WO2005045904A2

WO2005045904A2 - Notch-free etching of high aspect soi structures using a time division multiplex process and rf bias modulation

Info

Publication number: WO2005045904A2
Application number: PCT/US2004/034803
Authority: WO
Inventors: Sunil Srinivasan; David Johnson; Russell Westerman
Original assignee: Unaxis USA Inc
Current assignee: Oerlikon Management USA Inc
Priority date: 2003-10-21
Filing date: 2004-10-19
Publication date: 2005-05-19
Anticipated expiration: 2006-04-21
Also published as: US20070175856A1; EP1676302A2; JP2007509506A; EP1676302B1; ATE418157T1; DE602004018531D1; US20050112891A1; WO2005045904A3

Abstract

The present invention provides a method and an apparatus for reducing, or eliminating, the notching observed in the creation of SOI structures on a substrate when plasma etching through an alternating deposition/etch process by modulating the RF bias that is applied to the cathode. Modulation of the bias voltage to the cathode is accomplished either discretely, between at least two frequencies, or continuously during the alternating deposition/etch process.

Description

NOTCH-FREE ETCHING OF HIGH ASPECT SOI STRUCTURES USING A TIME DIVISION MULTIPLEX PROCESS AND RF BIAS MODULATION

Cross References to Related Applications

This application claims priority from and is related to commonly owned U.S. Provisional Patent Application Serial No. 60/512933 filed

October 21, 2003, entitled: Notch-Free Etching of High Aspect SOI

Structures Using a Time Division Multiplex Process and RF Bias

Modulation, this Provisional Patent Application incorporated by reference

herein.

Field of the Invention

The present invention relates generally to the manufacture of

silicon based micro-electro-mechanical-systems. More particularly, the

present invention relates to the manufacture of high aspect ratio silicon

structures using alternating deposition and etching steps with a

modulated RF bias.

Background of the Invention

The fabrication of high aspect ratio features in silicon is used

extensively in the manufacture of microelectromechanical (MEMS)

devices. Such features frequently extend completely through the silicon

wafer and may require etching in excess of 500μm into the silicon

substrate. Even "shallow" features require etch depths up to 30μm with feature widths as small as lμm, requiring the definition of structures with

aspect ratios (depth/width) in excess of 30:1. To ensure manufacturability,

these processes must operate at high etch rates to maintain reasonable

throughputs.

Conventional, single step, plasma etch processes cannot

simultaneously meet these needs, and alternating deposition etching

processes have been developed. Such processes are frequently referred to

as Time Division Multiplexed (TDM) processes, which more generally

consist of at least one group containing two or more process steps, where

the group(s) is periodically repeated. These processes (see for example

U.S. Patent 4,985,114 and U.S. Patent 5,501,893) are typically carried out

in a reactor configured with a high-density plasma source, such as an

Inductively Coupled Plasma (ICP), in conjunction with a radio frequency

(RF) biased substrate electrode. The most common process gases used in

the TDM etch process for silicon are sulfur hexafluoride and

octofluorocyclobutane. Sulfur hexafluoride (SFβ) is typically used as the

etch gas and octofluorocyclobutane (C F8) as the deposition gas. During

the etch step (Figure 1(b)), SFβ facilitates spontaneous and isotropic

etching of silicon (Si); in the deposition step (Figure 1(c)), C4F8 facilitates

protective polymer deposition onto the sidewalls as well as the bottom of

etched structures. Upon energetic and directional ion bombardment,

which is present in etch steps, the polymer film coated in the bottom of

etched structures from the previous deposition step will be removed to

expose silicon surface for further etching. The polymer film on the sidewall will remain because it is not subjected to direct ion bombardment,

inhibiting lateral etching. The TDM process cyclically alternates between

etch and deposition process steps enabling high aspect ratio structures to

be defined into a masked silicon substrate (Figures 1(d) & 1(e)). Using the

TDM approach allows high aspect ratio features to be defined into silicon

substrates at high Si etch rates. A complex TDM process may incorporate

more than one etch step, and more than one deposition step that are

cyclically repeated. Certain MEMS devices require that the silicon substrate be etched

down to a buried insulating layer such as silicon dioxide (Siθ2), which acts

as an etch stop (Silicon On Insulator, SOI structure), or which is required ( for functionality of the final device. When such structures are etched

using a TDM process a well-documented phenomenon, commonly referred

to as "notching", occurs. This is evidenced as a severe undercutting of the

silicon, localized at the silicon/insulator interface (Figure 2). It is

generally understood that this is caused by electrical charging effects

during the etching. Because of the different angular distributions of ions

and electrons in the plasma, ions tend to accumulate at the bottom of the

feature, and electrons at the top. During the bulk etch, because the silicon

substrate is sufficiently conductive, current flow within the substrate

prevents any charge separation (Figure 3A). However, when the etch reaches the silicon/insulator interface, the insulator is exposed and the

conductive current path is broken, which allows charge separation to

occur. The resultant electric field is strong enough to bend the trajectories of arriving ions into the feature sidewall where lateral etching (notching) occurs (Figure 3B). Note, for a full discussion see KP Giapis,

