JPH09129621A - Pulse corrugated bias electric power - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an SiO2 protective layer under polysilicon in a contact hole from being etched when the polysilicon is etched by providing a step in which a bias is given to a pedestal under a first voltage and another step in which a pulse of a prescribed frequency is given to the pedestal. SOLUTION: During plasma treatment, a bias is given to a pedestal 20 supporting a substrate 22 in a plasma chamber 14. For giving the bias to the pedestal 20, a step in which the bias is given under a first voltage which is at least equal to an arbitrary positive voltage VB and another step in which a pulse having a polarity which is opposite to that of the voltage VB and an amplitude which is larger than that of the voltage VB is regularly or intermittently given to the pedestal 20 at a prescribed frequency are provided. For example, the plasma treatment can be performed by plasma etching and the first voltage can be adjusted to a negative voltage. In addition, and a pulse shifting to the positive side can be used as the pulse and the prescribed frequency can be selected from the range of RF frequencies.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to plasma processing systems and, more particularly, to bias power supply to a substrate pedestal in a high density plasma processing chamber.

[0002]

2. Description of the Related Art In a typical etching system, a reactive gas (eg, SiCl _x ) above a wafer supported on a pedestal in a process chamber.
Plasma is generated in the atmosphere. This plasma is
Generates ions having a positive charge (for example, Cl ions). RF bias power is typically applied to the pedestal to cause self-biasing of the negative DC voltage present on the pedestal and the wafer. The negative bias voltage on the wafer attracts the positively charged ions, causing them to strike the wafer with sufficient energy to accelerate the chemical reaction and etch away the unmasked portions of the protective layer. The energy of the bombarding ions and thus the etch rate (etch rate) is determined in part by the negative bias produced on the wafer by the RF bias power.

[0003]

FIG. 1 is a cross-sectional view of an example of a semiconductor wafer subjected to etching by this method. The wafer is made of Si and is covered by a thin oxide layer 2 (eg ˜100 Å SiO ₂ ), a polysilicon layer 4 and a mask layer 6. The mask layer 6 is patterned to expose only selected portions of the polysilicon layer, such as contact holes and vias, which will be etched. The impinging reactive gas ions chemically react with the exposed polysilicon to remove it. However, the etching often does not proceed uniformly. Particularly in contact holes and other sub-micron sized structures, the regions outside the contact holes tend to etch faster than the center of the contact. Therefore, in the region outside the contact hole, the SiO ₂ protective layer thereunder may be hit by etching before the polysilicon is completely removed by etching at the center of the hole. If the etching process is continued to remove the polysilicon remaining in the contact holes, the exposed portions of the SiO ₂ layer will also be etched. If the etching process is not well controlled, the etching will actually proceed to the SiO ₂ layer and destroy the device or circuit to be manufactured. It is desirable to reduce the chances of this happening.

[0004]

SUMMARY OF THE INVENTION Broadly speaking, the invention, in one aspect, is a method of biasing a pedestal supporting a substrate in a plasma chamber during plasma processing. The method comprises the steps of biasing the pedestal with a first voltage that is at least greater than some positive voltage V _B ; periodically or intermittently applying a pulse of a predetermined frequency to the pedestal, which pulse comprises: A pulse having a polarity opposite to that of the first voltage and having an amplitude greater than V _B.

The preferred embodiment has the following features. The predetermined frequency is a range of RF frequency, for example, a range of megahertz, or 13.56M.
Hz. The first voltage is negative and the pulse is a positive-going pulse. During each cycle, the process further comprises increasing the magnitude of the first voltage between pulses from a first value to a second value between pulses. The amplitude and duration of this pulse are sufficient to neutralize the positive ions accumulated on the substrate surface by the previous pulse. Typically, the pulsed signal has a duty cycle of less than about 30%, for example about 15%. For example, for a 13.56 MHz signal, the pulse duration is less than about 10 ns (nanoseconds) and has a rise time on the order of 1 ns.

In general, the invention, in another aspect, is a method of etching a substrate supported on a pedestal in a plasma etching chamber. The method comprises the steps of: generating a plasma above the substrate that produces positively charged ions that impact the substrate; applying a bias waveform to the substrate; repeating the bias waveform at a frequency f _0. have. This bias waveform has a first portion and a second portion. The first part has a duration of t ₁ and is characterized by a voltage V ₁ that is even more negative than the negative baseline voltage V _B. The second part is a pulse of positive transition, it extends upward by V ₂ than its a duration t ₂ baseline voltage V _B, V ₂ is greater than the magnitude of the baseline voltage V _B.

