In situ resistivity measurements during the atomic layer deposition of ZnO and W thin films

Article abstract of DOI:10.1063/1.1490413

In situ monitoring of the electrical resistivity of ZnO and W thin films during atomic layer deposition (ALD) was performed using a four-point probe. Large oscillations were observed during the ZnO ALD resistivity measurements. The resistivity dramatically increased during each diethyl zinc exposure and decreased during each H₂O exposure. In contrast, the W ALD resistivity measurements exhibited a steplike pattern where the resistivity decreased during the Si₂H₆ exposures and remained constant during the WF₆ exposures. In situ resistivity measurements will be useful to monitor ALD processing and also may help to optimize and understand the properties of gas sensors.

Full text of DOI:10.1063/1.1490413

In situ resistivity measurements during the atomic layer deposition of ZnO and W thin
films
M. Schuisky, J. W. Elam, and S. M. George
Citation: Applied Physics Letters 81, 180 (2002); doi: 10.1063/1.1490413
View online: http://dx.doi.org/10.1063/1.1490413
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 1
1 JULY 2002
In situ resistivity measurements during the atomic layer deposition
of ZnO and W thin films
M. Schuisky and J. W. Elam
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215
S. M. Georgea)
Department of Chemistry and Biochemistry and Department of Chemical Engineering,
University of Colorado, Boulder, Colorado 80309-0215
͑
Received 4 March 2002; accepted for publication 6 May 2002͒
In situ monitoring of the electrical resistivity of ZnO and W thin films during atomic layer
deposition ͑ALD͒ was performed using a four-point probe. Large oscillations were observed during
the ZnO ALD resistivity measurements. The resistivity dramatically increased during each diethyl
zinc exposure and decreased during each H O exposure. In contrast, the W ALD resistivity
2
measurements exhibited a steplike pattern where the resistivity decreased during the Si H6
2
exposures and remained constant during the WF exposures. In situ resistivity measurements will be
6
useful to monitor ALD processing and also may help to optimize and understand the properties of
gas sensors. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1490413͔
2
Atomic layer deposition ͑ALD͒ is a growth technique
allowing highly conformal films to be deposited with atomic
level thickness control.1 ALD is a variant of chemical vapor
deposition ͑CVD͒. In ALD, a binary CVD reaction is divided
into two self-limiting half reactions. By repeating these half
reactions in an ABAB... binary sequence, a thin film can be
deposited one atomic layer per AB cycle.
the deposition substrate. A 5ϫ5 mm alumina substrate was
lithographically patterned with gold as shown in Fig. 1͑a͒.
,2
2
The four-point probe consisted of four 50ϫ50 ␮m gold
contacts in a symmetric van der Pauw geometry separated
3
diagonally by 1 mm. Gold lines with a width of 25 ␮m
2
connected the contacts to 1ϫ1 mm gold pads at the edges
of the substrate.
The sequential nature of ALD allows each half reaction
to be examined independently during film growth. However,
typical ALD growth conditions challenge traditional surface
science techniques. ALD is often performed under viscous
flow at pressures of 1–10 Torr and using AB cycle times of
only a few seconds. Nevertheless, a number of in situ tech-
niques have been implemented to explore surface chemistry
and growth during ALD including quadrupole mass
A constant current, I, was applied through contacts A and
B and the resulting voltage difference, V, was measured be-
tween contacts C and D. For symmetrical, circular, or square
probe geometries, the resistivity, ␳, is calculated from13
␳
ϭ4.532Rd,
͑1͒
4
5
spectrometry, optical reflectivity, and quartz crystal mi-
crobalance measurements.6
,7
In this letter, we report the application of in situ resistiv-
ity techniques to monitor ALD. In situ resistivity measure-
ments provide an excellent method for probing conductance
changes during thin-film growth.8 The electrical resistivity
of thin films is determined by both bulk and surface effects.
Electrical resistivity is influenced by electron scattering from
phonons, point defects, impurities, and grain boundaries.10
For film thicknesses less than the electron mean-free path,
the resistivity increases because of interfacial scattering. The
resistivity can also be influenced dramatically by adsorbed
species that alter charge carrier concentrations or intergranu-
lar barrier heights.11
,9
In situ resistivity measurements typically employ a four-
point probe that is brought into contact with the substrate.12
This method allows resistivity measurements at discrete
thickness intervals after particular deposition times. To en-
able continuous in situ resistivity monitoring, a special de-
vice was designed that integrated the four-point probe with
FIG. 1. Schematic diagram of ͑a͒ the gold four-point probe deposited on the
alumina substrate and ͑b͒ the four-point probe mounted on the conflat
flange.
a͒Electronic mail: steven.george@colorado.edu
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003-6951/2002/81(1)/180/3/$19.00 180 © 2002 American Institute of Physics
29.22.89.130 On: Mon, 01 Dec 2014 18:03:06
0
1