Fundamentals of Plasma Process-Induced Charging and Damage in

Handbook of Advanced Plasma Processing Techniques, RJ Shul and SJ

Pearton, Eds, Springer 2000. The notching effect is more prevalent in high density plasma,

because the ion density and therefore the charging effect due to the ions, is

greater. The effect can therefore be reduced by the use of a low density

plasma (conventional reactive ion etching (RIE)) which is employed only

after the insulator has been exposed (Donohue et al. U.S. Patent

6,071,822). The major drawback of such an approach is the low etch rate

attainable, which is a serious shortcoming when features with various

depths must be etched. This is a necessary consequence of etching devices

with various feature sizes, which will etch to different depths due to

Aspect Ratio Dependent Etching (ARDE).

Two groups have taught the use of TDM processes and novel RF

bias configurations' (U.S. Patent 4,579,623 and U.S. Patent 4,795,529).

Neither of these groups contemplate modulating the RF bias frequency in

conjunction with a TDM process.

The use of a low frequency (below 4 MHz) RF substrate bias with a

TDM deposition / etch process has also been described by Hopkins et al.

(U.S. Patent 6,187,685). The authors describe the use of amplitude-pulsed

RF bias (Figure 4) in a TDM process. Hopkins does not teach modulating the frequency of the RF bias. U.S. Patents 5,983,828, 6,253,704 and 6,395,641 by Savas teach the

use of a pulsed ICP to alleviate surface charging and subsequent notching.

However, none of the Patents by Savas teach the modulation of the

frequency of the RF bias to eliminate or reduce notching.

Ogino et al. (U.S. Patent 6,471,821) teach frequency modulation of

the RF bias power as an effective means of reducing charging of the wafer

surfaces during a plasma etch process. Ogino et al. consider frequency

modulation between two discrete frequency values as well as continuous

frequency modulation. Ogino et al. do not consider the application of

frequency modulated RF bias to a TDM process.

Arai et al. (U.S. Patent 6,110,287) also teach frequency modulation

of the RF bias power in order to relax charge formation on the substrate

during an etch process. Amplitude modulation of the frequency modulated

RF bias power is also disclosed, including the case of pulsing between

some power level and zero. Arai et al. do not consider the application of

frequency and/or amplitude modulated RF bias to a TDM process.

Otsubo et al. (U.S. Patent 4,808,258) also teach frequency or

amplitude modulation of the RF bias power to improve etch rate and

selectivity of plasma processes. Frequency modulation between 1 MHz

and 13.56 MHz is disclosed. Amplitude modulation of the RF bias

between two discrete levels is also discussed. Otsubo et al. do not consider

the application of frequency and/or amplitude modulated RF bias to a

TDM process. Therefore, there is a need for an alternating deposition and etch

process that reduces and/or eliminates notching.

Nothing in the prior art provides the benefits attendant with the

present invention.

Therefore, it is an object of the present invention to provide an

improvement which overcomes the inadequacies of the prior art devices

and which is a significant contribution to the advancement of the

semiconductor processing art.

Another object of the present invention is to provide a method for

etching a feature in a substrate comprising the steps of: placing the

substrate on a substrate support in a vacuum chamber, said substrate

support being a lower electrode; an etching step comprising introducing a

first process gas into the vacuum chamber, generating a first plasma from

said first process gas to etch the substrate; a passivation step comprising

introducing a second process gas into the vacuum chamber, generating a

second plasma from said second process gas to deposit a passivation layer

on the substrate; alternatingly repeating the etching step and the

passivation step; applying a modulated bias to the substrate though said

lower electrode; and removing the substrate from the vacuum chamber.

