1.1621713

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Modeling of fluorine-based high-density plasma etching of anisotropic silicon trenches

with oxygen sidewall passivation

M. A. Blauw, E. van der Drift, G. Marcos, andA. Rhallabi

Citation: Journal of Applied Physics 94, 6311 (2003); doi: 10.1063/1.1621713

View online: http://dx.doi.org/10.1063/1.1621713

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Modeling of fluorine-based high-density plasma etching of anisotropicsilicon trenches with oxygen sidewall passivation

M. A. Blauwa) and E. van der DriftDelft Institute of Microelectronics and Submicron Technology, Delft University of Technology,Feldmannweg 17, P.O. Box 5053, 2600 GB Delft, The Netherlands

G. Marcos and A. RhallabiLaboratoire des Plasmas et des Couches Minces, Institut des Materiaux Jean Rouxel (IMN), UMR 6502,

CNRS-Universitede Nantes, 2 rue de la Houssiniere, BP 32 229, 44 322 Nantes Cedex 3, France

Received 14 April 2003; accepted 2 September 2003

The kinetics of high aspect ratio, anisotropic silicon etching in a SF6 O2 plasma is investigated with

a combination of Monte Carlo simulations and inductively coupled plasma etching experiments. The

spontaneous reaction of atomic fluorine is dominant at room temperature and Knudsen transport of

the radicals is the only limitation in narrow structures. At low temperaturestypically between 125

and95Coxygen passivation becomes effective and anisotropic profiles are obtained because the

oxygen passivation can only be removed by the directional ion bombardment. The input parameter

settings for the Monte Carlo model are based on measurements with plasma diagnostics.

Simulations show that anisotropy is controlled by the oxygen sidewall passivation which depends on

the oxygen flux, the oxygen adsorption coefficient, and the aspect ratio. The simulated trench

profiles and the aspect ratio dependent etch rate are consistent with the experimental results.

Experimentally the etch rate behavior can be tuned from aspect ratio dependent to aspect ratioindependent by decreasing the ion flux. This effect can be described well by the recently developed

chemically enhanced ion-neutral synergy model. It turns out that aspect ratio independent etching is

obtained if the downwards depletion of fluorine radicals due to Knudsen transport is compensated

by an increase of the available reaction sites. 2003 American Institute of Physics.

DOI: 10.1063/1.1621713

I. INTRODUCTION

Deep anisotropic plasma etching of silicon is an indis-

pensable tool for microfabrication. High etch rates 5

m min1 with high selectivity and good profile control are

achieved in fluorine-based high-density plasmas due to the

independent control of the radical and ion fluxes.1 5 This

way on-chip applications such as microelectromechanical

systems MEMS and passive radio frequency rf compo-

nents coupled to the electronic circuitry become realistic. In

particular, exploiting the third dimension with high aspect

ratio structures, i.e., structures with a high depth to width

ratio, yields highly sensitive inertial sensors based on capac-

ity measurement at low area consumption. However, the sup-

pression of the lateral etch is of utmost importance for de-

vices with high aspect ratio structures.

Oxygen radicals play a crucial role as etch inhibitor in

deep anisotropic silicon etching. The controlled cooling ofthe substrate down to 125 C assures that a solid oxide

layer is formed on the silicon surface. It prevents the reaction

of fluorine radicals with silicon and can only be removed by

the directional ion bombardment. As such, the addition of

oxygen is necessary to obtain anisotropic profiles, because

even at these very low temperatures atomic fluorine by itself

may result in isotropic etching in high-density plasmas.6 The

amount of oxygen has to be adjusted precisely in relation to

the local fluorine concentration to obtain perfectly vertical

sidewalls in high aspect ratio trenches. The role of the oxy-

gen radicals is hard to quantify, especially in deep structures

where surface analysis of the bottom and sidewalls is ex-

tremely difficult.The surface chemistry is a delicate interplay between

ions, oxygen radicals, and fluorine radicals. Many aspects of

the process have already been investigated in detail, such as

the chemical reaction of fluorine radicals with plain silicon

and the etching of silicon with fluorine radicals under Ar ion

bombardment.7,8 Surface analysis with x-ray photoelectron

spectroscopyXPS and Rutherford backscattering spectros-

copyRBS has resulted in a thorough knowledge of surface

processes, however, many questions still exist about the ex-

act interactions of all three species simultaneously.912

In this work Monte Carlo simulations including both the

fluorine and oxygen flux and inductively coupled plasma

ICP etching experiments are combined in order to clarifythe underlying reaction mechanisms. In this unique situation

