Acta Polytechnica


Acta Polytechnica 53(2):113–116, 2013 © Czech Technical University in Prague, 2013
available online at http://ctn.cvut.cz/ap/

THE PROCESS OF PLASMA CHEMICAL PHOTORESIST FILM
ASHING FROM THE SURFACE OF SILICON WAFERS

Siarhei Bordusau∗, Siarhei Madveika, Anatolii Dostanko

Belorussian State University of Informatics and Radioelectronics, Minsk, Belarus
∗ corresponding author: bordusov@bsuir.by

Abstract. At present, the research for finding new technical methods of treating materials with plasma,
including the development of energy and resource saving technologies for microelectronic manufacturing,
is particularly actual.

In order to improve the efficiency of microwave plasma chemical ashing of photoresist films
from the surface of silicon wafers a two-stage process of treating was developed. The idea of the developed
process is that wafers coated with photoresist are pre-heated by microwave energy. This occurs because
the microwave energy initially is not spent on the excitation and maintenance of a microwave discharge
but it is absorbed by silicon wafers which have a high tangent of dielectric losses. During the next step
after the excitation of the microwave discharge the interaction of oxygen plasma with a pre-heated
photoresist films proceeds more intensively.

The delay of the start of plasma forming process in the vacuum chamber of a plasmatron
with respect to the beginning of microwave energy generation by a magnetron leads to the increase
of the total rate of photoresist ashing from the surface of silicon wafers approximately 1.7 times.

The advantage of this method of microwave plasma chemical processing of semi-conductor wafers
is the possibility of intensifying the process without changing the design of microwave discharge module
and without increasing the input microwave power supplied into the discharge.

Keywords: microwave plasma, microwave plasmatron, plasma chemical ashing of photoresist.

1. Introduction
At present, in most cases the technology of plasma
chemical photoresist ashing from the surface of semi-
conductor wafers uses high frequency or microwave
discharges the main drawback of which is inertia
of the process at the beginning of plasma forming
process. Ashing of photoresist begins with a delay
connected with warming up of the reaction-discharge
chamber’s construction elements and wafers. The du-
ration of the delay being dependent on the temper-
ature of the reaction-discharge chamber and wafers,
and, consequently, on the size and volume occupied
by semiconductor wafers [8]. The transition of the mi-
croelectronic industry to the use of 200 mm and even
larger diameter wafers needs reaction discharge cham-
bers of more than 0.005 m3 that significantly affects
the duration of photoresist ashing with plasma.
The formation of high volume and large area mi-
crowave discharges leads to certain construction dif-
ficulties as industrial microwave discharge plasma
sources are designed for 2.45 GHz frequency char-
acterized by a small electromagnetic wave length
(12.2 cm in open space [12]) and a small depth of elec-
tromagnetic field’s penetration into plasma.
Experiments to ash photoresist from the surface

of silicon wafers during batch processing led to finding
out a specific character of “loading” effect manifesting
itself in partial absorption of the electromagnetic wave

power entering a microwave resonator by the material
with a high tangent dielectric loss (silicon) [1, 4, 5].
Peculiar features of microwave energy and silicon
wafer interaction should be taken into consideration
as well as used in developing and their processing
in plasma microwave discharge.

The essence of photoresist plasma chemical ashing
lies in the interaction of active particles, as a rule,
oxygen plasma (atoms, excited molecules, ions, rad-
icals) with photoresist molecules which are organic
compounds [10]. The speed of these reactions is de-
termined by the streams of particles onto the wafer
and their temperature. Hence, with the increase
of reaction discharge chamber’s dimensions, quan-
tity and diameter of plasmatron’s semiconductor
wafers treated in the chamber, it is necessary
to increase the power of supplied microwave radia-
tion. Since medium power microwave magnetrons
for technological application have maximum power
about 0.8 ÷ 1.5 kW [6], it becomes necessary to create
additional stimulating effects on the process of batch
microwave plasma chemical treatment of materials.