Yet another object of the present invention is to provide an

apparatus for etching a feature in a substrate comprising: a vacuum chamber; at least one gas supply source for supplying at least one process

gas into said vacuum chamber; an exhaust in communication with said

vacuum chamber; a lower electrode positioned within said vacuum chamber; a substrate holder connected to said lower electrode; a plasma

source for generating a plasma within said vacuum chamber; a control

system for alternately etching the substrate and depositing a passivation

layer on the substrate; and a modulation signal generator for providing a

modulated bias to said lower electrode.

The foregoing has outlined some of the pertinent objects of the

present invention. These objects should be construed to be merely

illustrative of some of the more prominent features and applications of the

intended invention. Many other beneficial results can be attained by

applying the disclosed invention in a different manner or modifying the

invention within the scope of the disclosure. Accordingly, other objects

and a fuller understanding of the invention may be had by referring to the

summary of the invention and the detailed description of the preferred

embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.

Summary of the Invention

For the purpose of summarizing this invention, this invention comprises an improved method and an apparatus for deep silicon trench

etching using an alternating cyclical etch process or time division

multiplexed (TDM) process to eliminate the notching observed on SOI structures. In most plasma vacuum treatment processes (etching or deposition),

the main parameters that can be altered to optimize etching or deposition

performance are: the flow rates of the various gases, the working pressure,

the electromagnetic power coupled to the plasma to generate it, and the

energy with which the substrate is bombarded. As a general rule, to

optimize a deposition or etching process, the flow rates of the various

gases, the electromagnetic power coupled to the plasma and the substrate

bombardment energy are optimized at precise and constant values

throughout the treatment. Whereas, the present invention provides for an

improved method apparatus utilizing a modulated RF frequency o reduce

or eliminate notching during an alternating deposition and etch process.

A feature of the present invention is to provide a method for etching

a feature in a substrate. The substrate can be a semiconductor substrate

such as Silicon, Gallium Arsenide or any known semiconductor, including

compound semiconductors e.g., Group II and Group VI compounds and

Group III and Group V compounds. The substrate may also be a

conductor or a dielectric material such as glass or quartz. The method

comprising the steps of placing the substrate on a substrate support in a

vacuum chamber, generating a high density plasma using a first source of

RF energy. The substrate support is a lower electrode to which is

connected a second source of RF energy which provides a bias voltage to the substrate. An alternatingly and repeating process is performed on the

substrate. One part of the process is an etching step which is carried out

by introducing a first process gas, such as Sulfur hexafluoride, into the vacuum chamber. A first plasma is generated from the first process gas to

etch the substrate. The other part of the alternatingly and repeating

process is a passivation step which is carried out by introducing a second

process gas, such as octofluorocyclobutane into the vacuum chamber. A

second plasma is generated from the second process gas to deposit a

passivation layer on the substrate. The passivation layer consists of a

polymer or a fluorocarbon polymer, or can be silicon, carbon, nitride or any

other known passivating materials that can be deposited via a plasma. During the alternatingly and repeating process, one or more process

parameters can vary over time within the etching step or the passivation

step. In addition, during the alternatingly and repeating process, one or

more process parameters can vary over time from etching step to etching

step or from passivation step to passivation step. During the alternatingly

and repeating process a modulated bias is applied to the substrate through

the lower electrode. The bias can be voltage controlled. The bias can be

frequency modulated and it can be applied at or below the ion transit

frequency. The bias can be switched between two distinct frequencies

such that the switching is defined by a switching rate and a switching

duty cycle. The switching rate can be less than about 10kHz and the

switching duty cycle less than 50%. The RF bias frequency can be

continuously modulated and be defined by a mathematical function such as exp(k*sin(t)). The RF bias can additionally be amplitude modulated.

Further, the RF bias can be phase modulated or wave shaped modulated. Finally, upon completion of the etch process, the substrate is removed

from the vacuum chamber.

Still yet another feature of the present invention is to provide an

apparatus for etching a feature in a substrate. The apparatus comprising

a vacuum chamber having at least one gas supply source for supplying at

least one process gas into the vacuum chamber and an exhaust in

communication with the vacuum chamber. A lower electrode is positioned

within the vacuum chamber for applying a bias to the substrate that is

placed upon a substrate holder that is connected to the lower electrode. A

plasma source generates a plasma within the vacuum chamber. The

plasma that is generated is controlled through a control system, depending

on the plasma, alternately etching the substrate and depositing a

passivation layer on the substrate. A modulation signal generator

provides a modulated bias to the lower electrode. The bias can be powered

by RF or DC power. If the bias is powered by an RF source, it can be

frequency modulated and it can be provided at or below the ion transit

frequency. In addition, the frequency of the RF bias can be phase

modulated or wave shaped modulated in conjunction with the alternating

etching and deposition process.

