JP5227367B2 - Plasma processing method - Google Patents

Description

  In the manufacturing process of a semiconductor device, the present invention performs processing such as etching using plasma on materials such as silicon oxide, silicon nitride, low dielectric constant film (low-k film), polysilicon, and aluminum. The present invention relates to a suitable plasma processing method.

  In the manufacture of semiconductor devices, plasma processing apparatuses are widely used in processes such as film formation and etching. These plasma processing apparatuses are required to have high-precision processing performance corresponding to devices to be miniaturized and mass productivity. Here, a major problem during mass production is a decrease in yield due to foreign matter adhering to the wafer during plasma processing.

  If foreign matter adheres to the wafer during plasma processing, it will cause fatal defects for the device such as wire breakage or short circuit. In addition, as device miniaturization progresses, the influence of minute foreign matter that has not been a problem until now increases. These foreign substances can be removed by wet treatment after the plasma treatment, but this is not preferable because the number of steps increases and the manufacturing cost of the device increases. Therefore, when performing the plasma treatment, attention is paid to reducing the generation of foreign matter itself, removing the generated foreign matter, and preventing the foreign matter from dropping onto the wafer.

  An example of a technique for removing foreign matter during plasma processing is, for example, Patent Document 1. The publication states that “the magnetic field lines B that diverge upward are generated, and foreign substances are moved out of the upper region of the semiconductor wafer along the magnetic field lines B to be eliminated”.

  Moreover, as another example of the technique for removing foreign matters during plasma processing, for example, Patent Document 2 is available. This is because “a second plasma generating electrode is installed around the lower electrode”, “a high frequency voltage is applied to the second plasma generating electrode immediately before the plasma discharge is stopped, and a high density sub-electrode is formed on the outer periphery of the lower electrode. By forming plasma, a subpotential distribution is formed in the processing chamber to push out negatively charged foreign matter staying near the main surface of the semiconductor wafer to the outside of the semiconductor wafer. '' is there.

  Further, it has been known for a long time that foreign matters in plasma fall on a wafer not during plasma processing but when plasma is turned on and off. For example, in Non-Patent Document 1, during plasma processing, that is, while an RF bias is applied to the wafer, foreign matter is trapped at the boundary between the sheath formed immediately above the wafer and the bulk plasma, and the wafer is placed on the wafer. Shows that it does not fall too much.

  On the other hand, Non-Patent Document 2 describes an equation relating to the sheath thickness ds of the RF sheath formed immediately above the wafer when an RF bias is applied to the wafer in plasma. It describes the force Fg that a stationary foreign object receives from the surrounding gas flow.

JP 11-162946 A Japanese Patent Laid-Open No. 5-47712

H. H. Hwang, Appl. Phys. Lett. 68, p. 3716, 1996 Journal of Applied Physics 97, 043306, 2005 Clean Technology, January 2004, p. 9

  The technique described in Patent Document 1 utilizes the fact that foreign matter in plasma is charged. In general, when a foreign substance enters the bulk plasma, the foreign substance is negatively charged because the diffusion coefficient of electrons is much larger than the diffusion coefficient of positive ions.

  As is well known, a moving electric charge in a magnetic field receives a Lorentz force from the magnetic field and moves so as to be wound around the magnetic field, so that the direction of movement is restricted by the magnetic lines of force. If the mass is small like an electron (specifically, if the specific charge e / m is large, where e is the amount of charge and m is the mass), the number of Gauss to several hundred Gauss as used in the plasma processing apparatus. The magnetic field can sufficiently restrain the movement. However, the mass of ions (thousands of times that of electrons) can no longer be constrained by a magnetic field of several Gauss to several hundred Gauss.

  For example, in a plasma generally used for plasma processing, when a magnetic field of 75 Gauss is applied, the electron Larmor radius is 1 mm or less, but the ion Larmor radius is about 20 to 30 mm, and the mean free path of gas ( The value is an order of magnitude larger than a few millimeters). This is because electrons can swirl around the magnetic field lines several times before collision with gas molecules, that is, their movement can be constrained by the magnetic field, whereas ions collide with gas molecules before swirling around the magnetic field lines. Since the direction of motion changes, it means that the motion cannot be restrained by a magnetic field. Furthermore, even a foreign substance having a small diameter of 0.1 μm has a mass that is about eight orders of magnitude larger than that of ions, so that it is impossible to constrain its movement with a magnetic field, although it has a certain amount of charge. Further, since the mass of the foreign matter is proportional to the cube of the foreign matter radius, the charge amount of the foreign matter is proportional to the foreign matter surface area, that is, the square of the radius. Therefore, as the foreign matter diameter increases, the specific charge e / m becomes smaller. Get smaller. That is, with the technique disclosed in Patent Document 1, it is virtually impossible to remove foreign matter from the wafer.

  Moreover, the technique described in Patent Document 2 is not practical in terms of practical use. This is because the configuration of the apparatus is complicated because it is necessary to install a second plasma generating electrode for generating high-density subplasma on the outer periphery of the lower electrode and a power source for applying power to the electrode. There is a big drawback that the cost is considerably high. In addition, the electrode for generating the second plasma itself is consumed, and there is a high possibility that it becomes a foreign matter source or a contamination source. Furthermore, it cannot be expected as much as the original purpose of reducing foreign substances.

