Plasma processing apparatus
A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel to place a sample thereon, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher includes an electrostatic chuck electrode provided within the lower electrode to hold the sample, and a voltage measurement circuit provided within the lower electrode to measure a voltage at the electrostatic chuck electrode and output the measured voltage as a DC voltage.
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The present invention relates to a technique for manufacturing semiconductors. In particular, the present invention relates to a plasma processing apparatus suitable for conducting plasma processing on semiconductor wafers by using plasma.
As the degree of integration of semiconductor elements becomes higher in recent years, the circuit pattern goes on becoming finer. Accordingly, demanded working dimension precision is becoming stricter and stricter. Furthermore, the diameter of the wafer has become as large as 300 mm with the object of reducing the manufacturing cost of semiconductor elements. However, it is demanded to make plasma uniform in a wide range between the center of the wafer and the vicinity of the outer periphery and make high-quality uniform working possible with the object of increasing the yield. In the product processing, it is typical to apply a high-frequency bias in order to form a fine circuit pattern by using anisotropy working. At this time, values of a high frequency voltage and a self bias voltage generated on the wafer are important parameters in working. It becomes important to monitor them accurately.
In order to achieve such an object, it has heretofore been conducted to detect a high frequency voltage between a wafer and a matcher for a high frequency power supply (see, for example, JP-A-2003-174015 and JP-A-2002-203835 corresponding to U.S. Pat. No. 6,771,481).
Apart from this, as to the influence of a high frequency transmission path upon the high frequency voltage, current and phase difference, it is known that a high frequency waveform in an output part of the high frequency coupling circuit differs from that on the wafer and consequently a wafer potential probe technique of directly measuring a wafer potential to obtain information of the wafer potential is effective (see, for example, JP-A-2001-338917).
Furthermore, in the conventional technique, in a parallel plate plasma generation apparatus including an upper plate electrode of a metal material and a lower wafer (functioning as an electrode), a high frequency bias having the same frequency is applied to each of the upper electrode and the lower electrode (wafer). A technique of monitoring the voltages and phases of the upper electrode and the lower electrode in order to control the high frequency voltage phase between those biases is known (see, for example, JP-A-8-162292).
SUMMARY OF THE INVENTIONA phenomenon that poses a problem in the plasma processing apparatus is resonance caused by an inductance and a stray capacitance in a high frequency power feeding system or a capacitance of an ion sheath generated on a front face of an electrode capacitance-coupled to plasma, such as the wafer. Resonance caused by the stray capacitance and the inductance of the power feeding system and resonance caused by the capacitance of the ion sheath and the inductance of the power feeding system are independent of each other. In other words, the two resonance phenomena occur at the same time. This poses a problem that information such as a voltage obtained from a measurement point indicates a value that is widely different from a state such as a voltage that is being actually generated on the wafer or the electrode. A problem of the conventional technique is that these resonance phenomena are not taken into consideration essentially.
Clearly, the technique described in JP-A-2003-174015 has a precondition that information obtained from a measurement point, such as a voltage, is the same as the information concerning the wafer or has the same quality as the information concerning the wafer. If this precondition is not satisfied, the precision of the present technique is degraded remarkably.
In JP-A-2002-203835 corresponding to U.S. Pat. No. 6,771,481, attention is paid to the fact that the precondition is not satisfied in the typical plasma processing apparatus. In accordance with the present invention, it is possible to obtain information such as a load impedance seen from the wafer, besides the voltage, current and phase on the wafer, from the information at the measurement point by precisely specifying an equivalent circuit between a measurement point for the voltage or the like and the wafer. Even if the present technique is used, however, the influence of the resonance phenomenon in question cannot be avoided. Because the inductance component and the stray capacitance causing the resonance are incorporated in an equivalent circuit according to the present technique, but the capacitance of the paired ion sheath is not incorporated into the equivalent circuit. This resonance phenomenon caused by plasma is a phenomenon that cannot be predicted from the view point of the present technique.
In addition, it is very difficult, and substantially impossible, to incorporate the capacitance of the ion sheath into the equivalent circuit and evaluate it accurately. Because the capacitance of the sheath depends upon plasma characteristics (an electron density, an electron temperature and a gas density and distribution of them on the wafer), which in turn depend upon a large number of parameters such as the gas pressure and gas component and high frequency power for plasma generation, and high frequency power for bias applied to the wafer, and consequently the capacitance value of the sheath cannot be calculated accurately. As a matter of course, there is a theory for calculating the capacitance. However, it is not possible to know accurate values of numerical values to be substituted into the theory. In other words, precision assurance cannot be conducted.