Appl. Phys. Lett., Vol. 81, No. 1, 1 July 2002
Schuisky, Elam, and George
181
where RϭV/I is the resistance between contacts C and D
and d is the film thickness. The film thicknesses were deter-
mined using previously derived growth rates.
The ceramic substrate was glued to a supporting glass
plate that was clamped at the end of a stainless-steel arm as
shown in Fig. 1͑b͒. The four-point probe was mounted to a
2.75 in. conflat flange equipped with four electrical
feedthroughs. The gold pads were connected to the electrical
feedthroughs using 0.05 mm diameter wires. The in situ four-
point probe was then installed in a viscous flow ALD
1
4
reactor.
To prevent the electrical shorting or grounding of the
four-point probe by the conducting ALD films, the stainless-
steel arm was isolated using a ceramic spacer. In addition, a
flow of nitrogen gas purged the inside surface of the conflat
flange. The four-point probe was also coated with an insulat-
ing Al O ALD layer. Photoresist was initially placed over
2
3
2
the 50ϫ50 ␮m gold contacts. Al O ALD was then per-
2
3
formed using 800 trimethyl aluminum/H O cycles at 177 °C
2
to deposit an insulating Al O layer with a thickness of
2
3
1
5
ϳ100 nm. The photoresist was subsequently removed with
acetone to expose the 50ϫ50 ␮m gold contacts.
2
ZnO ALD films were grown using diethyl zinc ͑DEZ͒
and water at 177 °C with exposure times of 1 s and purge
times of 5 s. Under these conditions, the ZnO ALD growth
rate is 2.01(Ϯ0.01) Å/AB cycle. Figure 2͑a͒ shows the four-
point probe voltage measurements versus deposition time.
The voltage measurements commence at ZnO AB cycle num-
ber 89 at a film thickness of 179(Ϯ1) Å. The measurements
continued until 200 AB cycles when the ZnO film is 402
(
Ϯ4) Å thick. At 1550 s, the constant current was increased
by a factor of 100 to compensate for the decreasing resis-
tance and the measured voltage was subsequently divided by
1
00.
FIG. 2. ͑a͒ Voltage versus time measured using the in situ four-point probe
during ZnO ALD. ͑b͒ Expanded view of the resistivity values vs ZnO AB
cycle.
Figure 2͑a͒ demonstrates that the resistance of the ZnO
ALD film decreases with increasing thickness. By applying
Eq. ͑1͒ and the known ZnO ALD growth rate, Fig. 2͑a͒
yields a resistivity at 177 °C that drops from an initial value
of 0.1 ⍀ cm to a limiting value of ϳ0.002 ⍀ cm. This limit-
ing value is slightly lower than the bulk resistivity value of
ϳ0.008 ⍀ cm determined by ex situ four-point probe mea-
surements under ambient conditions at room temperature. A
lower resistivity value at 177 °C is expected because the re-
sistivity of the ZnO semiconductor film should decrease with
increasing temperature. The ZnO film resistivity may also
increase upon air exposure.
duction barrier heights at intergranular contacts.11 The DEZ
vapor will react with surface hydroxyl groups according to7
ZnOH*ϩZn͑CH CH ͒ →ZnOZnCH CH*ϩCH CH ,
2
3
2
2
3
3
3
͑2͒
where the asterisks denote surface species. The large increase
in resistivity upon DEZ exposure is associated with the con-
version of surface hydroxyl groups into alkyl species by the
covalent attachment of Zn–alkyl groups. The second half
reaction proceeds by7
The voltage measurements in Fig. 2͑a͒ also exhibit pro-
nounced oscillations. Figure 2͑b͒ shows an expanded view of
the corresponding resistivity values for AB cycles 89–92.
The resistivity increases dramatically during each DEZ ex-
ZnOZnCH CH*ϩH O→ZnOZnOH*ϩCH CH . ͑3͒
2
3
2
3
3
posure and subsequently drops with the next H O exposure.
Reaction 3 will restore the initial hydroxyl surface groups
and deposit additional ZnO. The resistivity subsequently
drops below the resistivity value measured before the ALD
cycle.
2
Oscillations in the resistivity of thin metallic films have pre-
viously been explained as resulting from either periodic
roughness changes or quantum size effects.9 These expla-
nations are unlikely in the current experiment.
,16