  First, since the secondary plasma is generated at the electrode outer peripheral portion where the plasma generating portion cannot be expected from the wafer, the formation of the secondary potential on the wafer from which foreign matter is to be removed is not sufficiently achieved. That is, even if the secondary plasma is generated, the effect does not reach the wafer. This is particularly noticeable in a high-pressure region where the plasma diffusion rate is low. On the other hand, even if the pressure is lowered and the plasma diffusion rate is increased, the secondary plasma density distribution on the wafer approaches to be uniform due to the high diffusion rate, and the desired subpotential shape is sufficiently formed. Not. Therefore, in the technique described in Patent Document 2, a question remains in the foreign matter removal effect itself.

  An object of the present invention is to provide a plasma processing method and a plasma processing apparatus capable of greatly reducing the amount of foreign matter adhering to a wafer during plasma processing.

  The reduction of foreign matter falling on the wafer in plasma processing depends on how foreign matter is not dropped at the non-stationary time when the plasma is turned on / off. Therefore, the present inventors control the shape of the boundary of the sheath (layer having positive ion space charge) / bulk (plasma layer in which positive and negative charges are equally distributed) in which the foreign substance is trapped to remove the foreign substance from the wafer. I thought it could be eliminated. As a result, it has been found that when the plasma is turned on and off, foreign matters can be removed from the wafer by gravity by making the sheath formed immediately above the wafer convex. Furthermore, when turning on and off the plasma, it has been found that by increasing the average thickness of the sheath formed immediately above the wafer, foreign substances can be removed from the wafer by the force of gas flow.

The present invention is a plasma processing method for processing a semiconductor wafer disposed on a stage in a processing chamber that has been evacuated by using plasma formed in the processing chamber, wherein the plasma is supplied from a high-frequency power source connected to the stage. After applying a bias power that does not cause the generation of gas, a gas is introduced into the processing chamber from above the semiconductor wafer with the bias power applied, and the density distribution in the processing chamber is increased at the outer periphery of the semiconductor wafer. A first foreign matter removing step for generating a high and low plasma at the central portion and forming a sheath layer immediately above the semiconductor wafer in a convex shape having a thickness at the central portion larger than the thickness at the outer peripheral portion of the semiconductor wafer; A wafer processing step for processing the semiconductor wafer by forming the plasma in the processing chamber under a predetermined condition after the foreign matter removing step; Characterized by comprising.
In addition, after the wafer processing step, bias power is applied to the stage from the high-frequency power source, and a gas is introduced into the processing chamber, and the density distribution in the processing chamber is high at the outer peripheral portion of the semiconductor wafer. Generating a low plasma at a step, and forming a sheath layer directly above the semiconductor wafer into a convex shape having a thickness at the central portion larger than a thickness at an outer peripheral portion of the semiconductor wafer, the first foreign matter removing step And a second foreign matter removing step of extinguishing the plasma while applying the bias power without generating the plasma after generating the plasma for a long time .

  According to the present invention, when plasma on / off is applied, a low source power and wafer bias power are applied, and a step of controlling the plasma distribution to an external height is added. A thinner sheath is formed, and foreign matter trapped at the sheath / bulk boundary can be discharged out of the wafer by the effects of gravity and gas flow. Thereby, the foreign matter adhering to a wafer can be reduced to 1/10 or less, and the yield improvement in manufacture of a semiconductor device can be expected. Furthermore, the present invention exerts a tremendous effect in removing foreign matters having a particle size of 0.1 μm or less, which will be a major problem in future device miniaturization.

It is sectional drawing which shows Example 1 of the plasma processing apparatus by this invention. It is a schematic diagram which shows the gravity concerning the foreign material trapped by the sheath / bulk boundary. It is a graph which shows the calculation result of the sheath thickness at the time of changing a plasma density and a sheath voltage. It is a figure which shows the calculation result of the gas flow velocity vector in a reactor. It is a schematic diagram which shows the force which the foreign material trapped by the sheath / bulk boundary receives from a gas flow. It is a graph which shows the radial position dependence of the radial direction component Vr of the gas flow rate in a reactor. It is a graph which shows the time dependence of the radial direction position of the foreign material when a sheath / bulk boundary is at z = 1 mm. This is the height dependency of the radial velocity component Vr at r = 100 mm. It is a schematic diagram which shows the foreign material removal by sheath shape control. It is a schematic diagram which shows an example of the discharge sequence by 1st embodiment of the plasma processing apparatus by this invention. It is a schematic diagram which shows another example of the discharge sequence by 1st embodiment of the plasma processing apparatus by this invention. This is the second high-frequency power dependency of the foreign matter increase number. This is the O ₂ flow rate dependence of the foreign matter increase number. When the plasma is turned on, the number of foreign particles increases when the timing of applying source power and bias power is changed. It is sectional drawing which shows Example 2 of the plasma processing apparatus by this invention. It is sectional drawing which shows Example 3 of the plasma processing apparatus by this invention. It is a schematic diagram which shows an example of the discharge sequence by Example 2 and Example 3 of the plasma processing apparatus by this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a first embodiment of the present invention. In Embodiment 1 of the present invention, a wafer mounting stage 2 is provided inside a vacuum-evacuated vacuum processing chamber 1 having a gas introduction means 10, and a substantially circular antenna 7 parallel to the stage on the surface facing the stage. The high frequency power is applied to the antenna from the first high frequency power source 11 via the first matching unit 12, and the electromagnetic wave radiated from the antenna is formed by the external coils 6.1 and 6.2 and the yoke 5. The plasma is generated by the interaction with the magnetic field and the plasma processing is performed by applying a high frequency bias to the wafer 3 to be processed by the second high frequency power source 13 and the second matching unit 14 connected to the stage 2. It has become.