Furthermore, the capacitance of the ion sheath is a major element that determines a value of the load impedance seen from the wafer. A high frequency voltage generated on the wafer depends upon a combination of a circuit ranging from a matching circuit to the wafer and the load impedance. However, the capacitance of the ion sheath has a property that it depends upon the high frequency voltage generated on the wafer. In other words, the capacitance and the wafer voltage depend on each other and they are related by a nonlinear relation. As for determination of the capacitance and the wafer voltage, therefore, it cannot be solved by using ordinary equivalent circuit simulation. They cannot be determined without executing a recursive calculation using a numerical computation method. It is very difficult to conduct the present calculation in real time from the viewpoint of both aligning numerical values of basic data for calculation start and the calculation time.
A conclusion obtained from the foregoing description is that the resonance phenomenon in question cannot be solved by using the technique of using the equivalent circuit. As a result, calculation cannot be conducted even if the equivalent circuit is used, or the precision assurance cannot be conducted.
As compared with the technique disclosed in JP-A-2003-174015 or JP-A-2002-203835 corresponding to U.S. Pat. No. 6,771,481, the technique disclosed in JP-A-2001-338917 is a technique of directly measuring the wafer potential and the resonance phenomenon in question can be avoided in principle. However, the present technique has a problem of reliability, and it is difficult to put the present technique to practical use. According to the present technique, an oxide film or a nitride film located on the back of the wafer is broken through by a hard needle of WC (tungsten carbide), and direct measurement of the wafer voltage is implemented. A problem is that it cannot be ensured to break through the film on the back of the wafer certainly and implement stable measurement in a semiconductor manufacturing apparatus for processing five hundred thousand to one million wafers one after another. It is very difficult to design such a structure.
As regards the phase as well, it is well known that there is a large change between phases before and after the resonance point and the phase is inverted in an extreme case. Also in the technique of conducting the phase control as described in JP-A-8-162292, the control performance is crippled by the resonance in question. The resonance in question is a phenomenon that the inductance of the high frequency transmission path and the capacitance of the ion sheath cause resonance. The resonance in question is a phenomenon that occurs not only when a high frequency bias is applied to the wafer but also when a high frequency bias is applied to the electrode opposed to the wafer as described in JP-A-8-162292. In JP-A-8-162292 as well, the resonance in question is not taken into consideration as regards the phase measurement point, and it is appreciated that the resonance phenomenon in question constitutes a serious hindrance in the same way as JP-A-2003-174015, JP-A-2002-203835 corresponding to U.S. Pat. No. 6,771,481, and JP-A-2001-338917.
Hereafter, a resonance phenomenon found by the present inventors will be described in detail. Here, an electrode mounting the wafer is taken as an example. However, these two resonance problems occur entirely in the same way, as regards any electrode capacitance-coupled to plasma. First, it will now be described that a resonance phenomenon is seen even if plasma is not present, by representing the electrode structure by the use of an equivalent circuit and taking measurement of a voltage (here, a peak-to-peak voltage Vpp) as an example. This is first resonance, i.e., resonance caused by a stray capacitance and an inductance of the high frequency transmission system. Subsequently, a resonance phenomenon in the case where there is plasma will be described. This is second resonance, i.e., resonance caused by the capacitance of the ion sheath and the inductance of the high frequency transmission system. The entirely same conclusion is obtained as regards the phase measurement as well.
The first resonance, i.e., resonance caused by the stray capacitance and the inductance of the high frequency transmission system will now be described.
A result obtained by measuring frequency characteristics with the configuration shown in
In the equivalent circuit shown in
If resonance occurs at 4 MHz as heretofore described, then the reliability of voltage measurement conducted when using a frequency lower than the resonant frequency but higher than at least 2 MHz in this case, although it also depends on the resonance bandwidth (Q value). It is important that the inductance Lt and the stray capacitance Ct are respectively 1.7 μH and 908 pF, and consequently they are not extremely large values. They are the inductance and stray capacitance generated easily by connecting a high frequency transmission path having a length of several meters to the electrode. According to the experience of the present inventors, it is necessary to take this resonance phenomenon into consideration when using a bias having a frequency of at least 1 MHz, although it depends upon the design technique and the apparatus configuration.