Additional in situ resistivity measurements explored W
ALD. The W ALD was performed using tungsten haxafluo-
ride (WF ) and disilane (Si H ) exposures of 2 s and purge
The resistivity oscillations may reflect changes in con-
ductance because of the adsorbed species. The adsorbed
chemical groups may act as electron donors or acceptors and
change the charge carrier concentration in the ZnO film. Al-
6
2
6
times of 5 s at 300 °C. These conditions yield a W ALD
growth rate of 5.5(Ϯ0.1) Å/AB cycle. Figure 3͑a͒ shows the
in situ four-point probe voltage measurements versus the W
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ternatively, the adsorbed chemical groups may alter the con-
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182
Appl. Phys. Lett., Vol. 81, No. 1, 1 July 2002
Schuisky, Elam, and George
In contrast, a discrete drop in resistivity occurs when the
subsequent Si H exposure converts the WF* surface spe-
2
6
x
1
7
cies to metallic W atoms
WF*ϩSi H ͑g͒→WSiH F*ϩSiH F ͑g͒ϩdH ͑g͒.
x
2
6
y
z
a
b
2
͑
5͒
The Si H exposures also deposit SiH F* surface species.
2
6
y
z
These functional groups must not influence the film conduc-
tance because their replacement by WF* surface species dur-
x
ing the following WF exposure does not change the resis-
6
tivity.
In conclusion, we report the application of in situ resis-
tivity measurements to monitor the ALD of both semicon-
ducting ZnO and metallic W thin films. Oscillations in the
thin-film resistivity were observed that correspond to the
ALD reactant pulse sequence. During ZnO ALD, the ethyl-
terminated ZnO film displayed a much higher resistivity than
the hydroxyl-terminated film. During W ALD, the resistivity
only decreased when the fluorinated tungsten surface species
were converted to metallic tungsten during the Si H expo-
2
6
sure. These results demonstrate that in situ resistivity mea-
surements will be valuable to probe ALD surface chemistry
and film growth. These results may also have important con-
sequences for the understanding of thin film electrical prop-
erties and the development of gas sensors.
This research was supported by the National Science
Foundation through grant CHE-9905812. The authors thank
Nils Hoivik and Ryan Linderman at the Department of Me-
chanical Engineering, University of Colorado at Boulder for
the design and construction of the four-point probes. One of
the authors ͑M.S.͒ acknowledges the Wenner–Gren Founda-
tions and the Sweden–America Foundation for financial sup-
port.
FIG. 3. ͑a͒ Voltage versus time measured using the in situ four-point probe
during W ALD. ͑b͒ Expanded view of the resistivity values vs W AB cycle.
1
T. Suntola, Thin Solid Films 216, 84 ͑1992͒.
2
M. Ritala and M. Leskel a¨ , in Handbook of Thin Film Materials, edited by
ALD growth time. The voltage measurements commence at
W AB cycle 50 at a film thickness of 275(Ϯ5) Å. The four-
point probe voltage is monitored for an additional 70 AB
cycles until the W film is 660(Ϯ12) Å thick. During this
interval, the W film resistivity at 300 °C dropped from 1.8
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1
7
3
20 Å. The slightly higher resistivity at 300 °C is expected
9
0
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with increasing temperature.
Figure 3͑a͒ shows a distinct, stepped structure in the
probe voltage. An expanded view of the corresponding resis-
tivity values for AB cycles 55–59 is displayed in Fig. 3͑b͒.
1
11
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1
2
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6
͑
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14
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behavior indicates that the resistivity does not change when
W is deposited as WF* surface species during the WF ex-
15
x
6
1
7
posures according to the following half reaction
16
17
WSiH F*ϩWF ͑g͒→WWF*ϩSiH F ͑g͒ϩcH ͑g͒.
y
z
6
x
a
b
2
͑4͒
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1