In the preferred embodiment, the plasma is a high density plasma, such as one that produces an ion density greater than about 10 ¹¹ ions / cm ³ . Furthermore, t ₁ << t ₂ and the frequency f ₀ is in the range of the RF frequency, for example about 13.56 MHz. Also, the method further comprising the step of changing the V ₁ as a function of time during the second part of the waveform, this is like step of increasing as a function of time the amplitude of example V _1. In other words, V ₁ to compensate for positive ions accumulating on the substrate surface during the second part of the cycle.
As a function of time, resulting in an overall plasma sheath above the substrate (between the edges).
The voltage drop at is substantially constant.

In general and in another aspect, the invention is a method of performing plasma treatment in a process chamber in which a pedestal supporting a substrate is present. The method comprises the steps of introducing a gas into the chamber; generating a plasma in the gas above the substrate in the chamber, the plasma generating ions of positive charge which impinge on the substrate surface. A step of generating plasma; a step of biasing the pedestal to a negative voltage so as to control the energy of positively charged ions colliding with the surface of the substrate; Periodically or intermittently on the pedestal at a frequency f of, the pulse having a duration sufficient to neutralize the positive ions accumulated on the substrate surface by the immediately preceding pulse, And applying a pulse.

In general and in another aspect, the invention relates to a plasma etching system having a plasma chamber having a gas that forms a plasma during operation to emit positively charged ions; A plasma etching system having a pedestal on which a substrate is placed during plasma processing; and an RF bias supply circuit electrically connected to the pedestal and supplying RF bias power to the pedestal during plasma processing. An RF bias supply circuit biases the pedestal to a negative voltage to control the energy of the positively charged ions impinging on the substrate surface during plasma processing, and the RF bias supply circuit supplies a positive voltage to the pedestal. Has a pulse generator that periodically or intermittently supplies a pulse that shifts to a predetermined frequency f, where f is in the RF frequency range,
This pulse has sufficient duration to neutralize the positive ions accumulated on the substrate surface by the previous pulse.

In general and in another aspect, the invention provides a plasma etching system, comprising a plasma chamber having a gas that forms a plasma during operation to emit positively charged ions; A plasma etching system having a pedestal on which a substrate is placed during plasma processing; and an RF bias supply circuit electrically connected to the pedestal. An RF bias supply circuit biases the pedestal with a first voltage that is at least as great as the positive voltage V _B ; the bias supply circuit periodically or intermittently pulses the pedestal at a predetermined frequency. Given, this pulse has a polarity opposite to that of the first voltage and has an amplitude greater than V _B.

The present invention, which uses a pulse bias waveform, provides a much narrower ion energy distribution than can be obtained using a sinusoid of the RF bias power signal, as used in conventional systems. can get.

The present invention is particularly effective for etching polysilicon. For such applications, an extremely narrow ion energy distribution (IED: ion energy distributi
on) (ie a distribution with a high degree of unity of energy) is highly desirable. In addition, a specific threshold (for example, 1
It is desirable to have an ion energy lower than 0-20 eV). In other words, there will be energy sufficient to cause the chemical reaction to remove the polysilicon, but not high enough to remove oxide from the wafer by spattering.

Other advantages and features will be apparent from the following description of the preferred embodiment and the claims.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described with reference to a plasma etching system 10 schematically illustrated in FIG. The plasma etching system 10 is
Mounted on the top of the metal vacuum chamber body 14,
It has a cylindrical body 12 made of a dielectric material. At one end of the cylindrical body 12 is a flange that is pressed against the O-ring to form a vacuum seal for the chamber body 14. A circular top plate 16 seals the other end of the cylinder 12 thereby forming a seal cavity 18 in which the plasma treatment takes place. Within the plasma cavity 18 is a pedestal 20 that holds a substrate 22 (eg, a semiconductor wafer) during processing.

RF power is supplied to the plasma cavity through a cylindrical coil antenna 24, which is wound on the outer wall of the cylindrical body 12 and is made of a conductive wire, such as copper. Cylinder 12 comprises a dielectric window through which RF power can be coupled into the chamber.