The second resonance, i.e., the resonance caused by the capacitance of the ion sheath and the inductance of the high frequency transmission system will now be described. If there is plasma, the wafer is capacitance-coupled to the plasma. Therefore, it becomes necessary to take a new capacitance generated by the plasma into consideration. In addition, when there is plasma, it is conceivable that there is a case where the resonant frequency further falls as compared with the case shown in
Here, λdb is the Debye length, e is the elementary charge, kB is the Boltzmann's constant, and Te is the electron temperature. An average voltage Vsh of the sheath can be defined by the following expression (3).
Here, τ is an angular frequency of the bias, Vs(τ) is a plasma space potential, and VB(τ) is a bias potential.
The final capacitance of the ion sheath is represented by the following expression (4) using the thickness dsh of the ion sheath.
Here, ε0 is the dielectric constant of the vacuum, and SW is an area of the wafer.
Since the area of the wafer is constant in the Expression 4, it is appreciated that the capacitance of the ion sheath is in inverse proportion to the thickness of the ion sheath. In other words, a condition under which the thickness of the ion sheath becomes thin is equivalent to a condition under which the resonant frequency becomes low. The Debye length is the basic length of the electric field shielding capability, and it becomes short in inverse proportion to the density of the plasma. In the plasma, the electron temperature changes only by several tens percents at most. Neglecting the electron temperature change accordingly, it is appreciated from the Expression 2 that a condition under which the thickness of the ion sheath becomes thin is satisfied when the plasma density is high and the bias voltage is low. A conclusion obtained from this is that the resonant frequency in question is not constant but it changes depending upon the plasma generation condition and wafer working condition even in the same apparatus or if the apparatus is different.
Typically, in plasma used for working on semiconductor products, the electron temperature is approximately 3 eV and the plasma density is in the range of 1010 to 1012 cm−3. Furthermore, the bias voltage is in the range of 100 to 4,000 Vpp. The capacitance of the ion sheath obtained from this is in the range of approximately 200 to 8,000 pF. By using these values, the resonance is simulated. A schematic equivalent circuit is shown in
This is verified by actually using the apparatus.
Conclusions and problems obtained heretofore are put together. First, there are two resonance phenomena in question. The first resonance phenomenon is generated by the inductance and the stray capacitance of the high frequency transmission line. The second resonance phenomenon is generated by the inductance of the high frequency transmission line and the capacitance of the ion sheath. On the basis of this principle, it is impossible that the resonance phenomenon itself disappears. The resonant frequency based on the capacitance of the ion sheath has strong dependence on the bias voltage and the plasma density. The resonant frequency based on the capacitance of the ion sheath changes greatly depending upon the wafer processing condition.
As a matter of course, it is more advantageous to lower these inductances and capacitances because the resonant frequency is raised in accordance with the (Expression 1). When the frequency of the high frequency used as the bias is in the vicinity of the resonant frequency, the voltage value measured at the measurement point becomes widely different from the voltage that is actually generated on the wafer. Furthermore, the ratio between the voltage at the measurement point and the voltage on the wafer changes according to the wafer processing condition, and the ratio does not assume a constant value. It is substantially impossible to quantitatively calculate the voltage generated on the wafer, by using the equivalent circuit. As regards the phase and the current measurement as well, the conclusion is the same.
Dimensions of wafers in the semiconductor processing apparatuses, liquid crystal substrates, and so on have been extended from the past to the present time. This aims at reducing the manufacturing cost. This tendency can be expected to continue hereafter as well, although it depends on the development of the technique as well. An increase in dimensions, i.e., in area of the substrate such as the wafer lowers the resonant frequency, because it increases the capacitance of the sheath as represented by the Expression 4. Therefore, the technique provided by the present invention becomes a technique indispensable to applying a high frequency in future semiconductor manufacturing.
An object of the present invention is to provide a technique by which voltage and phase measurement can be easily set to arbitrary goal precision even under presence of the resonance phenomenon.