the profile evolution of high aspect ratio trenches for

fluorine-based plasma etching with oxygen surface passiva-

tion is investigated. In previous work by Marcos et al.silicon

etching is the exclusive result of fluorine radicals and ions,

but the importance of oxygen radicals for lateral etch control

was also demonstrated.13 In Sec. II, the Monte Carlo simu-

lation model is described. Plasma diagnostics are used to

measure the density of fluorine radicals, oxygen radicals, and

ions for a quantitative approach of the simulations. WithaElectronic mail: [email protected]

JOURNAL OF APPLIED PHYSICS VOLUME 94, NUMBER 10 15 NOVEMBER 2003

63110021-8979/2003/94(10)/6311/8/$20.00 2003 American Institute of Physics


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these data as input for the parameter settings deep aniso-

tropic trench etching is realistically simulated. In particular,

the role of the oxygen radical flux in the profile evolution is

investigated, because the oxygen radicals along with the ions

are mainly responsible for the passivation mechanism. In

Sec. III the experimental etch rate as a function of aspectratio is investigated. Aspect ratio dependent etching ARDE

is observed, which is primarily caused by the depletion of the

fluorine radicals in narrow structures. This is described by

the Knudsen transport model. Aspect ratio independent etch-

ingARIEis obtained by tuning the ion flux to an extremely

low value. To some extent the Knudsen transport model fails

to describe ARIE correctly. To improve this, the chemically

enhanced ion-neutral synergy model has been developed,

which includes the fluorine radical, oxygen radical, and ion

flux. The latter model explains both ARDE and ARIE well.

The results of the plasma diagnostics are also used for an

independent verification of the model parameters.

II. PROFILE EVOLUTION WITH A MONTE CARLOSIMULATION MODEL

A. Monte Carlo simulation model

An etching simulator based on the Monte Carlo method

is used. Details of the model have been described in a pre-

vious article13 and here it is only briefly summarized. A two-

dimensional structure is considered composed of a mask and

a silicon substrate as shown in Fig. 1. It is defined by a

cellular discretization, which describes both geometry and

surface composition. The size of each cell is 1 nm and ac-

cording to the silicon density this corresponds to fifty atoms

per cell. The etching process is treated by the introduction ofinteractions between the incident plasma species fluorine,

oxygen, and ionsand the surface compounds (SiOxFy cells.

The Monte Carlo approach allows the introduction of chemi-

cal and physical mechanisms, each with its probabilistic con-

siderations. Typical examples are neutral adsorption and de-

sorption, spontaneous chemical etching, isotropic and

specular reflection of ions, preferential sputtering, passiva-

tion layer formation, and redeposition of etched species. All

these mechanisms play a role in the profile evolution. 1416

In a SF6 O2 plasma the two main reactive neutrals are

fluorine and oxygen atoms. For these neutrals, isotropic

fluxes are considered above the mask. Their impact is calcu-

lated by virtue of their adsorption probability. In literature

there is much contradiction about the fluorine adsorption

probability which is between 0.001 68 and 0.1 at room

temperature.7,13 Because of this variance the fluorine adsorp-

tion probability was determined independently in this work.

The oxygen adsorption probability is close to unity. In the

model the adsorption process is automatically followed by a

chemisorption mechanism creating a series of SiOxFy cells

on the surface. Only if a silicon cell is completely saturatedwith fluorine, the SiF4 cell is immediately desorbed thus

modeling spontaneous chemical etching. In all other cases

some ion assistance is needed to remove the cell which is

described in the next paragraph.

