23. The bypass line 21 and the exhaust line 23 are disposed in order to exhaust the heat transfer gas remaining in the heat transfer gas supply line 15 between the main valve of a heat transfer gas supply source (not shown in the drawing) and the supply valve 17 when the operation of the apparatus is stopped. The apparatus thus constructed is used in the following way. The sample 6 is placed on the sample bed 5 by the conveyor device not shown in the drawing. A processing gas is supplied at a predetermined flow rate into the vacuum processing chamber 1 through the gas introduction pipe 3. The vacuum processing chamber 1 is evacuated by the evacuator from the exhaust port 4 and the pressure in the vacuum processing chamber 1 is controlled to a predetermined degree of vacuum by an absolute pressure vacuum gauge (not shown) disposed in the vacuum processing chamber and a control valve (not shown) disposed in the exhaust system. Thereafter, high frequency power is supplied by the high frequency power supply 12 to the sample bed 5 through the matching circuit 11, and D.C. power is supplied, too, by the D.C. power supply 14 to the sample bed 5 through the high frequency cutoff circuit 13. When this high frequency power is applied, discharge takes place inside the vacuum processing chamber 1 and the processing gas is converted to plasma, so that etching of the sample 6 is started. When D.C. voltage is applied, an electrostatic attracting force takes place and the sample 6 is held on the dielectric film 7. After the sample 6 is held by the sample bed 5 through the dielectric film 7, the heat transfer gas is supplied under the back of the sample 6 through the heat transfer gas supply line 15. The sample bed 5 is cooled by a cooling medium kept at a predetermined temperature. Accordingly, the heat input to the sample 6 by the plasma processing is transferred to the sample bed 5 through the heat transfer gas on the back of the sample 6 and through the dielectric film 7 and is removed by the temperature regulator (not shown in the drawing) through the cooling medium-circulating through the sample bed 5. When the sample 6 is electrostatically held as in this embodiment, the gap between the sample 6 and the dielectric film 7 can be made sufficiently small. Accordingly, the heat transfer gas under the back of the sample 6 can be within the pressure range of from about 133 Pa (1 Torr) to around 1.33 kPa (10 Torr). In other words, the major proportion of the gap (except for the groove, and the like) between the back of the sample 6 and the upper surface of the dielectric film 7 are narrower than the mean free path of the heat transfer gas and the gap of such portions is a molecular flow region of the heat transfer gas. Therefore, the quantity of heat transfer is determined by the number of molecules of the heat transfer gas, that is, by the pressure of the heat transfer gas, and the heat transfer coefficient is approximately proportional to the heat transfer gas pressure on the back of the sample 6 as shown in Fig. 2. Accordingly, though depending on the performance of the heat regulator when the temperature of the sample bed 5 is lowered beforehand by the maximum capacity of the temperature regulator, the temperature of the sample bed 5 has the following relation with the quantity of heat inputted to the sample 6: (quantity of heat inputted to sample 6) = (heat transfer coefficient of heat transfer gas under back of sample 6) x (temperature of sample 6 - temperature of sample bed 5) where the difference between temperatures of the dielectric film 7 formed on the sample bed 5 and the sample bed 5 can be neglected. The temperature of the sample can be controlled appropriately by using the relation between the heat transfer gas under the back of the sample 6 and the heat transfer coefficient. When, for example, the sample 6 is processed while the sample bed 5 is cooled by a cooling medium at -60 °C and the sample 6 is cooled to -30 °C, this processing is carried out in the following way. The measured values and proportional expression are experimentally determined and stored in the controller 25 beforehand. The flow rate regulator 16 is controlled by the controller 25, while the value of the pressure measured by the absolute pressure gauge 18 being read, so that the pressure of the heat transfer gas supplied from the heat transfer gas supply line 15 may be equal to the one corresponding to the temperature of the sample, that is, -30 °C, in this case. Thus, the pressure is set to the predetermined pressure value by regulating the heat transfer gas flow rate. In this manner, the sample 6 is controlled to the set temperature. The result of the experiment, shows that the heat transfer coefficient was from about 50 to about 500 and the time constant was from 20 to about 2 seconds when the heat transfer gas pressure on the back