In order to achieve the object, in accordance with a first aspect of the present invention, a plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel to place a sample thereon, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher includes an electrostatic chuck electrode provided within the lower electrode to hold the sample, and a voltage measurement circuit provided within the lower electrode to measure a voltage at the electrostatic chuck electrode and output the measured voltage as a DC voltage.
In accordance with a second aspect of the present invention, a plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel so as to incorporate an electrostatic chuck electrode for holding a sample, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher includes a voltage measurement circuit provided under atmospheric pressure to measure a voltage at the electrostatic chuck electrode and output the measured voltage as a DC voltage, and a coaxial line for connecting the electrostatic chuck electrode to the voltage measurement circuit.
According to the present invention, it is possible to implement a detection circuit that is not susceptible to the influence of resonance even if the resonance is present. As a result, the high frequency voltage and phase can be detected accurately. Furthermore, it becomes possible to run the operation of the plasma processing apparatus stably in an optimum state.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.
As described above, the resonances do not disappear, and correction using a calculation or calibration cannot be conducted. Therefore, it is appreciated that it is important in attaining the object to configure the apparatus so as to cause the voltage and phase information at the measurement point to be equivalent to or have the same quality as the voltage and phase information at an electrode of the measurement subject (an electrode capacitance-coupled to plasma on the wafer or the like). Specifically, it is important to form a configuration having a detection circuit that is not susceptible to the influence of resonance even if the resonances are present.
Such a configuration can be achieved by incorporating the Vpp detector incorporated in the matcher shown in
Hereafter, a first embodiment obtained by making the structure shown in
A coaxial line 111 is provided over the antenna electrode 103. A high frequency power supply for plasma generation (first high frequency power supply) 108 (having, for example, a frequency of 200 MHz) is connected to the antenna electrode 103 via the coaxial line 111, a coaxial waveguide 125 and a matcher 109. A substrate electrode 115 on which a wafer 116 can be disposed is provided in a lower part in the vacuum vessel 101. In the same way as the antenna electrode 103, a coaxial line 151 is provided under the substrate electrode 115. A wafer bias power supply (second high frequency power supply) 119 (having, for example, a frequency of 4 MHz) is connected to the substrate electrode 115 via the coaxial line 151, a coaxial waveguide 152, a power feeding cable 153, and a matcher 118. The coaxial line 151 and the coaxial waveguide 152 are, for example, the high frequency transmission part in the electrode shown in
In the present configuration, a wafer voltage measurement circuit 154 is incorporated right under the electrostatic chuck electrode 124 in the vacuum. The influence of the resonance is eliminated by thus attaching the measurement circuit directly to a place where the voltage to be measured is generated, converting the measured voltage to a DC voltage on the spot, and taking out a resultant signal to the outside of the vacuum. A composite impedance of C6 and C7 in the voltage measurement circuit 154 shown in
A second embodiment in which the problems of the first embodiment are solved more thoroughly is shown in
In the present configuration, the voltage measurement point is the electrostatic chuck electrode 124 in the same way as
A special contrivance becomes necessary in the coaxial cable 157 and the voltage measurement circuit 154 in order to make the voltage at the electrostatic chuck electrode 124 equal to the voltage at the voltage measurement circuit 154.
An equivalent circuit for the apparatus shown in
The relation between Zp and Zs will now be described in detail. When seen from the RF power supply shown in
If, for example, the voltage detection precision is made at least 95%, then it is appreciated from this expression that Zs must have an impedance that is at least 9.3 times as large as Zp. It is also possible to replace C6 and C7 in the voltage measurement circuit by resistors. Unless resistances of the resistors are sufficiently high (for example, at least 10 MΩ), however, power loss is caused in the resistors. Accordingly, care should be taken.
This will now be described by using concrete numerical values. A composite impedance Zp obtained by seeing the plasma side from the place of V1 is calculated by using C5=2000 pF, R3=160Ω and other constants. As a result, |Zp|=approximately 15Ω is obtained.
A composite impedance Zs obtained by seeing the voltage measurement circuit side from the place of V1 will now be found. Supposing a coaxial cable corresponding to 3D2V from the viewpoint of the withstand voltage, inductance and capacitance per unit length become 0.27 μH/m and 103 pF/m, respectively. These correspond to L5, L6 and C9 shown in
Equivalent circuit simulation results obtained by using circuit constants heretofore described are shown in
In the V1/V3 ratio shown in
First, a composite impedance of L6, C6 and C7 shown in
A composite capacitance of this and C9 becomes 103 pF+8.005 pF=111.005 pF. Since this composite capacitance and L5 (=0.135 μH) cause serial resonance, the resonant frequency (hereafter referred to as Reso_Measure) becomes 41.113 MHz according to the Expression 1.
