Patent Number: 
Section: description

Referring now to the accompanying drawings, as an example of a preferred embodiment of a gas treatment system according to the present invention, a plasma deposition system for depositing a thin film on a substrate to be treated by utilizing the electron cyclotron resonance (ECR) will be described below. In this preferred embodiment, a gas in a vacuum vessel is extracted, and electrons are added to the gas to change particles, e.g., radicals, in the gas to negative ions. Then, the quantity of negative ions corresponding to specific radicals is analyzed by a mass spectrometer. On the basis of the results thereof, the density of radicals is estimated. In accordance with the estimated value, various process conditions for influencing radicals in plasma are controlled. FIG. 1 is a sectional view showing the whole construction of the preferred embodiment of a gas treatment system according to the present invention, and FIG. 2 is a side view schematically showing an electron adhesion type mass spectrometer for use in this gas treatment system. First, a plasma deposition system shown in FIG. 1 will be described. As shown in FIG. 1, the plasma deposition system has a vacuum vessel 1 of, e.g., aluminum. The vacuum vessel 1 comprises a first cylindrical vacuum chamber 11 arranged upward for producing plasma, and a second cylindrical vacuum chamber 12 arranged downward to be communicated with the first vacuum chamber 11. Furthermore, the vacuum vessel 1 is grounded to have zero potential. The upper end of the vacuum vessel 1 has an opening, in which a transmission window 13 formed of a material capable of transmitting microwaves, e.g., quartz, is airtightly provided to maintain vacuum in the vacuum vessel 1. Outside of the transmission window 13, a waveguide 15 connected to a microwave power supply part 14 serving as a high-frequency supply means for producing a plasma of, e.g., 2.45 GHz and 1.5 kW is provided. The microwaves produced by the microwave power supply part 14 are guided by the waveguide 15 in, e.g., a TE mode, or the microwaves guided in the TE mode are changed by the waveguide 15 to a TM mode, to be introduced into the first vacuum vessel 11 via the transmission window 13. On the side wall defining the first vacuum vessel 11, gas nozzles 16 are arranged at regular intervals in the circumferential directions thereof. A gas source (not shown), e.g., an Ar gas source, is connected to the gas nozzles 16 so as to uniformly supply Ar gas to the upper portion of the first vacuum vessel 11. In the second vacuum vessel 12, a wafer mounting table 17 having substantially the same size as that of a wafer W is supported on a supporting part 18 via an insulator (not shown) of, e.g., aluminum, so as to face the first vacuum vessel 11. An electrode is embedded in the mounting table 17, and connected to a high-frequency power supply part 19 so as to supply an ion drawing bias voltage thereto. On the other hand, as shown in FIG. 1, the upper portion of the second vacuum chamber 12, i.e., a portion communicated with the first vacuum chamber 11, is provided with a ring-shaped deposition gas supply part 20. The deposition gas supply part is designed to jet deposition gases, e.g., C4F8 and C2H4 gases, which are fed from a gas supply pipe (not shown), into the second vacuum chamber 12. Furthermore, the Ar gas and the deposition gases corresponds to treatment gases. On the side wall of the second vacuum chamber 12, a gate valve 21 for introducing wafers into the second vacuum vessel 12 is provided. To the other side of the side wall, an electron adhesion type mass spectrometer 3, which will be described later, is connected. To the bottom of the second vacuum chamber 12, exhaust pipes 22 are connected at, e.g., two positions which are symmetrical with respect to the central axis of the second vacuum chamber 12. On the periphery of the side wall defining the first vacuum vessel 11, a ring-shaped main electromagnetic coil 23 serving as a magnetic field forming means is arranged so as to be close to the first vacuum vessel 11. Beneath the second vacuum vessel 12, a ring-shaped auxiliary electromagnetic coil 24 is arranged so as to be close to the second vacuum vessel 12. Referring to FIG. 3, the electron adhesion type mass spectrometer 3 will be described below. The mass spectrometer 3 has a cylindrical body 30 comprising an introducing pipe 31, an ion passage part 32 and an ion detecting part 33, which are arranged in that order from the vacuum vessel 1. The introducing pipe 31 has an extracting port 34 on one end thereof. The extracting port 34 is arranged so as to face the vacuum vessel 1 via a hole 35 formed in the side wall of the vacuum vessel 1. The introducing pipe 31 is made of a new metal or permalloy, which is a material having a high permeability. The periphery of the introducing pipe 31 is surrounded by a metallic bellows body 36, both ends of which are airtightly mounted on a portion surrounding the base end portion of the introducing pipe 31 and a portion surrounding the hole 35, respectively. The bellows body 36 is connected to a driving part 37, such as an air cylinder, which is guided along a rail 38. Therefore, in accordance with the movement of the driving part 37, the bellows body 36 expands and contracts to allow the introducing pipe 33 into the vacuum chamber 1. Furthermore, the hole 35 may be open and closed by a lid (not shown). In this case, the first and second vacuum chambers 11 and 12 can be separated from the electron adhesion type mass spectrometer 3 by tightly closing the hole 35 by the lid, so that process conditions can be more easily controlled. In the introducing pipe 31, a first focus ring 40, a second focus ring 41, a filament 42 serving as a part of an electron adding means for adding electrons to radicals, and an electrode 43 for drawing ions are arranged in that order from the extracting port 34. The filament 42 is connected to a direct voltage source 44 capable of varying voltage. In the ion passage part 32, four rod-shaped electrodes 45 arranged in the vicinity of the periphery of the