To include ion-assisted effects, a model of the ion trans-

port through the plasma sheath, which is connected with the

surface module, calculates the angular and energetic distri-

bution function of the incident ions. When an ion strikes the

surface the preferential sputter yield is determined. The latter

depends on the ion energy, the angle of incidence, and the

chemical nature of the impacted surface site. The SiOxFysputter yield Yx ,y is calculated according to

Yx ,yx ,yAEIET 1

as a function of the ion energy EI . For the Si sputter yield

Y0,0 the results of SFx ion beam etching of silicon are used

where the threshold energy ETis 6 eV and the prefactor A is

0.15.17 For Si cells the factor 0,0 is equal to one. For other

SiOxFy cells the factor x ,y is modulated. The most extreme

values are the SiO2 factor of 0.4 and the SiF3 factor of 1.3.

Y0,0 is 0.79 for a typical ion energy of 60 eV. This is one

order of magnitude larger than for Si sputtering with Ar

ions. It incorporates the effect that the yield increases if fluo-

rine is involved in the sputtering process.

Redeposition of sputtered species is studied by consider-

ing their adsorption on the trench sidewalls. Local surface

displacement is thus modeled by the disappearance or reap-

pearance of a SiOxFy cell, when an etching or redeposition

process occurs, respectively.

B. Input parameters from experimental data

The equipment that has been used for the etching experi-

ments is an Alcatel MET ICP reactor and an Alcatel RCE

200 reactor.18 The latter is an electron cyclotron resonance

ECR system in which the ion flux can be tuned by placing

a quartz cylinder around the wafer, so that the direct path

between the ECR source region and the wafer is blocked.This effectively results in a downstream reactor geometry

with an extremely low ion-to-radical flux ratio at the wafer

level, because ion-electron recombination is much faster than

neutralneutral recombination. Ion densities are measured

with a Langmuir probe Scientific Systems Smart Probeand

radical densities are measured with actinometry optical

emission spectroscopy, ISA/Jobin Yvon-Sofie Digitwin

550.19,20 Radical fluxes are calculated assuming a plasma

temperature of 400 K.

The flux of ions, oxygen, and fluorine radicals have to be

known for a correct Monte Carlo simulation. The plasma

conditions for deep anisotropic trench etching in the ICP

FIG. 1. Two-dimensional cellular grid in the Monte Carlo simulation. The

structure is composed of a mask with an aperture and a silicon substrate.

6312 J. Appl. Phys., Vol. 94, No. 10, 15 November 2003 Blauw et al.


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reactor were as follows: 300 sccm SF6 , 5.5sccm O2 , 3.4 Pa

pressure, 2500 W source power, 25 V bias voltage, and

120 C substrate temperature. A fluorine radical partial

pressure of 1.27 Pa and an oxygen radical partial pressure of

0.032 Pa were determined for a plasma nearly identical to

these plasma conditions. The fluorine radical flux Fis given

by

Fp F

2m FkT, 2

where pF is the fluorine radical partial pressure, m F the fluo-

rine atom mass, k the Boltzmann constant, and T the plasma

temperature. The fluorine radical flux to the substrate is thus

approximately a factor 40 higher than the oxygen radical

flux. In other words the oxygen percentage is 2.5%. In aparticular experiment the flow of oxygen is adjusted to ob-

tain perfect anisotropy. With a Langmuir probe, the ion cur-

rent density was measured. It was 0.39 mA cm2 on the sub-

strate position directly below the source and it was 0.07

mAcm2 on the substrate position 8.0 cm downstream. The

fluorine and oxygen radical flux were constant. This gives

fluorine radical-to-ion flux ratios of 1.48103 8.26103,

depending on the position.

The silicon etch rate was measured without the interfer-

ence of the ion flux at a substrate temperature of 25 C to

investigate the etching kinetics of fluorine radicals. The ver-

tical ion flux is excluded in a structure consisting of horizon-

tal lines in 1.5 m thick silicon on insulator SOI coveredby thermal silicon dioxide. The fluorine radicals reach the

silicon core through a window at one of the sides, and an

empty tube of silicon dioxide remains after etching. The av-

erage etch rate in these tubes has been plotted as a function

of the aspect ratio in Fig. 2 together with a view of the

structure after etching. A least squares fit of the data with the

Knudsen transport model described by Eq. 11 in Sec. III

gives a reaction probability of 0.47 together with an initial

etch rate of 10.6 min1. A fluorine radical partial pressure

of 2.5 Pa was measured with actinometry for the plasma

conditions used in this experiment. The etch rate RE is given

by

REFS

rF, 3

whereSis the reaction probability, rF the average number of

fluorine atoms per etched silicon atom, and the atomic

density of silicon. It is assumed that all silicon is removed in

the form of SiF4( rF4) , because in literature it has been

shown that SiF4 is the major reaction product below

300 C.21

However, above this temperature SiF2 is also animportant reaction product. In that case the etch rate in-

