frequency response in pulsed dc reactive sputtering processes 2000 thin solid films

7/30/2019 Frequency Response in Pulsed DC Reactive Sputtering Processes 2000 Thin Solid Films

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Frequency response in pulsed DC reactive sputtering processes

L.B. Jonsson, T. Nyberg, I. Katardjiev, S. Berg*

The Angstrom laboratory, Uppsala university, P.O. Box 534, 751 21 Uppsala, Sweden

Received 30 April 1999; received in revised form 24 November 1999; accepted 24 November 1999

Abstract

By simple arguments as well as results from a recently developed computer simulation model we have found out that for high frequency

pulsed DC reactive sputtering the target poisoning does not reect the periodicity of the pulsed DC power supply. The degree of target

poisoning does not change markedly during a single duty cycle. The degree of poisoning essentially exhibits the same continuous time

independent behavior as observed for the conventional continuous reactive sputtering process. Furthermore, it is shown that the distribution

width of the transit times for sputtered atoms by far exceeds the period time for pulsed DC frequencies higher than 510 kHz. This causes a

large overlap between sputter eroded material between consecutive pulses during processing resulting in an essentially continuous arrival

rate of sputtered atoms to the substrate surface. This implies also that the deposition rate will be constant and will not follow the pulsed

sputter erosion variation from the target. These ndings show that, with respect to lm stoichiometry and homogeneity, the high frequency

pulsed DC reactive sputtering process behaves identically as the continuous reactive sputtering process. No chemical reaction effects or gas

gettering variations will follow the periodicity of the pulsed DC power supply at high frequencies. q2000 Elsevier Science S.A. All rights

reserved.

Keywords: Pulsed DC; Reactive sputtering

1. Introduction

Sputtering is probably the most popular metal thin lm

deposition process. There exist commercially available

codes [1,2] enabling the user to calculate the distribution

of sputtered material at the substrate surface. These

programs take into consideration the shape of the erosion

track at the magnetron target, the energy- and angular-

distributions of the sputtered atoms, the scattering of the

latter via gas-phase collisions as well as the geometry of

the processing chamber.

Reactive sputtering can be used to deposit insulating thin

lms. Performing DC magnetron sputtering from a pure

metal target in an ambient of Ar and oxygen may result in

the formation of a metal oxide insulating coating on the

substrate. However, the presence of oxygen in the proces-

sing chamber, unfortunately, may also result in some oxida-

tion of the metal target surface. The thin oxide coating at the

target may be charged by the bombarding ions. This will

result in microarcing at this surface. During such arcing

microparticles may be formed resulting in a coating with

microparticles inclusions. This is of course undesirable. To

overcome this problem it has become quite popular to

supply the DC power in the form of high negative pulses

interrupted by small positive pulses (arc suppression) [3,4].In this way the charge at the oxide surface on the target may

be neutralized by attracting electrons during the positive

part of the duty cycle. It has been shown that properly

matched positive and negative pulses may allow for arc

free reactive sputter deposition of insulating materials

from pure metal targets.

One might suspect that the DC-pulsed reactive sputter

deposition process may create different processing condi-

tions than those obtained in the traditional continuous RF-

or DC-magnetron sputtering. We have observed that there is

surprisingly little reported about the basic understanding of

the physical and chemical mechanisms for the DC pulsed

technique. In fact we have not found any article in the

literature that penetrates in detail how the pulsed DC reac-

tive sputtering process deviates from the continuous reac-

tive sputtering process.

Our group has worked intensively with modeling and

experimental studies of the reactive sputtering technique

for the last 1015 years. The computer simulation model

for the reactive sputtering process that was originally

suggested by Berg et al. [58] has probably become the

most generally accepted description of this process. As a

natural continuation of our work concerning reactive sput-

Thin Solid Films 365 (2000) 4348

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99) 01116-5

www.elsevier.com/locate/tsf

* Corresponding author. Tel.: 1 46-18-471-3084.