The RF generator 30 supplies RF power to the coil antenna 24. The RF generator 30 with an output impedance of 50 ohms is connected to the RF matching portion 34 via a 50 ohm coaxial cable 32. In general, the RF matching portion 34 has one or more variable reactance members (for example, inductors or capacitors), and the impedance of the RF matching portion is adjusted by this member so that the RF cable 32 and the coil antenna 24 can be adjusted. Achieving the matching condition allows the RF power delivered to the plasma in the chamber to be maximized.
An RF detection circuit within the RF matching section 34 monitors the transfer of power into the chamber, thereby generating a control signal,
The value of the variable reactance member in the RF matching unit 34 is adjusted to realize and maintain the matching condition.

The design and construction of RF matching sections that can be used in plasma processing systems are well known to those skilled in the art. A suitable RF matching circuit is described in US Pat. No. 5,392,018 by Collins et al.
A suitable RF matching control system is described in US Pat. No. 5,187,454 by Collins et al.

Bias power is supplied to the pedestal 20 by another RF generator 42 via a coaxial cable 44. A shunt resistor R is connected to ground in parallel with the pedestal.

Of course, although not shown in FIG. 2, there are other components that make up the complete plasma system.
A few examples of such members include, for example, vacuum pumps for degassing a chamber, a source of process gas, a mass flow controller and programmable control circuitry. These components are well known to those skilled in the art and therefore will not be discussed or illustrated here. However, the presence of such other components to provide a complete working system should be assumed.

The RF generator 30 supplies the antenna 24 with a sinusoidal signal of 13.56 MHz. In contrast to this, the RF generator 42 provides a 13.56 MHz waveform with the characteristics shown in FIG. In other words, this waveform is a pulse 60 to move to a series of positive rising from the base line voltage 62 of approximately -V _D. Since the amplitude of this positive going pulse is greater than V _0, it will result in a slight positive bias on the substrate during the duration of this positive going pulse.

Etching is carried out using the reactive atmosphere in the chamber. The reaction gas is, for example, SiCl
_x . Chlorine reacts with the polysilicon layer but does not spontaneously react with oxides. Other reactive atmospheres known to those skilled in the art may be utilized.

The system operates in the high density plasma region. Typically, the high density plasma region is characterized by an ion density of about 10 ¹¹ -10 ¹³ ions / cm ³ . By comparison, typical ion densities obtained by plasma in a conventional parallel plate discharge system are about 10 ⁹ to ¹⁰ 10 at a power input of about 1 kW.
Between ions / cm ³ .

In the embodiment described herein, the inductively coupled RF signal introduced by antenna 24 is used to generate a high density plasma. Other known alternative sources for high density plasma generation can also be utilized.
Examples of other sources that can be used to generate high density plasma include electron cyclotron resonance (ECR).
n reasonance) and helicon induced launching waves. By using such a source and setting the chamber pressure to only a few millitorr, 10 ¹¹ to 10 ¹² ions / c
It is possible to achieve a plasma density of m ³ . For comparison, 1 millitorr gas is about 3 × 10 ¹³ ions / c
Note that it gives m ³ . Therefore, when operating in the dense plasma phase, a high degree of conversion (ie, ionization) will occur.

Typical power operating conditions for etching polysilicon on a SiO ₂ layer are about 500 watts and about 20 watts bias power for the RF coil. This produces an ion energy in the range of about 10-20 eV, which is sufficient to facilitate the desired chemical etching reaction, but not SiO2.
Less than the energy that causes sputtering or damage to ₂ .

In the case of the specific example described here, RF
The pulse generator 42 has a rise time of about 1 ns
, A pulse having a duration of less than about 10 ns is generated. Generator 42 is of conventional design and utilizes circuitry well known to those skilled in the art.

As shown in FIG. 2, a current I ₀ is passed through the shunt resistor R during the process. The value of R is the current I ₀
Is chosen to be much higher than the current flowing into the plasma. In other words, the resistance R dominates the load presented to the RF pulse generator, and thus the impedance variation of the plasma that normally occurs in such systems does not significantly affect the load impedance presented to the pulse generator 42. In such cases, it is not necessary to use a matching network to couple power to the plasma.

As shown in FIG. 4, it may be desirable to use a matching transformer 45 to couple the pulse generator 42 to a load represented by a parallel combination of R and plasma impedance. Matching transformer 45 transforms the load impedance to about 50 ohms to match the output impedance of pulse generator 42, thereby providing a more effective coupling of power and pedestal 20.