The methods of controlling the processes of plasma
chemical treatment of semiconductor wafers with
the use of gas plasma microwave discharge may be
conventionally divided into two groups: by changing
the modes of treatment (power, frequency and form
of electric pulses etc.) and by changing non elec-
trical operation modes (pressure, type of gas, speed

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Siarhei Bordusau, Siarhei Madveika, Anatolii Dostanko Acta Polytechnica

of gas stream etc.) [3]. These ways are not always
efficient enough and since the increase of rate of pho-
toresist ashing from the wafer surface is proportional
to the value of sum power supplied to the discharge
from various energy sources, stimulating the treat-
ment process with such types of effects as bombarding
by inert gas ions, laser radiation, electron flow, ther-
mal heating before the treatment, heating up with
microwave radiation etc., is of great interest [9].
Wide use of additional methods of stimulating

the processes of plasma treatment processes of materi-
als is hindered by insufficiently developed technology
as well as construction differences in plasma chemical
equipment.
In order to stimulate the processes of microwave

plasma chemical photoresist ashing from the surface
of semiconductor wafers we suggest using microwave
warming up before the beginning of plasma treatment.
While plasma is absent in the reaction discharge cham-
ber during microwave heating, energy is absorbed
straight in the volume of the semiconductor material
without the mechanism of heat transfer from plasma
to wafers. The reaction of plasma chemical photore-
sist ashing is endothermic, so photoresist previously
warmed up during the stage of plasma treatment will
be ashed away quicker.

The purpose of the performed research was to study
the application of this method in developing highly
effective technologies and the possibility of its use
in microwave plasma chemical etching processes us-
ing the equipment already available at factories.

2. Method of experiment
The research was carried out on the base of microwave
discharge installation with a quartz tunnel-flow reactor
200 mm in diameter and 320 mm in length with a vol-
ume about 0.009 m3 placed in the center of a resonator.
An M-112 microwave magnetron was used for gen-
erating electromagnetic radiation of at least 650 W,
the efficiency coefficient being equal to 60 %. The to-
tal measured power consumed by the installation was
equal to 1300 W.
Monocrystal silicon wafers, 76 mm in diameter

and 0.3 mm thick, coated with of 1.4 ± 0.1 µm thick
S1813G2SP15 photoresist film, treated in accordance
with the standard modes of lithography, were used
as objects for treating. During the experiments
to study the rate of photoresist ashing the wafers were
placed in the reaction discharge chamber in pairs.

The moment of terminating the microwave plasma
chemical photoresist ashing from the surface of sil-
icon wafers was controlled with the spectrometer
SL 40-2-2048 ISA with 777.96 nm oxygen line inten-
sity. O2 from oxygen cylinders was used as plasma
forming gas.
The temperature of silicon wafers was measured

with the help of a pyrometer Testo 830-T1 (an infrared
thermometer).

3. Results and their discussion
In the result of experiments it was found out that
the delay of plasma forming process in a plasma-
tron vacuum chamber with respect to the beginning
of microwave energy generated by a magnetron leads
to the increased rate of photoresist ashing from the sur-
face of silicon wafers approximately by 1.7 times
and reaches 40 nm/s. It means that the efficiency
of plasma ashing of photoresist from the surface of sil-
icon wafers can be increased with two stage method
of treatment. The first stage is warming up of a semi-
conductor wafers with microwave energy, the second
is interaction between microwave plasma and warmed
up photoresist.
The essence of the developed two-stage method

of plasma chemical photorest ashing from the surface
of semiconductor wafers lies in a preliminary warming
up of photoresist with microwave energy. Initially,
microwave energy is not spent on exciting and main-
taining microwave discharge but is completely ab-
sorbed by silicon wafers with photoresist having high
tangent angle of dielectric losses thanks to which
they are warmed up. During further interaction of oxy-
gen plasma with preliminarily warmed up photoresist
the process of oxidizing destruction of photoresist
is more intensive and is accompanied by volatile com-
ponents of reaction products, which are removed from
the reaction chamber with a vacuum pump.
The delay of plasma formation with respect

to the beginning of microwave energy generation
by a magnetron was performed in the following way.
The reaction discharge chamber is pumped out to a suf-
ficient pressure and then is filled with plasma form-
ing gas to reach the pressure one/two times higher
than the working pressure at which plasma will
not ignite under the influence of microwave en-
ergy. Then the magnetron microwave generation is
switched on. The wafers being warmed up for a cer-
tain time under the influence of microwave en-
ergy, the required pressure of plasma forming gas
is set, which provides microwave discharge excitation
and the microwave plasma chemical treatment pro-
cess is performed. Semiconductor wafers are kept
in plasma till the photoresist is completely removed
from their surface.

Figures 1 and 2 show the dependencies of full cycle
duration (microwave warming up and plasma treat-
ment) and the rate of photoresist ashing from the sur-
face of silicon wafers calculated with respect to the to-
tal treatment process duration on the time of plasma
formation delay with respect to the beginning of mi-
crowave generating by the magnetron. Photoresist
was ashed from the surface of two Si wafers.