was from 0.133 to 1.33 kPa (1 to 10 Torrs). When the sample temperature is changed during processing, about 70 % of the desired temperature can be accomplished in a time which is approximately the sum of the control time of the pressure plus the time constant. Therefore, when the sample temperature is to be controlled to a lower temperature, so as to change rapidly the sample temperature to a desired temperature, the method is adopted of closing the exhaust valve 20 by the controller 25, opening the supply valve 17 so as to pass the heat transfer gas from the flow rate controller 16 at a maximum flow rate, speeding up the rise of the pressure of the heat transfer gas under the back of the sample 6 and increasing the heat transfer quantity. (Ordinarily, the valves 22 and 24 are kept closed.) Thereafter, the pressure of the heat transfer gas is returned to the predetermined pressure and is stabilized. In this manner, the sample temperature can be rapidly lowered to the predetermined temperature. When the sample 6 is changed to a further lower temperature after the processing performed at this temperature or at a certain time during this processing, the same operation as described above are carried out and the pressure is stabilized to the predetermined one corresponding to the lower temperature. When the sample temperature is to be adjusted to a higher temperature, on the contrary, the pressure of the heat transfer gas under the back of the sample 6 must be lowered. However, it is not easy that the pressure of the heat transfer gas under the back of the sample 6 electrostatically held hardly lowers because the gap between the back of the sample 6 and the dielectric film 7 is small and the gas in the gap cannot escape easily. For this reason, even if the supply of the heat transfer gas is stopped, the pressure drop is small. Accordingly, the flow rate regulator 16 decreases the flow of the gas by the operation of the controller 25 and the heat transfer gas in the heat transfer gas supply line 15 is once exhausted by opening the exhaust value 20. Then, the heat transfer gas is supplied at a flow rate where the predetermined pressure is attained and is kept stably under a lower pressure. In this manner, the sample temperature can be rapidly raised to the predetermined one. In this case, the temperature is raised by use of the heat input to the sample 6. The control operation described above can control arbitrarily and rapidly the temperature of the sample 6, even during the processing of the sample, by controlling the pressure of the heat transfer gas under the back of the sample 6 without changing the temperature of the sample bed. Since the temperature of the sample can be controlled rapidly during processing as described above, the selection ratio of the etched material to the base can be increased by lowering the temperature of the sample, for example, near the end of etching, if a large selection ratio is required at that point. In the embodiment described above, the sample temperature can be controlled more and sufficiently rapidly than the conventional technique in which the temperature of the sample bed is changed during the processing. Therefore, the present invention will be suitable for sheet-type dry etching for fabricating more miniaturized integrated circuitry by the use of low temperature technique. Since the temperature can be controlled quickly, and there is no need to carry out the processing on a plurality of sample beds having mutually different temperatures, the sample conveying time become unnecessary and the through-put can therefore be improved. Since the numbers of sample beds and cooling devices becomes small, the scale of the apparatus is small and the apparatus is more economical. The present invention is not particularly limited to the embodiment given above but can be applied suitably to the following cases: (1) The case where vacuum processing is carried out by changing and controlling the sample temperature. Here, the term "vacuum processing" includes not only processings such as an etching by use of plasma, CVD processing, cleaning processing, ashing processing, corrosion-proofing processing and sputtering processing but also vacuum processings which do not utilize plasma such as ion implantation processing, vacuum deposition processing, and MBE processing. (2) The case where the samples which are processed at different temperatures are placed sequentially on the same sample bed and are subjected continuously to the vacuum processing. For example, the sample is subjected to high precision plasma etching below a µm order at different temperatures below 0 °C. According to the present invention, only the sample temperature can be changed without changing the temperature of the sample bed and consequently, the sample temperature can be quickly controlled to different temperatures.