The bias circuit described herein is based on the substrate 22.
However, the assumption is that the pedestal 20 is electrically conductive and can make direct electrical contact with it, that is, there is no insulating layer on either the back of the conductive substrate or the top of the pedestal. If an insulating layer is present either on the back side of the conductive substrate or on top of the pedestal, as is the case with an electrostatic chuck, then a conventional RF generator, such as RF generator 30, may be used to create the desired substrate. It is necessary to generate a bias voltage.
In that case, the pulse generator need not produce a DC offset and can only output pulses rising from a baseline voltage of zero volts.

When the plasma system described above is operated in a high density situation and the pulsed waveform shown in FIG. 3 is used, the resulting ion energy distribution is as shown in FIG. The horizontal axis represents the energy of ions that pass through the plasma sheath and strike the substrate surface, and the vertical axis represents the ion flux that reaches the substrate surface.
It is expressed in cm ² / sec. As can be seen, the pulsed RF waveform results in an ion energy distribution with two narrow peaks, a low amplitude peak 50 at low energy E ₁ and a much higher amplitude peak 52 at high energy E ₂ . . Energy E
Ions with ₂ help the chemical etching process that takes place, but ions with energy E ₁ that is too low to help the etching process is a waste of energy.

To understand the improvement brought about by the pulse waveform of FIG. 3, it is useful to compare the ion energy distribution shown in FIG. 5 with the ion energy distribution obtained by a conventional high density plasma etching system. In a conventional system, the RF generator used to bias the pedestal outputs an RF waveform 70 as shown in Figure 6 (a), which results in an ion energy distribution as shown in Figure 7. . This ion energy distribution, like the IED of FIG. 5, again has two peaks, a low energy peak 54 and a high energy peak 56. However, the total number of high energy ions is small and the total number of low energy ions is large as compared with the IED obtained by the pulse waveform. In addition, both peaks are much broader and relatively long-tailed. In short, this ion energy distribution is
It is much less concentrated than the IED obtained by the present invention. In conventional systems, ions with energies above the high energy peaks generally tend to cause some sputtering of the protective oxide layer once they penetrate the polysilicon. Referring to FIG. 1, this causes erosion 80 in the protective oxide,
If too terrible, erosion will destroy the device being manufactured. Ions with energies lower than the high energy peaks, especially those with energies around the low energy peaks, all contribute little to the etching process and, in fact, are mostly energy wasted.

Low energy ions are believed to affect a phenomenon known as notching. Referring again to FIG. 1, when the etch reaches the bottom of the contact hole, it tends to preferentially erode the corners of the bottom, which causes a notch in the oxide, forming a "notch" 90, This is highly undesirable.
The cause of notching is not well understood. One theory believes that the etching products that are removed during the etching process adhere to the sidewalls of the etched holes and that these redeposited products tend to protect the sidewalls. When the etching reaches the bottom, there is no more material to be etched and no more material deposits on the sidewalls. Therefore, the sidewalls of the hole that are very close to the bottom will not be protected. Also, slow ions (ie low energy ions)
Are believed to potentially form notches.
This can improve sidewall etch rate, since slower ions will generally deflect more easily and strike the sidewalls.

By the above-mentioned RF pulse bias waveform,
The number of low-energy or slow ions is greatly reduced,
Therefore, one of the mechanisms believed to contribute to notching is weakened.

The reason why this pulsed waveform gives an ion energy distribution having a much higher degree of unity of energy can be understood by reexamining the mechanism that determines the energy of the ions reaching the substrate surface. . When plasma is generated in the chamber, a plasma sheath is formed above the substrate. The sheath represents a region where slow moving positively charged ions are more abundant than fast moving electrons that are repelled from the substrate by the negative bias at the cathode. Ions generated in the plasma reduce the potential across the sheath. Therefore, the energy imparted to the ions is proportional to the voltage drop across this plasma sheath.

The sheath thickness x is generally related to the density as follows:

(Equation 1) Where η _i is the ion density in the plasma sheath, V is the voltage across the sheath, and e
Is the charge of the electron and ε ₀ is the dielectric transmittance of free space. This equation assumes a uniform charge distribution within the sheath. As is clear from this approximation, the plasma sheath becomes progressively smaller as the ion density increases.