  The frequency of the first high frequency power supply 11 is selected between 50 MHz and 500 MHz. By using the frequency band, it is possible to generate plasma with high uniformity and efficiency on the wafer in a low and medium pressure region (about 0.2 to 50 Pa) suitable for fine processing. In the first embodiment, the frequency of the first high-frequency power source is 200 MHz.

  The frequency of the second high-frequency power source 13 for applying a high-frequency bias to the wafer is preferably 100 kHz so as not to affect the plasma generated by the first high-frequency power and efficiently draw ions into the wafer. To 20 MHz, more preferably 400 kHz to 13.56 MHz. In the present embodiment, a frequency of 4 MHz is used.

  A magnetic field is generated by flowing predetermined currents through the two external coils 6.1 and 6.2. Plasma can be generated more efficiently by the interaction between the electromagnetic wave radiated from the antenna 7 into the processing chamber and the magnetic field, in other words, it is possible to generate medium density plasma that is optimal for processing with lower source power. Further, the shape of the plasma density distribution can be controlled by adjusting the value of the current passed through the coils 6.1 and 6.2 and adjusting the magnetic field strength and the magnetic line shape.

  In the first embodiment, a shower plate 9 is provided on the surface of the antenna 7. The shower plate 9 has hundreds of fine holes with a diameter of about 0.3 to 0.8 mm. Furthermore, between the shower plate and the antenna body 7, there is installed a gas dispersion plate 8 having about several hundred fine holes with a diameter of about 0.3 to 1.5 mm. A processing gas buffer chamber is provided between the gas dispersion plate 8 and the antenna 7, and the processing gas supplied from the gas supply system 10 is uniformly introduced into the processing chamber via the dispersion plate 8 and the shower plate 9. The

  The gas buffer chamber described above is divided into two regions, an antenna central portion and an antenna outer peripheral portion, so that a processing gas can be supplied independently to the central portion and the outer peripheral portion. By changing the flow rate ratio and gas composition of the gas flowing inside and the gas flowing outside, more precise processing can be performed uniformly.

  In addition, each of the high-frequency power sources, gas introduction systems, coil power sources, and the like described so far are all computer-controlled, and can be controlled by an operator via dedicated control software. In addition, a storage medium for storing a processing recipe that defines processing conditions (each high-frequency power value, coil current value, gas flow rate value, etc.) including a plurality of steps for performing a series of plasma processings is also provided.

Below, the grounds of the present invention will be shown that gravity and gas flow, which until now have been considered to have a very small influence on the behavior of foreign matter, can actually sufficiently affect the behavior of foreign matter.
First, the force applied to the foreign material when the foreign material is trapped at the sheath / bulk boundary will be described. First, here, the flow of processing gas is ignored in order to consider only the influence of gravity.

During normal plasma processing, the plasma density on the wafer is generally uniform from the viewpoint of in-plane uniformity of processing speed, and the thickness of the sheath formed on the wafer is also substantially constant on the wafer, that is, the sheath / The bulk boundary is horizontal. As shown in FIG. 2 (a), a foreign substance having a mass M negatively charged to the charge amount q is applied to an ion drag that is a Coulomb force qE received from the sheath electric field E and a force received from positive ions accelerated by the sheath electric field. Three forces of force Fid and gravity Mg are applied (g is the velocity of gravity), and trapped at the sheath / bulk boundary where these three forces are balanced. Here, the relationship between the magnitudes of the three forces is
_{F id + Mg = qE (1} )
It has become.

FIG. 2B shows the force applied to the foreign matter when the sheath / bulk boundary is inclined by a minute angle θ from the horizontal plane due to some factor. As can be seen from the figure, the Coulomb force qE and ion drag force F _id relates to a direction parallel to both the sheath electric field E, the direction of the direction of gravity mg of the sheath electric field E is an angle of theta. Therefore, the balance of forces parallel to the electric field E is
F _id + Mg cos (θ) = qE (2)
It becomes. In other words, the foreign object has a direction perpendicular to the electric field,
Mg sin (θ) (3)
Will be applied.