E-mail address: [email protected] (S. Berg)


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tering we have also initiated both experimental and theore-

tical studies of the pulsed DC process. It is the purpose of

these studies to obtain a more detailed understanding of

some of the basic physical and chemical processes that

take place with this relatively new processing technique.

In pulsed DC reactive sputtering erosion of the target only

takes place during the negative part of the duty cycle. One

might suspect that the positive part of the duty cycle offers

an additional time for reactions with the sputter eroded

metal atoms and the reactive gas at the substrate as well

as at the target surface. This is a signicant difference in

processing conditions as compared to the continuous sput-

tering process. It is the purpose of this article to clarify how

the conditions at the target and substrate will differ during

pulsed DC reactive sputtering as compared to continuous

reactive sputter deposition. The differences (and similari-

ties) between the two cases are explained and illustrated

by simple and well known accepted physical and chemical

effects.

2. Arc suppression

During DC reactive sputter deposition of e.g. an oxide

from an elemental metal target in a mixture of argon and

oxygen an insulating oxide lm may be formed on the target

surface. This insulating layer will be charged by the positive

ions that are accelerated in the plasma and collected on this

surface. If the insulating layer could sustain an innite elec-

tric eld strength this layer would ultimately be charged to

the applied power supply voltage Va. As a consequence,

there would be no potential left to accelerate ions and elec-trons in the plasma and the plasma would therefore disap-

pear and the sputtering process will stop.

However, the insulating lm at the target can normally

withstand an electric eld strength of up to 110 MV/cm

before electrical breakdown in the layer will occur. During

such a breakdown (micro-arcing) small particles may be

ejected from the arcing tracks. These micro particles may

reach the substrate and become imbedded in the growing

lm. This will most probably be detrimental to the intended

lm properties.

A DC pulsed power supply can create conditions that

eliminate the arcing at the target. The voltage build up VD

on the insulating layer may be limited by interrupting thenegative voltage and applying a short positive pulse to the

target. This positive pulse will attract electrons to the target.

This ux of negative particles will discharge fully or

partially the insulating layer on the target. In this way arcing

can be avoided. In Fig. 1 is shown a simplied drawing of

the surface of the metal target during reactive DC sputter-

ing. Included in Fig. 1 is also a schematic drawing of the

time dependent target bias during pulsed DC reactive sput-

tering.

We recall the simple arguments for selecting a suitable

pulse frequency when operating the reactive sputtering

process with a pulsed DC power supply. As a rst order

approximation the dielectric layer on the target may be

described as a parallel plate capacitor where the dielectric

layer represents the thickness (d) between the parallel

plates. The voltage build up (VD) on this capacitor will

follow the simple relation

Q VDC 1

where Q is the charge on the capacitor C formed by the

dielectric. The capacitor C can be expressed as

C 110A=d 2

and the voltage VD can be written as

VD Fd 3

where F is the electric eld strength in the dielectric layer.

Due to the ion current density J(during the negative part

of a duty cycle) charge will be collected in the capacitor.

The charge build up can be written

Q JAt 4

where t is the time that the constant current density J has

passed current to the dielectric layer.

Combining Eqs. (1)(4) will result in an expression for F

F J=110t 5

Assuming a relative dielectric constant 1< 5, an average

ion current density J< 20 mA/cm2 and that arcing will

occur when the dielectric has been charged to a critical

eld strength F< 5 MV/cm will result in a maximum char-

ging time t1024 s before the critical eld strength is

L.B. Jonsson et al. / Thin Solid Films 365 (2000) 434844

Fig. 1. Schematic of the target surface during reactive DC sputtering of an

insulating material. The dielectric layer is charged by positive ions causing

a voltage build up, VD, across the insulating layer with thickness d. Also

shown is a schematic of the time dependent target voltage, Va, during pulsed

DC reactive sputtering.


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reached. This corresponds to a minimum pulse repetition

frequency of approx. 10 kHz to avoid arcing. However, to

assure absolute arc-free operation practical operating DC

pulse frequencies use to be in the region of 50100 kHz.

These simple `school-book calculations' serve to clarify

why it is necessary to choose a relatively high operating

frequency for the pulsed DC reactive sputtering process.