The foregoing has outlined rather broadly the more pertinent and

important features of the present invention in order that the detailed description of the invention that follows may be better understood so that

the present contribution to the art can be more fully appreciated.

Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by

those skilled in the art that the conception and the specific embodiment

disclosed may be readily utilized as a basis for modifying or designing

other structures for carrying out the same purposes of the present

invention. It should also be realized by those skilled in the art that such

equivalent constructions do not depart from the spirit and scope of the

invention as set forth in the appended claims.

Brief Description of the Drawings

Figures l(a-d) is a pictorial example of one type of the TDM process

for deep silicon etching;

Figure 1(e) is a typical scanning electron microscopy photograph of

an etch performed using a TDM process for deep silicon etching;

Figure 2 is a scanning electron microscopy photograph of an etch

performed using a TDM process for deep silicon etching showing the notch

formation at the surface of the substrate;

Figures 3A and 3B are a pictorial of charge buildup at the surface of

the semiconductor substrate during a typical TDM process;

Figure 4 is a graph of amplitude versus time showing a pulsed

amplitude RF bias in a TDM process;

Figure 5 is a graph of amplitude versus time showing frequency

switched modulation using two discrete frequencies for a TDM process;

Figure 6 is a graph of amplitude versus time showing continuous

frequency modulation for a TDM process; Figure 7 is a schematic of a plasma reactor configured for providing

a modulated bias to a substrate; Figure 8 is a schematic of a plasma reactor configured for providing

a modulated RF bias in conjunction with a modulated high density source

to a substrate for a TDM process; and Figure 9 is a scanning electron microscopy photograph of an etch

performed using a modulated bias as taught in the present invention for a

TDM process for deep silicon etching showing no notch formation at the

surface of the substrate.

Similar reference characters refer to similar parts throughout the

several views of the drawings.

Detailed Description of the Invention

We disclose an improved method and apparatus for reducing, or

eliminating, the notching observed when etching SOI structures, by using

an alternating deposition/etch process in conjunction with a modulated RF

bias.

The present invention provides a method and apparatus for

improved etching of silicon on insulator (SOI) structures through the use

of a frequency modulated RF bias, phase modulated RF bias, or a wave

shaped modulated RF bias in conjunction with a TDM process. By frequency modulation of the RF bias, for the present invention,

it is meant that bias voltage applied to the cathode is changed between at least two frequencies, either discretely switched (Figure 5) or continuously

modulated (Figure 6) during the time division multiplex process.

By wave shaped modulation it is meant that the bias voltage

waveform to the cathode is changed between at least two shapes, either

discretely or continuously modulated. Such waveform shapes may be, for

example, a sine wave and a square- wave or any arbitrary waveforms.

By phase modulated bias it is meant that the bias waveform is

changed between at least two states, either discretely or continuously

modulated, where the difference between the two states is a phase

relationship.

A reactor 10 configured for frequency modulated RF bias is shown

in Figure 7. The reactor 10 shown comprises a vacuum chamber 12, a gas

inlet 14, an exhaust 16, an inductively coupled plasma power source (RF

generator) 20 that is connected to a first impedance matching network 21

that provides power to a coil 22, a lower electrode 24 and a substrate

holder 26 for a substrate 28. Provided, as part of the reactor 10, is a

modulation signal generator 30 that is connected to a voltage control

oscillator 32 that is connected to a broadband amplifier 34 that is

connected to a second impedance matching network 36 that supplies

power to the lower electrode 24. It should be noted that applying a

modulated RF bias is not limited to this configuration and one skilled in

the art will appreciate that alternate approaches are possible. Thus the

waveform can be digitally synthesized and applied to the broadband

amplifier 34 using an arbitrary waveform generator. The RF Bias frequency is preferentially modulated during the etch sub-cycle of the

deposition/etch process, since this is when the notching primarily occurs.

However, the RF bias frequency can also be modulated through both deposition and etch sub-cycles.

The duration of the RF Bias high-frequency state should be short enough that charge build-up on the feature surfaces has not reached a

steady state, or is at a steady state for only a short period of time. This

time scale is typically on the order of a few microseconds to a few milliseconds. The duration of the low-frequency RF bias state should be

long enough that charge bleed off can occur, but not so long that reduced etch rates are produced. The low-frequency duration should also be of the

order of a few microseconds to a few milliseconds. In the case of switching the RF bias frequency between two discrete values, the duty cycle, as

defined by the high frequency duration divided by the total modulation

cycle period, should be in the range of 5-90%, preferably 10-50%

The RF bias frequency for the high-frequency state should be below

the ion transit frequency to allow ions to follow the voltage on the cathode

over time.