ion passage part 32 so as to extend in longitudinal directions thereof. Two pairs of the electrodes 45 facing each other serve as a quadrupole. In the ion detector 33, a third focus ring 46 and a detector 47 for detecting a current value due to negative ions are arranged in that order from the ion passage part 32. Furthermore, the body is evacuated to a predetermined degree of vacuum by means of a vacuum pump 48. The value (current value) detected by the detector 47 is fed to a kind determining part 47a, which derives the relationship between the mass number of the negative ions and the measured value (relative intensity) of the number of the negative ions, i.e., a mass spectrum, to determine the kind of the negative ions on the basis of the mass spectrum. This determination is carried out on the basis of data which are obtained by deriving the mass number at the peak of the measured value of the negative ions and deriving a correspondence between the previously prepared mass number and the kind of the negative ions on the basis of the derived mass number. The detected value is fed to a density estimating means 49. The density estimating means 49 has the function of grasping the relationship between the value of the energy of electrons emitted from the filament 42 and the measured value when the voltage of the direct voltage source 44 is varied, deriving the peak of the measured value, and estimating the density of specific radicals in plasma on the basis of the peak value. The results estimated by the density estimating means 49 are fed to the control part 5. FIG. 4 is a block diagram of a control system for controlling process conditions influencing the density of specific particles, e.g., radicals in this example, in plasma, on the basis of the estimated results obtained by the density estimating means 49. FIG. 4 shows signal lines extending from the control part 5. This point will be described later. In this preferred embodiment, an example where control signals outputted from the control part 5 control only a pulse generating part 51 for modulating the output power of the microwave power supply part 14 will be described. The operation of this preferred embodiment will be described below. First, the magnetic field formed by the electromagnetic coils 14 and 15 is associated with microwaves to cause electron cyclotron resonance, so that Ar gas supplied from the nozzles 16 and, e.g., C4F8 and C2H4 gases, supplied from the gas supply part 20 are activated to plasma, respectively. On the other hand, during a deposition treatment, the extracting port 34 of the body 30 of the electron adhesion type mass spectrometer 3 protrudes above the center of a wafer W, and the interior of the body 30 of the electron adhesion type mass spectrometer 3 is maintained to be higher vacuum than the vacuum vessel 1. Therefore, a part of plasma is drawn into the extracting port 34 to be incorporated into the body 30 via the first and second focus rings 40 and 41. Then, electrons emitted from the filament 42 are added to particles, such as radicals, contained in the plasma, so that the radicals are ionized. For example, C4F7 radicals become negative ions of C4F7xe2x80x94. As described above, a superimposed voltage of a positive or negative direct voltage U (volts) and a high-frequency voltage Vxe2x80x2 (volts) [frequency f (MHz) ] is previously supplied from power supply parts (not shown) to the electrodes 45 of two pairs of hyperbolic cylindrical rods (quadrupole). If Vxe2x80x2 is continuously varied while U/Vxe2x80x2 is maintained to be constant, ions corresponding to the respective masses can be detected by the detector 47. The kind determining part 47a prepares a mass spectrum on the basis of the detected signal from the detector as described above, and selects a mass number contained in a predetermined range of mass number, from the mass numbers at the peak values in the mass spectrum. Then, the values of U and Vxe2x80x2 are set every negative ions of the selected mass number so as to accelerate the negative ions, to vary the filament voltage to vary electron energy emitted from the filament 42, to acquire data relating to a correspondence between the value of the electron energy and the measured value of the number of ions. FIG. 5 shows an example of the acquired data. It can be seen from this figure that the peak value varies in accordance with pressure. The inventor has grasped that the peak value of the measured value of negative ions corresponds to the density of target radicals. In this preferred embodiment, it is previous grasped how much the power of microwaves increases (or decreases) with respect to the peak value of the number of negative ions (e.g., C4F7xe2x80x94), and the peak value is inputted to an automatic control circuit, which supplies a control signal to the pulse generating part 51 to control the state of plasma. In this case, the relative value of the density of radicals is grasped to control the density of radicals. FIGS. 7 and 8 show examples where the peak value varies the magnitude of microwaves with respect to radicals C3F7xe2x80x94 and C4F9xe2x80x94 obtained by negative ionizing C4F8 gas used as a treatment gas. In each of these figures, microwaves of 500 W (solid line) and 600 W (dotted line) are measured at a pressure of 20 Torr. Furthermore, data relating to the peak value and the density of specific radicals, e.g., C4F7 radicals, may be previously prepared, and the detected peak value may be applied to the data to estimate the density of radicals corresponding to the peak value to supply a control signal corresponding to the estimated value to, e.g., the pulse generating part 51. The density of radicals thus estimated can be controlled to a target value by controlling the electronic temperature of plasma. The electronic temperature of plasma can be adjusted by pulse-modulating microwaves outputted from the microwave power supply part 14. The adjusting way in the case of radicals having a density increasing as the energy increases is different from the adjusting way in the case of radicals having a density decreasing as the energy