creases for a given fluorine flux and reaction probability,

because 2rF4. An initial etch rate of 10.2 m min1 can

be calculated from the independently measured fluorine radi-

cal partial pressure and the fitted reaction probability of 0.47

using Eqs.2 and 3 with rF4. This is in good agreement

with the fitted initial etch rate, which is derived for a sub-

strate temperature of 25 C.

C. Profile evolution results

For the profile simulations a radical-to-ion flux ratio of

0.950103 was chosen. It is of the same order of magnitude

as for the experiments and already high in comparison with

previous simulations.13 The pressure was 2.8 Pa. The adsorp-

tion probability of fluorine and oxygen was set to 0.5 and

0.7, respectively. The value for fluorine is based on the

Knudsen transport results of horizontal lines given in Sec.

II B. For oxygen it has been shown that the adsorption prob-

ability is near unity on bare silicon.22 For a covered silicon

surface the oxygen adsorption probability is reduced. Corre-

spondingly, an average value of 0.7 takes into account the

reduction of the oxygen adsorption probability in the simu-

lation model. Two sites per silicon atom are available for

oxygen adsorption.

The simulated trench profiles for an increasing oxygenpercentage of 0%, 5%, and 7.5% of the total neutral flux are

given in Figs. 3a, 3b, and 3c, respectively. The adsorp-

tion probability for sputtered species is initially set to 0.5.

The mask width and thickness are 0.50 and 0.25 m, respec-

tively. Intermediate profiles are shown for each 0.50 m etch

step. The depth of the profiles is 2 m except for the trench

in Fig. 3a, because the limits of the simulation grid are

reached in the lateral direction. The simulation shows that

oxygen is necessary for anisotropy. Without oxygen there is a

high lateral etch rate corresponding to purely isotropic

etching.13 The best anisotropy is obtained for an oxygen per-

centage of 5%, because most lateral etching is suppressed.

This percentage is rather high compared to the 2.5% in theexperiments. However, in the simulation the ion flux is

higher leading to more sidewall erosion, and moreover, in the

experiment molecular oxygen plays a role in the surface pas-

sivation. Spikes appear on the trench bottom for 7.5% oxy-

gen. This is similar to the formation of silicon grass due to

micromasking, which is experimentally observed as a result

of over-passivation. Reducing the adsorption probability for

sputtered species to 0.0 improves the trench profile, which is

shown in Fig. 3d for 5% oxygen.

With an adsorption probability for sputtered species of

0.0, the etching of a deep trench was simulated. The final

aspect ratio of the 0.5 m wide trench is 10. In Fig. 4 this

FIG. 2. Average etch rate as a function of the aspect ratio for horizontal

lines. The inset shows the hollow silicon dioxide structure after etching. The

continuous curve is a least squares fit of the Knudsen transport model with

a reaction probability of 0.47 for fluorine. The point at zero aspect ratio is

independently measured on a plain silicon sample.

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trench is compared with deep anisotropic trenches etched in

the ICP reactor with the conditions given in Sec. II B. The

etch rate of the pattern consisting of 6.0 m lines on a 12.0

m pitch was measured by in situ laser interferometry. The

results are very similar regarding anisotropy and lateral etch-

ing emphasizing the accuracy of the Monte Carlo simulation.

The quantitative agreement of the simulated and experimen-

tal initial etch rate is very good with 4.62 and 5.20

m min1, respectively. Although, the lateral etch rate is

0.14m min1 for the simulation and 0.03 m min1 for the

experiment. In the simulation the trench bottom becomes

gradually faceted. A decreasing ion flux in the corners of a

trench due to the angular distribution of ions could be the

cause. Angle dependent sputter yield and crystal orientation

dependent etching are not included in the simulation. How-

ever, these effects may play a role in the plasma etching

experiments. The etch rate as a function of aspect ratio is

presented in Fig. 5 both for the Monte Carlo simulation and

the ICP etching experiment.