The ion transit frequency, cϋ i, is described by: ω_p = ( e n₀ / ε₀M )^ Where e - charge on an electron n₀ - ion density ε₀ - permittivity in a vacuum M - mass of the ion

For a typical high density plasma etcher for semiconductor applications, the ion transit frequency is approximately 2 MHz. Notching performance is improved by using a high frequency state in the range of

50kHz to 2MHz. A preferred embodiment uses a high frequency state in the range of 50kHz to 300kHz and a low frequency state in the range of

1kHz to 10kHz which significantly improves notching performance.

In a time division multiplex process, the etch step proceeds in two

stages. During the 1^st stage of the etch step, the passivating film from the

previous deposition step is removed from the horizontal surfaces. This passivation removal process typically follows an ion assisted etch

mechanism with the polymer removal rate being a function of the ion

energy. Ion energy (and passivation removal rate) increases with

increasing bias voltage. Thus it is important to control the magnitude of

the bias voltage. Once the passivation layer has been cleared, the exposed

Si then proceeds to etch by a primarily chemical etch mechanism.

As described above, the amplitude of the modulated bias is

maintained constant as the frequency is changed. It is also possible to

additionally change the amplitude of the waveform. This may be done in

such a manner that the amplitude is high when the frequency is high or

alternately such that the amplitude is low when the frequency is high. As

with the frequency, the amplitude can be switched discreetly between two

levels or continuously varied between two levels.

It is important to note for all the systems and methods described above that the TDM finish etch method can be implemented as a single

sequence or multi-sequence process. A sequence refers to a group of deposition and/or etch steps. In the single sequence implementation,

modulated RF bias is used during the entire process. In the multi-sequence implementation, the first sequence can be

any suitable method (TDM or conventional plasma etch process) that

results in the required etch profile and etch rate, but which is terminated

before the underlying insulator is exposed. The RF bias need only be

modulated for the period when the insulator film is exposed, since this is

when the maximum benefit from charge reduction and, therefore, notch

reduction is expected. The etch is completed using a modulated RF bias

"finish" etch to avoid notching at the silicon/insulator interface.

Etch endpoint detection methods, such as laser reflectance and

optical emission spectroscopy (OES), are helpful in determining when the

insulating film has been exposed.

For all of the modulated RF bias systems and methods described

above, the ICP power can be maintained "on" continuously during the etch

cycle, or it to can be pulsed. This pulsing can be either in phase with the

RF bias modulation (i.e., the ICP is "on" when the RF bias frequency or

amplitude is high), can be out of phase with the RF Bias modulation (i.e.,

ICP is "on" when the RF bias frequency or amplitude is low) or can bear

some other phase relationship to the RF bias (i.e., can be phase shifted).

A reactor 10 configured for a modulated RF bias in conjunction with a modulated high density source is shown in Figure 8. The reactor 10

shown comprises a vacuum chamber 12, a gas inlet 14, an exhaust 16, an

inductively coupled plasma power source (RF generator) 20 that is connected to a first impedance matching network 21 that provides power

to a coil 22, a lower electrode 24 and a substrate holder 26 for a substrate

28. Provided, as part of the reactor 10, is a modulation signal generator 30 that is connected to both the RF Generator 20 and an RF Bias

Generator 50 that is connected to a second impedance matching network

36 that supplies power to the lower electrode 24. The high density source

can be operated at a preferred frequency of 2 MHz or can be at higher

frequencies (e.g., 13.56, 27, 40, 60, 100 MHz, 2.45 GHz) or at lower

frequencies (e.g., 50 kHz — 2 MHz). While the examples below were

performed using an ICP source to generate the high density plasma, other

sources such as ECR, helicon, TCP, etc. could also be used.

Example

The result of etching an SOI structure using a standard TDM etch

process (prior art) with an approximate 2 minute over-etch (sufficient to

etch other smaller structures) is shown in the cross section of Figure 2.

The notch at the silicon-insulator interface is evident, and extends ~ 3μm

into the silicon. Other features with widths of ~ 4μm were undercut to an

extent that they were no longer attached to the substrate.