The presented dependencies show that they have
an extreme character. We believe it may be connected
with the structural changes going on in the photore-
sist film under the influence of both the temperature
factor [7, 13], and the result of processes caused by mi-
crowave field [11].

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vol. 53 no. 2/2013 The Process of Plasma Chemical Photoresist Film Ashing

Figure 1. Dependence of total T duration of photore-
sist ashing from the surface of Si wafers on the delay
time t of the beginning of plasma formation with re-
spect to the beginning of microwave energy generation.

Figure 2. Dependence of the calculated speed V
of photoresist ashing from the surface of Si wafers
on the delay time t of the beginning of plasma for-
mation with respect to the beginning of microwave
energy generation.

The experiments on the investigation of silicon wafer
temperature depending on the time of treating them
under the influence of microwave field (Fig. 3) showed,
that in the time range of 20 seconds the dependence
looks like linear.

According to the data shown in Fig. 3 it is evident
that during microwave warming up of silicon wafers
their temperature reaches and exceeds the thresh-
old working temperature for this type of photore-
sist (130 ÷ 140 ◦C). Analyzing the data of Figs. 2
and 3 it is possible to make a conclusion that with re-
spect to the conditions of the experiment after 15 s
of microwave energy application, structural changes
in the photoresist film may take place. It results
in the increase of film resistivity to the process
of plasma destruction [2] and, consequently, to the re-
duction of plasma chemical photoresist ashing rate.
In order to define the rule of revealing the process

acceleration effect in case of preliminary microwave
warming up of Si wafers, a series of experiments
was carried out to investigate the “loading” effect
with reference to the conditions of treatment under
investigation. The experiments were carried out load-
ing 2, 5, 10 and 15 wafers into the reaction discharge
chamber. The delay of plasma formation with re-

Figure 3. Temperature of Si wafers θ depending
on the time t of their warming up under the influence
of microwave energy.

Figure 4. Rate of photoresist film ashing from the sur-
face of Si wafers depending on the number of Si wafers
in the reactor (the delay duration of the beginning
of plasma formation with respect to the beginning
of microwave energy generation t = 15 s).

spect to the beginning of microwave generation in ac-
cordance with the data of Fig. 2 was chosen equal
to 15 seconds.
Figure 4 shows the data on the rate of photore-

sist ashing from the surface of Si wafers depend-
ing on the quantity of Si wafers in the reaction dis-
charge chamber. Analyzing the obtained data and
the data given in [5], we can arrive at the conclusion
that the character of “loading” effect in case of two
stage treatment is analogous to the classical microwave
plasma chemical treatment. We suppose in both cases
the process of photoresist film ashing takes place in ac-
cordance with the same mechanism but with a higher
(up to 1.7 times) rate in case of preliminary activation
owing to the microwave warming up.
The experimental data on the investigation of de-

pendence of the rate of plasma chemical photoresist
film ashing from the surface of Si wafers on the pres-
sure of O2 in the reaction discharge chamber is shown
in Fig. 5.

Figure 5 shows that the investigated two-stage pro-
cess of microwave plasma chemical photoresist film
ashing is characterized by the presence of an evi-
dent extreme value of photoresist film ashing rate
in the range of O2 pressure which qualitatively coin-
cides with the analogous dependence obtained in [5].

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Siarhei Bordusau, Siarhei Madveika, Anatolii Dostanko Acta Polytechnica

Figure 5. Rate values of photoresist film ashing from
the surface of two silicon wafers depending on the pres-
sure O2 (the delay duration of the beginning of plasma
formation with respect to the beginning of microwave
energy generation t = 15 s).

These results can also confirm the conclusion
about the identity of microwave plasma chemi-
cal photoresist film coating processes’ mechanisms
for the studied two-stage method of microwave treat-
ment and a standard single stage process.

4. Conclusions
The performed experimental investigation shows
that microwave energy can be successfully employed
for intensifying plasma chemical processes running
at plasma chemical photoresist film coating ashing
during the production of electronic devices.

The effect of acceleration the treatment is achieved
by absorption of microwave energy in silicon plates
and their heating during processing without plasma.
The advantage of such microwave plasma chemi-

cal treatment of semiconductor wafers is the possi-
bility of a significant reduction of the process dura-
tion without changing the construction of microwave
discharge module for technological application and
without increasing the microwave power supplied
to the discharge.

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	Acta Polytechnica 53(2):113–116, 2013
	1 Introduction
	2 Method of experiment
	3 Results and their discussion
	4 Conclusions
	References