III. ASPECT RATIO DEPENDENT ETCHING

A. Chemically enhanced ion-neutral synergy model

The etch rate is evaluated with the chemically enhanced

ion-neutral synergy model. This model is obtained by con-

sidering the surface site balance for both fluorine and oxygen

radicals given by

dF

dt FF1OFFFFJF

4a

and

dO

dt OO1OFOJO , 4b

where is the surface site area density, F the surface cov-

erage of fluorine, O the surface coverage of oxygen, F the

fluorine flux, O the oxygen flux, J the ion flux, F the ad-

sorption probability for fluorine, O the adsorption probabil-

FIG. 3. Simulation results for a 0.5 m wide trench with different percent-

ages of oxygen. aWhen no oxygen is added the profile is strongly isotro-

pic; b the passivation is optimal for 5% oxygen and the profile is aniso-tropic; andcmicromasking appears for an increase to 7.5% oxygen; difthe adsorption probability of the sputtered species is reduced from 0.5 to 0.0

the sidewall taper changes from slightly positive to vertical for 5% oxygen.

FIG. 4. Comparison of deep trenches obtained with aICP etching and b

Monte Carlo simulation under comparable conditions. Both results show a

lateral etch in the top part directly below the mask, in the middle part a

straight anisotropic profile and an apex at the bottom of the trench.

FIG. 5. Aspect ratio dependent etching: a the etch rate is measured two

times byin situlaser interferometry during ICP etching; bthe etch rate for

the Monte Carlo simulation. The data points have been fitted with the Knud-

sen transport model continuous curves and the chemically enhanced ion-

neutral synergy model dotted curves.



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ity for oxygen, F the spontaneous reaction rate for fluorine,

Fthe ion-induced desorption coefficient for fluorine, and Othe ion-induced desorption coefficient for oxygen. For quasi-

steady state the derivatives are negligible and the set of linear

equations can be solved for F and O . The etch rate is

obtained by substituting these quantities in

RE1

FF

4

YFJFYOJO

, 5

where is the silicon atom density andYFand YOthe sputter

yield for silicon fluoride and silicon oxide, respectively.

When one silicon atom is sputtered, four surface sites are

cleared (F4YF and O4YO). Equation 5 is very

comparable to ion-induced etching with an etch inhibitor, but

an important difference is the incorporation of the spontane-

ous etch term.2325 The spontaneous reaction term is respon-

sible for chemical etching of silicon with volatile SiF 4 as the

reaction product. In high-density plasmas the ion flux is

much lower than the radical flux. If it is assumed that the

spontaneous reaction is much faster than the ion-induced de-

sorption of fluorine (

F

F

J) the etch rate can be writ-ten as

RE1

FF4OO

YO

O OJ

1 FFF

1 OJOO

. 6

In cryogenic ICP etching the fluorine flux is typically much

higher than the oxygen flux (FFOO). Actinometry

confirms this. It can also be assumed that the spontaneous

reaction is much faster than the supply of fluorine (FFF), because the reaction with silicon is found to be

proportional to the fluorine radical partial pressure.