In general, considering the mass of foreign matter in the order of 0.05 μm to 5 μm, which is a problem during plasma processing, the amount of charge on the foreign matter caused by plasma, and the strength of the sheath electric field, the magnitude of gravity against Coulomb force and ion drag force is large. It is said to be negligibly small and hardly affects the behavior of foreign matter. For example, if a plasma with a plasma density of 1 × 10 ¹⁰ (cm ⁻³ ) and an electron temperature of 3 (eV) and a foreign substance having a diameter of 1 μm and a density of 2.4 g / cm 3 is floating at the sheath / bulk boundary, the foreign substance is applied. Coulomb force and ion drag force are on the order of 1 × 10 ⁻¹³ (N), while gravity is 1 × 10 ⁻¹⁴ (N), which is an order of magnitude smaller. However, this is a situation where the sheath / bulk boundary is horizontal and the Coulomb force, ion drag force and gravity all work in the same direction.

  Here, if the angle θ is attached to the sheath / bulk boundary and the horizontal plane due to some influence, the gravitational force according to the equation (3) acts on the foreign matter, which affects the behavior of the foreign matter. For example, if the sheath thickness is reduced at the outer periphery of the wafer and the sheath / bulk interface at the position where the foreign substance exists can be tilted by θ from the horizontal plane, the equation (3) The indicated force is applied to the foreign object.

Here, it is estimated whether or not the force shown in the equation (3) can remove foreign matter outside the wafer on a realistic time scale. If the initial velocity of the foreign matter at 0 s is 0, the distance r that the foreign matter can move after t seconds is
r = 1/2 g sin (θ) t ² (4)
It becomes. Here, Table 1 summarizes distances r1 and r2 at which the foreign matter can move at t = 1 s and 2 s at a certain angle θ1, θ2,.

  Thus, if the sheath thickness at the center of the wafer is maximized, the sheath thickness at the wafer edge is minimized, and the angle θ formed by the sheath / bulk boundary and the horizontal plane can be set to 2 °, the interval is 1 s. It can be seen that foreign matter can be excluded from the wafer radius (r = 15 cm). It can also be seen that even if θ is 0.5 °, foreign matter can be eliminated if it takes about 2 seconds. Furthermore, since the foreign substance mass M is not included in the equation (4), the above-described method for controlling the sheath shape and eliminating the foreign substance is effective for foreign substances having all particle diameters trapped at the sheath / bulk boundary. It turns out that it is a means.

ここで、ｅは電荷素量、Ｖｓはシースを横切る際の電位差、ｋＢはボルツマン定数、Ｔｅは電子温度である。また、λＤはデバイ長であり、電子密度をＮｅ，真空の誘電率をε０とすると、以下の式で表される。
λ_Ｄ＝（ε_０ ｋ_Ｂ Ｔ_ｅ ／ Ｎ_ｅ ｅ^２ ）^１／２ （６）
（５）式、（６）式中でウエハ面内である程度制御可能であるパラメータは電子密度分布Ｎｅ（＝プラズマ密度分布）であり、これを制御すればよい。 Next, it was estimated whether it was practically possible to control the angle between the sheath / bulk boundary and the horizontal plane. Non-Patent Document 2 describes that when an RF bias is applied to a wafer in plasma, the sheath thickness ds of the RF sheath formed immediately above the wafer is expressed by the following equation (5).

Here, e is the elementary charge amount, Vs is the potential difference across the sheath, kB is the Boltzmann constant, and Te is the electron temperature. Also, λD is the Debye length, and is expressed by the following equation, where the electron density is Ne and the vacuum dielectric constant is ε0.
_{λ D = (ε 0 k B} T e / N e e 2) 1/2 (6)
The parameters that can be controlled to some extent within the wafer surface in the equations (5) and (6) are the electron density distribution Ne (= plasma density distribution), which may be controlled.

FIG. 3 shows the electron density dependence of the sheath thickness ds when the sheath voltage Vs is used as a parameter. It can be seen that the lower the plasma density and the higher the sheath voltage, the greater the change in sheath thickness with respect to the change in plasma density distribution. In addition, this figure shows a graph of plasma density and sheath voltage that can be normally realized by a plasma processing apparatus generally used for semiconductor manufacturing. For example, with an RF bias applied so that Vs = 300 V, the plasma density at the center (r = 0 mm) is 1 × 10 ⁹ cm ⁻³ , and the plasma density at the outer periphery (r = 150 mm) is 2 In the case of × 10 ⁹ cm ⁻³ , the difference in the sheath thickness between the central portion and the outer peripheral portion is about 5 mm, and the angle θ formed by the sheath / bulk interface and the horizontal plane is 1.9 °, which is on the wafer within 1 s. The trapped foreign matter can be sufficiently removed outside the wafer.

  Although the above discussion is a discussion in a very simplified system, it can be seen that gravity influences the behavior of foreign matter, and that foreign matter can be eliminated by making the sheath shape convex and using gravity. It can also be seen that the plasma density distribution and the sheath voltage for making the sheath shape convex can be sufficiently realized.