At 13.56 MHz and low pressure (ie,
For a parallel plate RF discharge with a conventional RF waveform, performed in a low pressure, low density plasma), the typical sheath thickness is compared to the distance ions travel during one period of the RF bias signal. And thick. Therefore, in that case, the ions will take many RF cycles to traverse the sheath.
Therefore, the ion energy is dominated by the time-averaged electric field in the sheath. One of the results is
When implemented at low pressure and low density, the ion energy distribution at the RF cathode tends to have a long tail and one characteristic spike.

On the other hand, in the case of low pressure and high density plasma,
The sheath thickness is much thinner. If the density is high enough,
The faster the ions pass through it, the thinner the sheath, compared to the length of one period of the RF signal.
In other words, ion energy tends to be an example of the instantaneous voltage drop across the sheath. Therefore, as the voltage drop varies, so does the energy of the ions passing through the sheath.

To see this more clearly, reference is made to FIG. 6, which shows a substrate potential 70 and a plasma potential 66, respectively, which illustrates a conventional system. Plasma potential 66
Oscillates at a slightly positive potential; the substrate potential 70 is V _A
It oscillates with an amplitude of V _A volts near a DC bias of V _DC which is slightly less than the magnitude of the volts. For a short period
The potential of the wafer is slightly positive, during which time the plasma sheath completely collapses and electrons flow to the surface of the substrate, neutralizing the ions stored therein. The potential drop across the sheath changes continuously from a small value ΔV ₁ to a large value ΔV ₂ . The number of ions at any given energy depends on the duration of the potential drop associated with that energy. In the case of an ion, which is an example of an instantaneous voltage drop across the sheath, if the relative voltage drop oscillates from a large value to a small value, the voltage to which the ion is exposed will spread continuously, thus spreading the ion energy continuously. be able to.

In summary, for high density implementations, the sinusoidal RF voltage signal at the cathode will produce a wide range of kinetic energy for the ions reaching the cathode.

On the other hand, in the case of a pulsed waveform, there is no continuously varying voltage drop between the ends of the plasma sheath. On the contrary, the voltage drop that the ion encounters is
One of two values is shown: a small value for the duration of the pulse and a large drop value for the rest of the cycle (eg ~ V ₀ ). This tends to result in two narrow "bands" of ion energy at the cathode.

Ions cross the sheath during both the high and low levels of the waveform. The number of high energy ions and low energy ions reaching the cathode is
Proportional to the duration of the low and high voltage levels. That is, the amplitude of the two peaks is proportional to the relative duration of the two phases of the waveform. The duration of the pulse is t _low ,
Assume the duration of the rest of the waveform is t _high, and
Assuming the heights of the two peaks are P _low and P _high , respectively, the following relationship applies;

(Equation 2) Therefore, to minimize the height of the low energy peaks, the duration t _low of the positive going pulse should be as short as possible.

In general, the duration of the positive going pulse is such that there are enough electrons in the sheath to offset the positive charge transferred to the wafer surface by the ions that are continually escaping for the remainder of the cycle. It need only be long enough to allow it to cross. This time should be extremely short, since the electrons move much faster (ie by the square root of their mass ratio) than the heavier ions. If the sine wave used as the bias signal is 13.56 MHz (ie, in conventional systems), the sheath will completely collapse in approximately a few nanoseconds, but the electron will reach the electrode and neutralize it. That's all you need to do it. Therefore, a pulse on the order of nanoseconds should be long enough. However, the pulse duration is actually 10 nanoseconds (or 20 nanoseconds) in order to avoid having to utilize the advanced circuitry necessary to generate these short, fast pulses.
Even nanoseconds) may be more practical. Pulse repetition rate is 13.56MHz, pulse duration is 10n
For s, the total cycle length is about 73 ns, so the positive going duty cycle is less than about 15%.

During the low level of the pulse waveform (ie, the pedestal voltage is -V ₀ ), the ions move to the cathode and
It neutralizes the negative charge delivered to the cathode by pumping at the beginning of the low voltage. Therefore, the sheath electric field collapses from the constant voltage to some extent as the cycle progresses. This is done by pumping a negative charge to the cathode so as to create a negative slope ramp 69 (see FIG. 3) during the low voltage portion of the waveform (ie, the region outside the pulse). It is possible to prevent it. The gradient of the negative gradient ramp is selected to compensate for this tendency to slowly collect positively charged ions at the substrate surface. When properly selected, the negative gradient ramp keeps the sheath field at a more stable value, thus keeping the energy of the arriving ions constant (ie, a more energy-unity IED is obtained). ).