26

Withthese assumptions Eq. 6 can be simplified to

RE1

FF4OO

OJ1

OJ

OO

. 7

Apart from the factor FF over OO the result is very

similar to the well-known ion-neutral synergy model. How-

ever, the ion-neutral synergy applies to the oxygen passiva-

tion mechanism only. The etch rate is strongly enhanced due

to the spontaneous chemical reaction of fluorine with silicon

and the subsequent desorption of SiF4

, which is expressed

by the factor FFoverOO . With this amplification the

extremely high etch yield in the order of 1000 observed in

cryogenic etching can be understood better.18 The etch rate

can also be expressed as

REFF

41O, 8a

where

O1

1OJ

OO

. 8b

So it can be seen that the etch rate is proportional to the

fluorine flux and the fraction of uncovered surface sites. The

fluorine surface coverage is negligible. When this model is

used for etch rate evaluation in deep trenches the decrease of

the neutral fluxes Fand Owith depth should be taken into

account. For this purpose F and Oare expressed as a con-

stant flux at zero aspect ratio times a factor as a function of

aspect ratio.27 These functions are defined by

ii0i

AR iF,O 9a

and

iAR

KAR

KARi1OKAR

i1O, 9b

where KAR is the Knudsen transport coefficient and i the

adsorption probability of species i. The parameters F0, O

J/O0 , F , and O are unknown. To reduce the number of

known parameters F is set to 0.5 and O is set to 0.7 as in

Sec. II. After substitution of Eq. 9 in Eq. 8 the etch rate

function is transformed to a function with the fit parameters

a and b given byREF

ARa1O, 10a

where

O1

1b

OAR

. 10b

The fit parameters are defined by

aFF

0

410c

and

bOJ

OO0

. 10d

One more step has to be taken before fitting can be per-

formed. The oxygen flux in Eq. 9 and the oxygen surface

coverage in Eq. 10 are interdependent. By substituting Eq.

9b in Eq. 10b a quadratic expression for O is obtained.

This gives a solution for O as a function ofKAR , b and O .

The solution is substituted in Eq. 10a. By varying a and b

in this final expression the least-squares fit is obtained.

B. Modeling of experimental and simulated results

Etch rate results obtained from etching experiments and

Monte Carlo simulations were fitted with both the Knudsen

transport model and the chemically enhanced ion-neutral

synergy model. The Knudsen transport model is given by

REFF

0

4

KAR

KARFKAR

F. 11

The main difference is that the Knudsen transport model

does not take into account the role of oxygen. The etch rate

is proportional to the fluorine flux and the adsorption prob-

ability of fluorine. For the Knudsen transport model the ad-



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sorption probability is equal to the reaction probability. In

fitting the Knudsen transport model the fluorine flux at zero

aspect ratio F0 and the adsorption probability F are varied.

In the limit of zero oxygen surface coverage the chemically

enhanced ion-neutral synergy model is identical to the Knud-

sen transport model with an adsorption probability of 0.5.

Deep anisotropic trenches were made with ICP etching.

The etch rate was measured by in situ laser interferometry

during the ICP etching experiments. The results for thetrenches shown in Fig. 4aare plotted as a function of aspect

ratio in Fig. 5a. Corresponding results for the deep trench

Monte Carlo simulation are given in Figs. 4b and 5b.

For the ICP etching experiments the reaction probability

of fluorine at zero aspect ratio for the Knudsen transport

model and the chemically enhanced ion-neutral synergy

model is 0.26 and 0.32, respectively. Except for small differ-

ences the models fit equally well due to measurement inac-

curacies. For the simulation the reaction probability of fluo-

rine at zero aspect ratio is 0.41 and 0.44, respectively. There

are no visible differences between the two models. This is

due to the low oxygen surface coverage of 0.12 in the chemi-

cally enhanced ion-neutral synergy model. In this case theetch rate behavior is mainly governed by Knudsen transport

of fluorine.

With actinometry a fluorine radical partial pressure of

1.27 Pa was measured. An initial etch rate of 3.5 m min1

has been calculated with Eqs. 2 and 3 using the fitted

reaction probability of 0.32. This is lower than the experi-

mental value of 5.2 m min1. The discrepancy can be ex-

plained if it is assumed that the plasma temperature is equal

to the substrate temperature, because in that case the fluorine

flux and the calculated initial etch rate become a factor 1.6

higher. For the simulations the input is known, so that the

reaction probability can be calculated exactly from the etch

rate and the fluorine flux. In this case it is 0.333. This value

is lower than the fitted values of 0.41 and 0.44. Some effects

are not incorporated in the models. For example, sidewall

reactions that are responsible for the small lateral etch reduce

the fluorine flux. The etch rate decreases more quickly as a

function of aspect ratio, which effectively results in a higher

reaction probability in the fitted models.

Another remark concerns the fit parameter b. For the

simulation it is 7.3. The oxygen-to-ion flux ratio is 47.5. In

this case the ion-induced desorption coefficient of oxygen

must be 243 to obtain the right value ofb. However, in the

simulation this coefficient is only about 2.22 taking into ac-

count the sputter yield of the SiOxFy cells so a lot of oxygenmust be removed by other means. For example, the oxidized

cell can also be removed if the cell under it is etched away.