Further, a general relationship between the sheath voltage Vs and the electron density Ne (≈plasma density) used in the above discussion and the control parameters of the plasma processing apparatus will be supplemented. First, the electron density Ne increases as the source power Ps (first high frequency power) increases, but does not depend much on the bias power Pb (second high frequency power). In other words, when expressed very roughly,
Ne ∝ Ps (7)
The relationship is established. In order to control the plasma density and the ion energy incident on the wafer independently, the source frequency is a relatively high frequency of several tens of MHz to several hundreds of MHz, and the bias frequency is a comparison from several hundred KHz to about 14 MHz. This is because a low frequency is used. The bias power Pb does not affect the plasma density and controls the ion energy, that is, Vs. Here, it goes without saying that increasing the bias power Pb increases the sheath voltage Vs. In other words, roughly speaking,
Vs ∝ Pb (8)
The relationship is established. On the other hand, when the source power Ps is increased, the sheath voltage Vs is decreased. This is because as the source power increases, the plasma density increases and the bias current Ib flowing through the plasma increases. In other words, the bias power Pb is roughly expressed as follows:
Pb = IbVs∝NeVs∝PsVs (9)
Therefore, it can be seen that when the source power Ps is increased while the bias power Pb is fixed, the sheath voltage Vs decreases.

  Next, the effect of gas flow is estimated. First, in order to estimate the gas flow rate at the sheath / bulk boundary, the gas flow rate distribution in the processing chamber was calculated using a general fluid calculation code. FIG. 4 shows the calculation result of the gas flow velocity vector and the outline of the calculation system when the processing gas flow rate is 200 sccm and the pressure is 5 Pa. It can be seen that immediately above the wafer, the velocity vector of the processing gas has a large radial component Vr with respect to the component Vz perpendicular to the wafer surface.

  FIG. 6 shows the radial dependence of the Vr component of the flow velocity vector at each height position. Vr is 0 m / s at the central portion r = 0 mm, and increases linearly toward the outer peripheral portion. Here, the average value at the radial position of the radial component Vr of the gas velocity at z = 1 mm is 0.11 m / s in this calculation result.

According to Non-Patent Document 3, the force Fg that a stationary foreign object receives from the surrounding gas flow is as follows:
F _g = N V ² m πr _p ² (10)
It can be expressed. Here, N is the gas density, V is the gas flow velocity, m is the mass of the gas particles, and rp is the radius of the foreign matter. Equation (10) represents the force that a stationary foreign object receives from the surrounding gas flow. As the foreign object begins to move along the flow and approaches the velocity of the gas flow, the force that the foreign object receives from the flow decreases. That is, when the speed of the foreign matter is also taken into consideration, the force that the foreign matter receives from the gas flow is expressed by the following equation (11).
F _g = N (V−Vp) ² m πr _p ² (11)
Here, Vp is the speed of the foreign matter. The expression (11) means that the force applied to the foreign matter is proportional to the cross-sectional area of the foreign matter and the gas density, and is proportional to the square of the relative velocity between the foreign matter and the gas.

Now, assuming that the mass of the gas particles is the same as Ar, the foreign matter diameter is 1 μm, and the gas flow rate is 0.11 m / s, even the Fg when the foreign matter is stationary is about 2 × 10 ⁻¹⁴ (N) Thus, it is an order of magnitude smaller than the ion drag force and Coulomb force described above. However, the direction in which the force Fg due to the gas flow acts is generally perpendicular to the direction in which the ion drag force and the Coulomb force are balanced, as shown in FIG. Will be affected.

Here, the acceleration αg when a foreign substance of mass M receives Fg is:
αg = Fg / M (12)
It becomes. When the density of the foreign matter is 2.4 g / cm 3, the foreign matter mass M is obtained from the particle diameter. Now, assuming that the initial position of the foreign matter is r0 = 0.01 (m) and the initial speed of the foreign matter is Vp (0) = 0 m / s at t = 0, the result of FIG. 6 and the equations (11), (12) Can be used to calculate the time evolution of the position of the foreign matter.

  Here, FIG. 7 shows the time dependence of the radial position of the foreign material when the sheath / bulk boundary is z = 1 mm. As can be seen from this figure, the position of the foreign substance that is moved by the gas has a foreign substance diameter dependency. The foreign material mass M in the equation (11) is proportional to the third power of the particle size of the foreign material, and the force Fg by the gas is proportional to the cross-sectional area of the foreign material, that is, the square of the particle size of the foreign material from the equation (10). Smaller is more susceptible to gas flow. The removal of foreign matters by gas flow will exert a great effect on removing foreign matters having a particle diameter of 0.1 μm or less, which will become a problem in the future.

  From FIG. 7, the foreign matter diameter of 0.1 μm is excluded outside the wafer range (r> 0.15 m) within 2 s, but the foreign matter diameters of 1 μm and 10 μm are both in the range of r <0.15 m. It can be seen that it cannot be excluded outside the wafer. However, if the gas flow rate is increased, it is clear that the gas flow rate in the vicinity of the wafer is increased and the foreign matter removal effect is enhanced. However, when the gas flow rate is kept constant and the pressure is lowered, the gas flow rate is increased, but the gas density is decreased, so that the Fg in the equation (10) does not change, so the foreign matter removal effect does not change.