In the following sections we will predict in some detail

how the target and substrate conditions will vary during one

duty cycle of the operating pulse frequency.

3. Target erosion

3.1. Negative period

It is interesting to calculate how much the conditions at

the target may change during a single period of the DC pulse

duty cycle. It has been claimed that it is possible to prefer-

entially `clean' the target (sputter removal of possible oxide

formation) during the negative voltage period of the duty

cycle [9]. There are reasons to believe, however, that this

statement is not correct for the frequencies normally used in

pulsed DC processing.

It is quite easy to estimate the possible degree of change

of target poisoning by calculating the possible maximum

sputter erosion from the target during one single period of

the DC pulse repetition frequency.

At moderate target to substrate distances and low pres-

sures the target erosion rate needed to supply a substrate

deposition rate of 104 A/min may be ten times greater


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takes a certain time to reach steady state processing condi-

tions. We dene that steady state has been reached when twoconsecutive periods become identical. In this calculation it

takes


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atoms and the gas scattering will contribute to a broadening

of the transit time distribution. A Monte Carlo based compu-

ter simulation of the distribution of transit times for sput-

tered atoms for a 0.1 m distance between the target and the

substrate has been carried out. Only atoms arriving at the

substrate within a radii of 5 cm from the symmetry axis have

been considered. The calculations have been carried out for

two different targets, Al and W. The energy distribution of

the sputter eroded atoms used in the calculations are shown

in Fig. 3. Due to the higher sublimation energy of W the

escape energy is somewhat higher for W atoms than for Al

atoms. The Ar pressure during processing was assumed to

be 5 mTorr. The results of the calculations of the transit time

distribution from sputter erosion from an Al or W target,

respectively, are shown in Fig. 4. The latter indicates that

sputtered Al atoms are more rapidly thermalized by gas

scattering with Ar atoms than the W atoms. Therefore, the

transit time distribution for sputtered Al atoms is shifted

towards higher values. Note, however, that the width of

these distributions


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However, this is certainly the case for a pulse repetition

frequency of 50100 kHz normally used in pulsed DC reac-

tive sputter processing.

5. Conclusions

By some simple but straightforward arguments and calcu-

lations we have illustrated that for high frequency pulsed

DC reactive sputtering the target poisoning as well as the

substrate deposition rate and lm composition do not reect

the periodicity of the pulsed DC power supply. These

processing parameters will exhibit the same continuous

time independent behavior as previously observed for the

continuous reactive sputtering process. This nding indi-

cates that the processing behavior (reactive gas gettering,

hysteresis etc.) at high pulse repetition rates will be very

similar to the continuous reactive sputtering. Results frommore sophisticated computer simulation calculations

conrm this prediction.

We agree to, as earlier have been suggested, that the

positive pulses primarily serve to supply electrons to the

target surface to prevent the harmful charging of possible

formed insulating layers on the target surface. In this way

arc free reactive sputter deposition of insulating thin lms

may be carried out.

On the other hand, however, one cannot rule out that the

intense short electron bombardment of the target does not

inuence the reaction kinetics between metal target atoms

and adsorbed molecules of the reactive gas. Furthermore,

for identical average power supplied to the target the peak

power is always higher for the pulsed DC process as

compared to the continuous process. This will give rise to

a higher plasma density during the sputter erosion part of the

duty cycle in the pulsed plasma system. These two effects

may generate conditions that have not been taken into

account in this work. These effects can be simulated by

modifying some of the parameters in the computer simula-

tions. As far as we have found out, however, it is unlikely

that these effects should change the process behavior to

deviate signicantly from what has been described in this

article.

Acknowledgements

This work has been nancially supported by the Swedish

Research Council for Engineering Sciences (TFR) and theSwedish Foundation for Strategic Research (SSf).

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L.B. Jonsson et al. / Thin Solid Films 365 (2000) 434848

Fig. 6. The net arrival time distribution (in steady-state) for aluminum

atoms sputtered during the dashed pulses (f 5 kHz).

frequency response in pulsed dc reactive sputtering processes 2000 thin solid films

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