Claims: A vacuum processing method for processing a sample (6) having front and back surfaces under a reduced pressure, comprising: cooling a sample bed (5,7) by a cooling medium kept at a predetermined temperature lower than a processing temperature of the sample; holding the sample (6) on a sample installation surface of said sample bed (5,7), said back surface of said sample (6) being spaced from said sample installation surface by a gap; supplying a heat transfer gas into the gap between the back surface of said sample (6) and said sample installation surface of said sample bed (5,7); characterised by: controlling the pressure of said heat transfer gas to a first pressure sufficient to establish and maintain a first processing temperature of said sample (6) by means of a control means storing the relationship between the pressure of said heat transfer gas and said sample temperature; processing said sample (6) at said first processing temperature; changing the pressure of said heat transfer gas to a second pressure sufficient to establish and maintain a second processing temperature of said sample different from said first temperature by means of said control means storing the relationship between the pressure of said heat transfer gas and said sample temperature; and processing said sample (6) at said second temperature; said predetermined temperature of said sample bed being maintained during processing of said sample at both said first and second temperatures. A vacuum processing method according to claim 1, wherein said sample (6) is held electrostatically. A vacuum processing method according to claim 1 or 2, wherein said second processing temperature is lower than said first processing temperature and is established by increasing the pressure of said heat transfer gas from said first pressure to said second pressure. A vacuum processing method according to claim 1 or 2, wherein said second processing temperature is higher than said first processing temperature and is established by first evacuating said gap and then providing said heat transfer gas in said gap at a second pressure lower than said first pressure. A vacuum processing method according to any one of the preceding claims, wherein said gap is arranged to be narrower than a mean free path of said heat transfer gas, so that a plurality of temperatures can be established by controlling a quantity of molecules of said heat transfer gas in said gap. A vacuum processing apparatus for processing a sample (6) having front and back surfaces under a reduced pressure, comprising: means (10) for cooling a sample bed (5,7) by a cooling medium kept at a predetermined temperature lower than a processing temperature of said sample; means (13,14) for holding said sample (6) on a sample installation surface of said sample bed (5,7), said back surface of said sample (6) being spaced from said sample installation surface by a gap; and means (15,15a,17) for supplying a heat transfer gas into the gap between said back surface of said sample (6) and said sample installation surface of said sample bed (5,7); characterised by: control means (25) adapted to control the pressure of said heat transfer gas supplied into said gap to a first value of said pressure so as to establish and maintain a first processing temperature of said sample (6) and to change and control said pressure to a second value of said pressure so as to establish and maintain a second processing temperature of said sample (6) different from said first processing temperature; and processing means for processing said sample (6) at said first processing temperature to achieve a first processing condition and for processing said sample at said second processing temperature to achieve a second processing condition different from said first processing condition; said control means (25) being arranged to store the relationship between the pressure of said heat transfer gas and said sample temperature. A vacuum processing apparatus according to claim 6, wherein said sample bed (5,7) includes an insulating film (7) on the installation surface, and the apparatus has means (13,14) for applying a D.C. voltage to the sample bed (5,7). A vacuum processing apparatus according to claim 6 or 7, wherein said control means (25) includes means (19,20) to evacuate said gap. A vacuum processing apparatus according to any one of claims 6 to 8, comprising a flow rate regulator (16) for regulating flow of said heat transfer gas.

Inventors: ITOU ATSUSHI, KATO SHIGEKAZU, NISHIHATA KOUJI, TAMURA NAOYUKI, TSUBONE TSUNEHIKO, ITOU, ATSUSHI, KATO, SHIGEKAZU, NISHIHATA, KOUJI, TAMURA, NAOYUKI, TSUBONE, TSUNEHIKO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01L21
IPC codes: H01L21, C23C14, C30B33, C23C16