  FIG. 8 shows the height dependence of the radial velocity component Vr at r = 100 mm. As can be seen from this figure, the gas flow velocity in the vicinity of the wafer increases rapidly as the distance from the wafer increases. That is, foreign matter removal by gas flow can be more effectively performed by lowering the plasma density or raising the sheath voltage and raising the position in the z direction where the foreign matter is trapped.

  In summary, by controlling the sheath shape to be convex, the foreign matter trapped at the sheath / bulk boundary on the wafer can be removed to the region outside the wafer by gravity. Further, by controlling the average thickness of the sheath to be thick, foreign substances trapped at the sheath / bulk boundary on the wafer can be excluded to a region outside the wafer by the force of the processing gas flow. Needless to say, if both are used in combination, further effects can be expected. By executing these foreign substance removal steps at the time of plasma On and Off, foreign substances that cause a decrease in yield can be reduced.

Next, a method for reducing foreign matter when the plasma processing apparatus is used will be described. First, FIG. 9 shows a basic conceptual diagram. During normal wafer processing (main step), uniform plasma is generated on the wafer as shown in FIG. In this state, the sheath width on the wafer is also uniform and the sheath / bulk interface is horizontal. In this case, the foreign matter generated during processing is trapped at the sheath / bulk boundary and stays on the wafer. Moreover, since a plasma with a medium density (1 × 10 ¹⁰ to 1 × 10 ¹¹ cm ⁻³ ) is usually used from the viewpoint of processing speed, the sheath thickness formed on the wafer is thin. There is not much gas flow force acting on the foreign material.

  Wafer processing conditions are determined by factors such as processing performance, processing speed, selection ratio, and uniformity, so it cannot be changed from the standpoint of reducing foreign matter. Since foreign matter is trapped, it is unlikely to fall on the wafer.

  Therefore, as shown in FIG. 9B, immediately before the plasma is turned off, the source power (the power of the first high-frequency power supply) is lowered, the plasma density is lowered, the coil current is increased, and the plasma density distribution is increased. The height of the sheath formed on the wafer is increased by controlling the height on the wafer outer peripheral portion and lowering on the wafer central portion (so-called outer height), and the sheath shape becomes convex. (Foreign matter removal step). Since the gas flow rate for processing increases as the distance from the wafer increases, the thickness of the sheath increases, so that the foreign matter receives a greater force from the gas flow and is removed from the wafer. Further, by making the sheath shape convex, gravity acting on the foreign substance acts in the direction of removing the foreign substance outside the wafer. Thus, the foreign matter trapped on the wafer is removed out of the wafer by the action of the gas flow and gravity, and even if the plasma is turned off, it does not fall on the wafer. Here, it goes without saying that increasing the processing gas flow rate in the step immediately before turning off the plasma is desirable from the viewpoint of removing foreign matter.

  In addition, it is also effective for reducing foreign matter to execute the foreign matter removing step shown in FIG. This is because it is not necessary for foreign matter flying on the wafer to fall on the wafer between the start of the plasma processing and the stable formation of the sheath on the wafer. In a normal processing sequence, source power (first high-frequency power) is first applied, and bias power (second high-frequency power) is applied after plasma has arrived. In the present invention, plasma is ignited by source power. By applying a bias power before this, a desired sheath can be formed on the wafer when the plasma is ignited. In addition, the foreign matter removal step during ignition also suppresses the plasma density, so that the processing result of the main step is not affected.

Hereinafter, an actual plasma processing sequence will be described in detail with reference to FIG.
First, as a first step, current is applied to the coil and gas is introduced into the processing chamber before the plasma is turned on (usually about 1 to 5 seconds before), and the pressure is adjusted so as to become the processing pressure. Complete it. At this time, for example, a current of 7 A is applied to the coil 6.1 in order to generate an outer high plasma. Of the two gas introduction systems, the inner and the outer, for example, a processing gas of 800 ml / min is introduced into the inner gas introduction system. It is because the foreign substance removal effect by a gas flow can be heightened more by introduce | transducing gas from an inner side.

  Next, as a second step, a bias power of about 5 to 100 W from the second high-frequency power source, for example, 30 W, immediately before starting the supply of source power from the first high-frequency power source (about 0 to 1 s before) Apply power. Since 4 MHz is used as the frequency of the bias power, plasma is not generated with this level of power. The reason why the bias power is applied before the source power is that a sheath is immediately formed on the wafer when the source power is applied in the next step to turn on the plasma.

  Next, as a third step, a source power of about 100 W to 400 W is applied to generate plasma. At this time, in order to keep the plasma density as low as possible, the value of the source power is preferably near the lower limit value at which the plasma can be stably turned on, for example, about 200 W. The plasma generated in this step has a low density due to a low source power, and has an outer high distribution due to magnetic field control. Since a bias is applied in advance, the plasma is generated on the wafer from the moment of plasma On. A thick convex sheath is formed. This step is the foreign matter removal step described above. Moreover, this step is usually about 0.5 to 1 s.