The preferred embodiment is a significant improvement over the

prior art in terms of notch performance. The reactor is a commercially

available Unaxis VLR modified according to the requirements of the present invention. Figure 7 represents the preferred embodiment of the

present invention, namely, frequency modulation of the RF bias for improved notch performance. The modulated RF bias is amplified and is applied through an impedance matching network to the electrode.

The test pattern used to characterize the notch performance has ~

45μm of Si before a buried oxide layer of lμm. The oxide layer acts as an

etch stop layer. The lines are 4μm wide and spaces range from the

smallest opening of 2.5μm to the largest opening of lOOμm. The resist is ~

1.7μm thick and the percentage of exposed to unexposed Si on a 6" wafer is

~ 15%. The finish etch process recipe with ideal notch performance

corresponding to the preferred embodiment of the present invention is

detailed below. The Si etch before exposure of the oxide layer is non-critical for

notch performance. The transition between the standard TDM Si etch and

the TDM Si finish etch (present invention) is made using an optical

emission end point technique. Finish Etch with Notch performance - Present invention

1* - RF Bias modulation

High Frequency value / duration — 100 kHz / 660 μsec Low Frequency value / duration — 1 kHz / 1340 μsec Total time for 1 cycle - 2000 μsec Results

The result of etching an SOI structure using a frequency modulated TDM

Si finish etch is shown in Figure 9. The over etch on the 15μm, lOμm, 9μm

and 8μm (from L-R in Figure 9) is sufficient to etch the smallest feature

(2.5μm opening) to the buried oxide layer. The absence of any notching at

the Si — oxide interface seen in the cross section is a significant improvement over the prior art.

The present disclosure includes that contained in the appended

claims, as well as that of the foregoing description. Although this

invention has been described in its preferred form with a certain degree of

particularity, it is understood that the present disclosure of the preferred

form has been made only by way of example and that numerous changes

in the details of construction and the combination and arrangement of

parts may be resorted to without departing from the spirit and scope of the

invention.

Now that the invention has been described,

Claims

What is claimed is:

1. A method for etching a feature in a substrate during a time

division multiplex process comprising the steps of: placing the substrate in a vacuum chamber; subjecting the substrate to the time division multiplex process; generating a plasma in at least one step of the time division multiplex

process; applying a modulated bias to the substrate; and removing the substrate from the vacuum chamber.

2. The method of claim 1 wherein the substrate to be etched is a

semiconductor substrate.

3. The method of claim 1 wherein said application of modulated

bias is voltage controlled.

4. The method of claim 1 wherein said modulated bias is an RF

bias.

5. The method of claim 4 wherein said RF bias is frequency

modulated.

6. The method of claim 5 wherein said bias frequency is applied to

the substrate below the ion transit frequency.

7. The method of claim 5 wherein at least one RF frequency is

below the ion transit frequency.

8. The method of claim 5 wherein the frequency of the RF bias is

switched between at least two values.

9. The method of claim 8 wherein the duty cycle is less than about

50%.

10. The method of claim 8 wherein the frequency of the RF bias is

switched between a low frequency state of less than about 10 kHz and a high frequency state of about 100kHz.

11. The method of claim 4 wherein said RF bias frequency is

continuously modulated.

12. The method of claim 4 wherein said RF bias is amplitude

modulated.

13. The method of claim 4 wherein said RF bias is phase modulated.

14. The method of claim 4 wherein said RF bias is wave shaped

modulated.

15. An apparatus for etching a feature in a substrate comprising: a vacuum chamber; at least one gas supply source for supplying at least one process gas

into said vacuum chamber; an exhaust in communication with said vacuum chamber; a lower electrode positioned within said vacuum chamber; a substrate holder connected to said lower electrode; a plasma source for generating a plasma within said vacuum chamber; a control system for alternately etching the substrate and depositing a passivation layer on the substrate; and a modulation signal generator for providing a modulated bias to said

lower electrode.

16. The apparatus of claim 15 wherein said modulated bias is an RF bias.

17. The apparatus of claim 16 wherein said RF bias is frequency

modulated.

18. The apparatus of claim 17 wherein said bias frequency is

provided to the lower electrode below the ion transit frequency.

19. The apparatus of claim 17 wherein the frequency of the RF bias

is switched between at least two values.

20. The apparatus of claim 19 wherein the frequency of the RF bias

is switched between a low frequency state of about less than 10 kHz and a

high frequency state of about 100kHz.

21. The apparatus of claim 16 wherein said RF bias is amplitude

modulated.

22. The apparatus of claim 16 wherein said RF bias is phase

modulated.

23. The apparatus of claim 16 wherein said RF bias is wave shaped

modulated.