increases. For example, in the former, assuming that the microwaves are pulse-modulated by a pulse having a certain duty ratio, if the density of radicals exceeds a preset value, the duty ratio of the microwave power is increased to increase the energy of microwaves supplied to the gas, so that the density of radicals is controlled so as to decrease. In addition, in order to control the energy (power) of microwaves, the output power value of the microwave power supply part 14 may be controlled in place of the control of the duty ratio, or these controls may be combined. According to this preferred embodiment, the density of, e.g., C4F7 radicals, in plasma in the vacuum vessel 1 can be estimated, and the power of the microwave power supply part 14 is controlled on the basis of the estimated density, so that the density of radicals can be set to be an appropriate value. Therefore, it is possible to carry out a treatment wherein the dispersion in wafer W is small, e.g., the thickness and quality of the wafer W are uniform. In addition, since a gas is extracted from the gas extracting port to give electrons to the gas to ionize the gas to count negative ions, there is no problem in that precision is decreased due to soil of the window provided in the vacuum vessel. Furthermore, in the above described preferred embodiment, the density of radicals is derived during the treatment of product wafers W to be fed back to the real time control part 5 to control process conditions. However, the present invention should not be limited thereto. After a predetermined number of wafers are treated, a treatment may be carried out using a test wafer to measure the density of radicals during the treatment to set process conditions, such as the duty ratio of microwaves, on the basis of the measured value during the subsequent treatment of product wafers W to control the process conditions. In order to control the process conditions, in addition to the microwave power, current control parts 52 and 53, which are shown in FIG. 4, for controlling the current values of the main electromagnetic coil 23 and the auxiliary electromagnetic coil 24, respectively, may be controlled to change the intensity and shape of a magnetic field. Alternatively, gas flow-rate adjusting parts 54 and 55 connected to the gas nozzles 16 and the gas supply part 20, respectively, may be controlled so as to adjust the flow rates and mixing ratio of treatment gases, or the opening and closing of a butterfly valve of a pressure adjusting part 56 provided in the middle of the exhaust pipe 22 may be controlled so as to adjust the pressure in the vacuum vessel 1. Also with respect to the high-frequency power supply part 19, the power value or bias may be controlled. When a pulse modulation is carried out by means of a pulse generating part 57, the duty ratio may be controlled by the control signal via the pulse generating part 57. This is particularly effective in etching of a thin film on a wafer W. Moreover, these controls of process conditions may be combined. As the way of adjusting the process conditions, the process conditions may be previously adjusted to change the density of radicals, and a program may be prepared on the basis of the obtained data. FIG. 9 is a graph showing the variation in peak value of radicals negative-ionized by changing the flow rate of C4F8 serving as a treatment gas. In this figure, C3F7 and C4F9 are measured as examples of radicals. This figure shows that the peak value of ions decreases as the flow rate of C4F8 increases. Therefore, it can be seen that the density of radicals varies in accordance with the flow rate of the treatment gas. In the above described preferred embodiment, the estimation of the density of radicals using the density estimating means 49 has been carried out on the basis of ion count data obtained by negative ionizing target radicals, e.g., C4F7, in the electron addition type mass spectrometer 3. However, in some kinds of radicals, e.g., CF4 radicals, F-ions are dissociated by adding electrons. In such a case, the density of CF4 radicals is estimated on the basis of the measured value of the dissociated negative ions, e.g., F-ions. The present invention also includes this case. In addition, the kind of radicals should not be limited to CF4, and the specific particles should not be limited to radicals, but the particles may be molecules or atoms. Furthermore, FIG. 6 is a characteristic diagram showing the measured value of the above described F-ions. Furthermore, in this preferred embodiment, a method for estimating a distribution of concentration of radicals above a wafer may be used. In this method, the bellows 36 is expanded and contracted by means of the driving part 38 of the electron addition type mass spectrometer 3, and the counted values of negative ions at a plurality of places in radial directions of a wafer are derived by changing the position of the extracting port 34. Furthermore, according to the present invention, a nozzle may be airtightly inserted into the extracting port 34 to provide a bellows between the outside of the extracting port 34 and the periphery of the hole 35, to reciprocate the nozzle while the body 30 is fixed. The present invention may be applied to a helicon wave type system, a parallel plate type system, an inductively coupled plasma (ICP) system and so forth, other than the ECR. In addition, the invention may be applied to plasma treatments other than deposition and etching, e.g., the ashing of a resist. Moreover, the present invention may be applied to any systems for treating substrates using treatment gases, other than the plasma treatment system, e.g., a thermal CVD system. According to the treatment system of the present invention, the density of particles, e.g., radicals, in a vacuum vessel can be estimated, and factors (process conditions) influencing the state of plasma can be controlled on the basis of the estimated results, so that it is possible to carry out a good treatment. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modification to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.