  Next, as a fourth step, there is a step of plasma processing an actual wafer. At the start of this step, the source power, bias power, coil current, inner / outer gas flow rate, and the like are changed to conditions for performing normal plasma processing. The normal processing condition is that the source power is usually larger than that of the foreign substance removal step, for example, about 1000 W, the bias power is also large, for example, about 800 W, and the magnetic field conditions for uniform plasma processing are, for example, coil current 4A The uniform inner / outer gas flow rate is, for example, an inner flow rate of 400 ml / min and an outer flow rate of 400 ml / min. Further, the time required for this step depends on the content of the process and is usually about 10 s to 300 s.

  Next, as a fifth step, a foreign matter removal step is entered. At the start of this step, each discharge parameter is changed to the same conditions as in the third step, and the discharge is continued for 1 to 5 seconds. The purpose of the third step is to prevent foreign matter flying in a very short time during plasma on from adhering to the wafer, so that the time is short. However, in the fifth step, the foreign matter staying on the wafer is removed from the wafer. In order to eliminate it outside, it is necessary to spend a little longer time. At the end of this step, the source power is set to 0 and the plasma is turned off. At this time, the wafer bias is still applied.

  Next, in the sixth step, the wafer bias is turned off, in the seventh step, the coil current is turned off, and the introduction of gas is stopped.

  By using the plasma processing apparatus and the plasma processing method described above, it is possible to significantly reduce foreign substances that fall on the wafer during the plasma processing. Here, it is mentioned that the value of each high-frequency power, the coil current value, the gas flow rate, the step time, and the like used in the description are merely examples and do not limit the present invention.

  Next, FIG. 11 shows a slightly modified plasma processing sequence shown in FIG. The third step and the fifth step, which are foreign matter removal steps, are performed when plasma is turned on / off, which is the same as in the case of FIG. The difference from the plasma processing sequence shown in FIG. 10 is that the source power, bias power, and coil current are gradually changed in the foreign substance removal step. By performing such control, a rapid change in the plasma state can be suppressed, and a higher foreign matter removal effect can be expected.

  Next, the result which shows the foreign material reduction effect at the time of performing the sheath shape control by this invention is demonstrated in FIGS. 12-14. The number of foreign particles increased in these figures was measured before and after exposing the wafer to the plasma for the foreign material adhering to the Si wafer surface for foreign material evaluation, and the measured value before exposure was subtracted from the measured value after plasma exposure. The value is shown.

  First, FIG. 12 shows how much foreign matter reduction effect can be expected when the sheath shape is changed. In FIG. 12, the plots indicated by ◆ and the solid line are the results of investigation on the foreign particles having a particle size larger than 0.16 μm, and the plots indicated by ▲ and dotted lines have the foreign particle size larger than 0.3 μm. It shows the survey results about things. Common conditions are: source power (first high frequency power) is 1000 W, coil current is 4 A, the total flow rate of the mixed gas of Ar / CHF3 / N2 is 800 ml / min, pressure is 4 Pa, bias power (second high frequency power) The change in the number of foreign objects is shown when V is changed to 600 W, 1300 W, and 2000 W. At this time, the wafers Vpp (Vpp∝Vs) were 655V, 1172V, and 1605V, respectively. From FIG. 12, it can be seen that the number of foreign matters decreases as the bias power is increased. One of the reasons is that the sheath voltage Vs increases as the bias power increases, so the sheath voltage Vs increases, and the sheath thickness increases as shown in FIG. It is.

  Another reason is that due to the increase of the bias power, the plasma distribution becomes an outer height distribution, so that the sheath shape becomes convex and the effect of removing foreign matters by gravity appears. (Plasma distribution changes depending on the value of wafer bias even with the same coil current, and the uniformity of plasma distribution is slightly medium / high distribution 5% for bias 600W, outer height 10% for 1300W, outer height distribution 20 for 2000W. In addition, it can be seen from this result that the number of foreign matters can be reduced to about 1/3 by increasing the plasma distribution to the outside height and increasing the sheath thickness.

  FIG. 13 shows the result when only the gas flow rate is changed without changing other conditions when the plasma is turned off. In FIG. 13, the plots indicated by ◆ and the solid line are the results of investigation on the foreign particles having a particle size larger than 0.10 μm, and the plots indicated by ▲ and dotted lines are the foreign particles having a particle size larger than 0.2 μm. It shows the survey results about things. The common conditions were a source power of 1000 W, a bias power of 200 W, a coil current of 5 A, an O2 flow rate of 800 ml / min, and a pressure of 4 Pa. The O2 flow rate was changed to 200 ml / min or 1600 ml / min 1 s before turning off the plasma. Result. The plot of 800 ml / min in the figure is the result when the gas flow rate is not changed. From this figure, it can be seen that when the gas flow rate is increased to 1600 ml / min when the plasma is turned off, the number of foreign matters is reduced to about 1/3 to 1/5. It goes without saying that this result shows that the effect of removing foreign matters by the gas flow has increased with an increase in the gas flow rate. Conversely, if the O2 flow rate is lowered to 200 ml / min, the number of foreign matters will increase by a factor of about two.