Note that for illustration, the simplest form of compensation, a linear negative slope ramp, has been shown. Of course, more complex (eg, exponentially decaying) waveforms can be utilized to more accurately compensate for gradual ion accumulation on the wafer surface during this period of the cycle.

As pointed out herein, the pulse bias technique is particularly useful for polysilicon etching processes. Other etching processes that would benefit from this technique include aluminum and oxide (eg, SiO ₂ ) etching. Implementation conditions are adjusted as appropriate for these other processes. For example,
When etching aluminum with a reactive gas, the RF power to the coil is in the range of about 1.5 kW with a bias power of about 100-200 watts. For oxide etch processes, the RF power to the coil is about 1
With a bias power of kW, it is about 2 kW. These numbers are only intended to give a rough estimate of acceptable practice conditions, and are not meant to imply that other conditions are not met.

Of course, as is obvious, this technique
It is also applicable to other plasma processes where the wafer / substrate is biased to control the energy of the bombarding charged particles (eg, ions).

As is apparent from FIG. 6A, the ion energy is determined not only by the potential of the cathode (more accurately, the wafer surface) but also by the plasma potential on the other side of the cathode sheath. Therefore, a stable plasma potential can further improve (ie, narrower) the ion energy distribution. There are known techniques for stabilizing this plasma potential. For example, Faraday shielding, isolation transformers, balanced RF inputs, and extending the distance between the coil and the cathode can be used, to name a few.

[0047]

As described above in detail, according to the present invention, in the etching of a contact hole or the like, before the entire polysilicon is removed by etching in the region outside the contact hole, the polysilicon is removed at the center thereof. Of S
It is possible to prevent the iO ₂ protective layer from being hit by etching.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a semiconductor structure to be etched.

FIG. 2 is a block diagram of a high density plasma etching system embodying the present invention.

3 is a voltage waveform for an RF signal applied to the pedestal shown in FIG.

4 is a diagram showing an alternative configuration for a bias circuit connected to the pedestal shown in FIG.

5 is a diagram showing an ion energy distribution (IED) resulting from using the waveform shown in FIG.

6A is a diagram showing a plasma potential and a pedestal potential when a conventional RF bias waveform is used, FIG.
(B) is a figure which shows the RF pulse waveform relevant to the conventional RF bias waveform of (a).

FIG. 7 is a diagram showing ion energy distribution for a conventional high density plasma etching system using a sinusoidal RF bias waveform.

[Explanation of symbols]

2 ... Oxide layer, 4 ... Polysilicon layer, 6 ... Mask layer, 1
0 ... Plasma etching system, 12 ... Cylindrical body, 14
... Vacuum chamber body, 16 ... Circular upper plate, 18 ...
Sealed cavity, 20 ... Pedestal, 22 ... Substrate, 24
... Cylindrical coil antenna, 30 ... RF generator, 3
2 ... coaxial cable, 34 ... RF matching section, 42 ... RF generator, 44 ... coaxial cable, 45 ... matching transformer,
90 ... notch.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Valentin N. Todorov United States, California, Fremont, Walcott Common 3300, Number 202 (72) Inventor Sue-Uuchan, Milpitas, California, United States, California Rose Drive 230 (72) Inventor Gerald Zie. Inn United States, California, Cupertino, Villich Place 10132

Claims (26)