The chance of this is high because of the low oxygen surface

coverage and the high fluorine-to-oxygen flux ratio. The

boiling point of Si2OF6 is 23.3 C compared to 86.0 C

for SiF4 , so this oxyfluoride compound is slighty volatile

and can be removed without interference of ions during cryo-

genic etching.28

C. Aspect ratio independent etching

Plasma etching with a high and low ion flux was carried

out in the ECR reactor, i.e., without and with the quartz

cylinder, respectively. The plasma conditions were

40.0 sccm SF6 , 4.4sccm O2 , 0.32 Pa pressure, 400 W source

power, 13 V bias voltage and 100 C substrate tempera-

ture. The average etch rate for trenches in the range 0.4 10.0

m is given as a function of aspect ratio in Fig. 6. With a

high ion flux the etch rate is high and decreases relatively

fast with aspect ratio, whereas for a low ion flux the etch rate

is nearly constant.

The two data sets are fitted by the Knudsen transport

model using the least-squares method. For each data set aseparate reaction probability has been taken, but the fluorine

flux at zero aspect ratio is the same for both situations. The

best fit is obtained for a reaction probability of 0.42 and 0.17

and an initial etch rate of 1.92 and 0.78 m min1, respec-

tively. The result is plotted in Fig. 6a. The strong decrease

of the etch rate for the high ion flux is modeled well. How-

ever, the Knudsen model for the low ion flux still shows a

decrease for high aspect ratios.

The best fit of the chemically enhanced ion-neutral syn-

ergy model is plotted in Fig. 6b. The reaction probability of

fluorine at zero aspect ratio is 0.39 and 0.14 and the initial

etch rate is 1.79 and 0.62 m min1 for the high and the low

ion flux, respectively. In this case the aspect ratio indepen-dent data are modeled very well, because the decreasing

fluorine flux is compensated by a decreasing oxygen surface

coverage. In a deep trench the oxygen flux also decreases,

but the ion flux is constant, which leads to a larger fraction of

empty surface sites available for reaction with fluorine.

The fit results of the chemically enhanced ion-neutral

synergy model are tested with independent plasma diagnos-

tics. The fit parameterb is 3.55 and 0.39 for the high and the

low ion flux, respectively. The ion current densities measured

by the Langmuir probe are 0.083 and 0.005 mA cm2, re-

spectively. This makes clear that the difference found by the

model is largely in agreement with the change of the ion

FIG. 6. Aspect ratio dependent and independent etching by tuning the ion

flux. The results have been fitted by athe Knudsen transport model and bthe chemically enhanced ion-neutral synergy model.



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current density in the plasma see Eq.10d. A fluorine radi-

cal partial pressure of 0.31 Pa was determined with actinom-

etry. An initial etch rate of 1.05 and 0.38 m min1 has been

calculated with Eqs. 2 and 3 using this independent mea-

surement of the fluorine radical flux and the fitted reaction

probabilities for the high and low ion flux cases, respectively.

Compared to the fitted initial etch rate, it seems that the

calculated initial etch rate is systematically underestimated

with this method for the low temperature process. So someinaccuracy either in the actinometry or in the fitted adsorp-

tion probabilities must be supposed. The plasma temperature

decreases near the cooled substrate, so that the fluorine flux

increases see Eq. 2. If the plasma is locally in thermal

equilibrium the plasma temperature is 173 K 100 C in-

stead of the assumed 400 K. In that case the calculated initial

etch rate is 1.60 and 0.58 m min1, respectively, which

gives a much better agreement with the fitted results.