  FIG. 14 shows a result when the timing of applying the source power and the bias power is changed when the plasma is turned on. In FIG. 14, the plots indicated by ◆ and the solid line are the results of investigation on the foreign particles having a particle size larger than 0.10 μm, and the plots indicated by ▲ and dotted lines have the foreign particle size larger than 0.2 μm. It shows the survey results about things. “Time” on the horizontal axis indicates the time difference from the application of the source power to the application of the bias power. That is, the plot of 0.5 s shows the result when a source is applied, a plasma is ignited, and a bias is applied after 0.5 s. Further, the plot of 0 s is a result when the plasma power is ignited by applying the source power after applying the bias power of 30 W in advance. From this result, it can be seen that by applying a bias in advance at the time of plasma On and forming a sheath on the wafer, foreign matter is reduced to about 1/3 compared with other cases.

  As described above, from the results shown in FIGS. 12 to 14, it is possible to form a sheath on the wafer at the time of plasma on and plasma off, to keep the sheath shape convex, and to increase the gas flow rate. It can be seen that foreign matter can be reduced. Further, by using all of the above-mentioned means together, it can be expected that the foreign matter adhering to the wafer is reduced to 1/10 or less. Furthermore, the removal of foreign matter by gas flow is expected to be effective as the particle size of the foreign matter is smaller, so it will be even more effective for removing foreign matter with a particle size of 0.1 μm or less, which will be a major problem in future device miniaturization. Demonstrate.

  Next, Embodiment 2 of the present invention is shown in FIG. The description of the basic apparatus configuration common to the first embodiment is omitted. In Example 2, in order to control the plasma distribution, the antenna portion is separated into an inner side and an outer side by an insulator, and the first high-frequency power source 11 and the third high-frequency power source 16 are connected to the inner antenna and the outer antenna, respectively. doing. In this embodiment, the plasma distribution control method is different from that of the first embodiment. That is, an external high plasma can be realized by increasing the power applied to the outer antenna. As a result, the sheath shape can be controlled to a convex shape at the time of plasma On and Off.

  Next, Embodiment 3 of the present invention is shown in FIG. The description of the parts common to the embodiments shown so far is omitted. The present embodiment is different in that the antenna for generating and maintaining plasma is an induction type antenna. An upper portion of the vacuum processing chamber 1 is provided with a dielectric plate 19 that holds a vacuum and allows an induction electric field to pass therethrough, and two induction antennas 7 and 21 are provided on the upper portion of the dielectric plate. Yes. A first high frequency power source 11 is connected to the inner induction antenna, and a second high frequency power source 16 is connected to the outer induction antenna. Also in the present embodiment, an external high plasma can be realized by increasing the power applied to the outer antenna 21. As a result, the sheath shape can be controlled to a convex shape at the time of plasma On and Off.

Next, FIG. 17 shows a foreign matter reduction method when Example 2 or Example 3 is used. The basic idea is the same as in the first embodiment, except that the third high-frequency power is applied so that the plasma distribution becomes an external height when the plasma is turned on and off.

DESCRIPTION OF SYMBOLS 1 Vacuum processing chamber 2 Wafer mounting stage 3 Wafer 5 Yoke 6.1 Coil 1
6.2 Coil 2
7 Antenna 8 Gas Dispersion Plate 9 Shower Plate 10 Gas Supply System 11 First High Frequency Power Supply 12 First Matching Device 13 Second High Frequency Power Supply 14 Second Matching Device 16 Third High Frequency Power Supply 17 Third Matching Device 19 Antenna outer peripheral insulating ring 20 Lid portion 21 First DC power source 22 Second DC power source

Claims (2)

A pulp plasma processing method to process using a plasma formed in the processing chamber of a semiconductor wafer disposed in the vacuum evacuated processed on chamber stage,
After applying a bias power that does not generate the plasma from a high-frequency power source connected to the stage, a gas is introduced from above the semiconductor wafer into the processing chamber while the bias power is applied, and the processing chamber the thickness of the outer peripheral portion generates a low plasma with high Ku Chuo unit sheet over scan layer directly the semiconductor wafer at the central portion than the thickness at the outer peripheral portion of the semiconductor wafer of the density distribution of the semiconductor wafer A first foreign matter removing step formed into a large convex shape,
Plasma processing method and a wafer processing step of processing the semiconductor wafer to form the plasma in the processing chamber under a predetermined condition after the foreign matter removing step. The plasma processing method according to claim 1,
After the wafer processing step, bias power is applied to the stage from the high-frequency power source, and the density distribution in the processing chamber is high in the outer peripheral portion of the semiconductor wafer and low in the central portion while introducing gas into the processing chamber. Generating a plasma and forming a sheath layer directly above the semiconductor wafer in a convex shape having a thickness at the central portion larger than a thickness at an outer peripheral portion of the semiconductor wafer, A plasma processing method comprising: a second foreign matter removing step of extinguishing the plasma in a state where the bias power that does not generate the plasma is applied after the plasma is generated for a long time .

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