Because of a voltage variation caused by the resonance in the voltage measurement circuit, there must be a definite relation between the frequency of the voltage to be measured and the resonant frequency.
In order to hold down the voltage measurement precision to ±5%, a frequency satisfying the relation V1/V3>0.95 is checked particularly in the graph representing the V1/V3 ratio shown in
Denoting an inductance and a capacitance that determine the resonant frequency of the voltage measurement circuit respectively by L and C, therefore, it is necessary to satisfy the following expression (6) on the basis of the Expression (1).
LC<(9.2 τfB)−2 (6)
The coefficient 9.2 need not be this value necessarily. Since this coefficient depends on the voltage measurement precision, this coefficient should be determined with respect to a required measurement precision by using simulation or actual measurement. For example, if the measurement precision is set to +10% under the same condition as that in
Phase measurement will now be described. If a rectifier circuit using a diode D1 included in the voltage measurement circuit shown in
The circuit concerning the voltage measurement and phase measurement heretofore described can be applied to not only the electrode having a wafer mounted thereon, but also all electrodes capacitance-coupled to plasma. This embodiment will now be described.
In order to raise the reliability of the control, the matcher 109 incorporates a filter 110 for cutting off the frequency of the antenna bias power supply 113. In the same way, the matcher 112 incorporates a filter 121 for cutting off the frequency of the high frequency power supply 108 for plasma generation. Outputs of the two matchers 109 and 112 are combined by using a coaxial cable 158, and a resultant signal is coupled to the coaxial line 111 which is the high frequency transmission system for the antenna electrode.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.
Claims
1. A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel to place a sample thereon, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher, the plasma processing apparatus comprising:
- an electrostatic chuck electrode provided within the lower electrode to hold the sample; and
- a voltage measurement circuit provided within the lower electrode to measure a voltage at said electrostatic chuck electrode and output the measured voltage as a DC voltage.
2. The plasma processing apparatus according to claim 1, wherein said voltage measurement circuit is installed within a vessel that intercepts at least a corrosive gas.
3. The plasma processing apparatus according to claim 1, wherein said voltage measurement circuit can detect a phase signal.
4. A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel so as to incorporate an electrostatic chuck electrode for holding a sample, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher, the plasma processing apparatus comprising:
- a voltage measurement circuit provided under atmospheric pressure to measure a voltage at the electrostatic chuck electrode and output the measured voltage as a DC voltage; and
- a coaxial line for connecting the electrostatic chuck electrode to said voltage measurement circuit.
5. The plasma processing apparatus according to claim 4, wherein a composite impedance of said voltage measurement circuit and said coaxial line is greater than a load impedance between the electrostatic chuck electrode and plasma.
6. The plasma processing apparatus according to claim 4, wherein said voltage measurement circuit can detect a phase signal.
7. A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel so as to incorporate an electrostatic chuck electrode for holding a sample, an upper electrode provided in a position opposed to the lower electrode, a first matcher connected to the lower electrode, a first power supply for supplying power to the lower electrode via the first matcher, a second matcher connected to the upper electrode, and a second power supply for supplying power to the upper electrode via the second matcher, the plasma processing apparatus comprising:
- a first phase measurement circuit provided under atmospheric pressure to measure a phase of a voltage applied to the electrostatic chuck electrode;
- a first coaxial line for connecting the electrostatic chuck electrode to said first phase measurement circuit;
- a second phase measurement circuit provided under atmospheric pressure to measure a phase of a voltage applied to the upper electrode;
- a second coaxial line for connecting the upper electrode to said second phase measurement circuit; and
- a controller for controlling the first power supply and the second power supply based on output signals of said first phase measurement circuit and said second phase measurement circuit.
Type: Application
Filed: Aug 31, 2006
Publication Date: Oct 11, 2007
Applicant:
Inventors: Ryoji Nishio (Kudamatsu), Tsutomu Iida (Hikari)
Application Number: 11/513,233
International Classification: C23F 1/00 (20060101);