D. Further discussion

Only ARDE has been observed in ICP etching, but it

remains unclear why tuning of the plasma parameters does

not result in ARIE. In general, the fluorine radical flux deter-

mines the oxygen radical flux that is needed to prevent lat-

eral etching. Tuning the oxygen radical-to-ion flux ratio

causes the switching between ARDE and ARIE. However,

the ion flux and the ion energy are also factors that are im-

portant for profile control. Higher ion flux and ion energy

lead to a more negative taper due to sidewall erosion deeper

in the trench. So the ion flux and ion energy cannot be cho-

sen arbitrarily. Possibly, the oxygen radical-to-ion flux ratio

has to be lower in anisotropic ICP etching due to different

plasma conditions such as pressure and bias voltage. ARIE

has only been observed for the lowest bias voltages in ECRetching. The average angle of incidence is larger for lower

ion energy, so that erosion of the sidewall passivation is rela-

tively stronger. In that case the oxygen radical-to-ion flux

ratio has to be increased to prevent lateral etching, which

favors ARIE. In ICP etching it has not been possible to make

anisotropic profiles with lower bias voltages due to the form-

ing of surface roughness. Ion-neutral collisions in the plasma

sheath, which play a role in ECR etching, also increase the

average ion angle.18

Tuning of the etching process is also possible with the

substrate temperature. At a lower temperature species are

less volatile so a lower oxygen flux is sufficient for sidewall

passivation. However, surface species are also more difficultto remove by ion bombardment deeper in the trench and it is

hard to estimate whether the sidewall taper becomes more

positive or negative. In practice, a more negative taper is

observed for decreasing temperatures while keeping the oxy-

gen flux constant.29 This is explained by the depletion of

oxygen radicals, which is stronger than the depletion of fluo-

rine radicals, so that the sidewall passivation becomes

weaker in deep trenches. This is illustrated by the following

etching simulation with a radical-to-ion flux ratio of 0.1

103 and a fluorine-to-oxygen radical flux ratio of 9. The

fluorine adsorption probability was taken 0.1. The pressure

was 2.8 Pa and the bias voltage was 30 V. The mask aper-

ture was 0.50 m and the total etched depth was 10 m.

Table I gives the fraction of the incident ion, fluorine radical,

and oxygen radical fluxes that reach the bottom of a trench.

Values are given for etched depths between 0.5 and 2.5 m.

In the beginning of the process the passivation is important

with a fraction 0.31 of the oxygen flux impacting the trench

bottom. When the depth increases the plasma surface inter-

actions change. The oxygen radical-to-ion flux ratio becomes

lower. In the same time the depletion of fluorine radicals is

much less pronounced than the depletion of oxygen radicals.

The fluorine-to-oxygen radical flux ratio increases from 28 to

65. The passivation becomes thus weaker for larger depth. In

this simulation the surface is mostly covered by SiFx species

due to the low oxygen radical-to-ion flux ratio. In the begin-

ning this leads to a significant decrease of the simulated etch

rate due to Knudsen transport of fluorine radicals. In this

particular simulation preferential sputtering dominates for

depths larger than 2.5 m due to the very low fluorine

radical-to-ion flux ratio, which results in a smaller decrease

of the simulated etch rate.

IV. CONCLUSIONS

Monte Carlo simulations show that oxygen passivation

is necessary to obtain anisotropic profiles. Depletion of the

fluorine radical flux plays an important role in high aspect

ratio trenches both for the simulations and high-density

plasma etching experiments. The Knudsen transport model

describes the depletion of the fluorine radical flux in narrow

structures during etching. Simulated and experimental results

for comparable plasma conditions agree very well. Switching

from ARDE to ARIE is observed if the ion flux is decreased

in ECR etching. This behavior is explained by the chemically

enhanced ion-neutral synergy model, which takes into ac-count the depletion of both fluorine and oxygen radicals. For

a high ion flux the oxygen surface coverage is low and the

depletion of fluorine radicals dominates. On the other hand,

for a low ion flux the oxygen surface coverage is high, and

the depletion of fluorine radicals is compensated by a de-

crease of the oxygen surface coverage leading to ARIE.

ACKNOWLEDGMENTS

This work is supported by Technology Foundation STW,

the Applied Science Division of NWO Netherlands Organi-

zation for Scientific Research, under Project No. DEL 4577.

TABLE I. The fraction of ions I , fluorine radicals F , and oxygen radicals

O , that reach the bottom of a trench with a 0.50 m wide mask aperture

for depths between 0.5 and 2.5 m. The fluorine-to-oxygen radical flux ratio

increases with depth.

Depth

m I F O

0.5 0.48 0.97 0.31

1.0 0.35 0.62 0.15

1.5 0.42 0.60 0.162.0 0.24 0.31 0.06

2.5 0.28 0.29 0.04



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