[Claims] 1. A method of biasing a pedestal supporting a substrate in a plasma chamber during plasma processing, the pedestal being biased with a first voltage at least equal to any positive voltage V _B. And a step of applying to the pedestal periodically or intermittently at a predetermined frequency, a pulse having a polarity opposite to the polarity of the first voltage and having an amplitude larger than V _B. Method. 2. The predetermined frequency is RF (radio frequency)
cy) The method of claim 1 in the frequency range. 3. The predetermined frequency is about 13.56 MHz
3. The method according to claim 2, wherein 4. The method of claim 2, wherein the first voltage is negative and the pulse is a positive going pulse. 5. The method of claim 4, further comprising increasing the magnitude of the first voltage between pulses from a first value to a second value between pulses in each cycle. 6. The method of claim 4, wherein the pulse amplitude and duration are sufficient to substantially neutralize the positive ions accumulated on the substrate surface since the last pulse. 7. The method of claim 4, wherein the pulse has a duration of less than about 10 nanoseconds (ns). 8. The method of claim 4, wherein the pulse has a rise time on the order of nanoseconds (ns). 9. The method of claim 4, wherein the pulsed signal has a duty cycle of less than about 30%. 10. The method of claim 9, wherein the pulsed signal has a duty cycle of less than about 15%. 11. A method of etching a pedestal-supported substrate in a plasma etching chamber, the method comprising: generating a plasma above the substrate that produces positively charged ions that will impact the substrate. , A bias waveform having a first portion and a second portion on the substrate, the first portion having a duration t ₁ and having a negative baseline voltage V _B. Is further characterized by a voltage V ₁ which is also negative, said second portion having a duration t ₂ and having an amount equal to V ₂ greater than the magnitude of said baseline voltage V _B said baseline voltage. A method comprising extending above V _B and repeating the bias waveform at a frequency f ₀ . 12. The method of claim 11, wherein the plasma is a high density plasma. 13. The plasma is about 10 ¹¹ ions / c.
The method of claim 12 for generating a large ion density than m ^3. 14. The plasma produces an ion density on the order of about 10 ¹¹ to 10 ¹² ions / cm ^3.
The method described in. 15. The method according to claim 12, wherein t ₁ <t ₂ , that is, t ₂ is greater than t ₁ . 16. The method according to claim 12, wherein t ₁ << t ₂ , that is, t ₂ is very large compared to t ₁ . 17. The method of claim 11, wherein the frequency f ₀ is in the range of RF frequencies. 18. The method of claim 17, wherein the frequency f ₀ is in the megahertz range. 19. The frequency f ₀ is about 13.56 MH.
19. The method of claim 18, which is z. 20. The method according to claim 11, further comprising the step of varying the V ₁ as a function of time during the second portion of the waveform. 21. The method of claim 20, wherein the step of varying V ₁ as a function of time comprises increasing the amplitude of V ₁ as a function of time during the second portion of the waveform. the method of. 22. The V ₁ to compensate for positive ions accumulating on the substrate surface during the second portion of the cycle.
21. The method of claim 20, wherein V is varied as a function of time to provide a substantially constant voltage drop across the plasma sheath above the substrate during the second time of the cycle. 23. The method of claim 20, wherein V ₁ is varied as a function of time so that ion influx at the substrate surface having substantially the same energy occurs. 24. A method of performing plasma treatment in a plasma process chamber having a pedestal for supporting a substrate therein, the method comprising the steps of introducing a gas into the chamber and introducing positively charged ions impinging on the substrate surface. Generating plasma to be generated by the gas above the substrate in the chamber and biasing the pedestal to a negative voltage to control the energy of the positively charged positive ions impinging on the substrate surface. A step of periodically or intermittently applying a positive going pulse to the pedestal at a predetermined frequency f, said f being in the range of RF frequencies, said pulse being the substrate surface since the last pulse. And d. Having a duration that is sufficient to substantially neutralize the cations accumulated above. 25. A plasma etching system comprising: a plasma chamber containing a gas forming a plasma that, in operation, produces ions having a positive charge; and a substrate above the plasma chamber in the chamber during plasma processing. And an RF bias supply circuit electrically connected to the pedestal, the RF bias supply circuit supplying bias power to the pedestal during plasma processing, the bias power being negative to the pedestal. Bias to control the energy of the positively charged ions that collide with the surface of the substrate during plasma processing.
The bias supply circuit comprises a pulse generator that periodically or intermittently provides a positive going pulse to the pedestal at a predetermined frequency f, said f being in the RF frequency range, said pulse being An RF bias supply circuit having a duration sufficient to substantially neutralize the positive ions accumulated on the substrate surface. 26. A plasma etching system comprising: a plasma chamber containing a gas forming a plasma that, in operation, produces positively charged ions; and a substrate above the plasma chamber in the chamber during plasma processing. And an RF bias supply circuit electrically connected to the pedestal, the RF bias supply circuit biasing the pedestal with a first voltage at least equal to any positive voltage V _B. The bias supply circuit applies to the pedestal at a predetermined frequency periodically or intermittently,
A plasma etching system comprising: the RF bias supply circuit for providing a pulse having a polarity opposite to that of the first voltage and having an amplitude larger than V _B.