A Comprehensive Characterization of Stress Relaxed ZnO …
497
to develop cracks or fracture in multilayer stack due to the thermal coefficient of
expansion (TCE) mismatch. Therefore, an industrial process is required to deposit
the ZnO thin film at room temperature (Iborra et al. 2004; Engelmark et al. 2000).
Usually, thin-film stress is estimated by X-ray diffraction technique (Conchon et al.
2010) and this process is only applicable to crystalline films. X-ray diffraction is an
indirect method and only provides local information about a small area of a sample.
Nevertheless, wafer curvature method is a direct measurement technique and provides
overall information about the measuring wafer. The resulting information is more
precise and based on bow of the wafer. Moreover, the wafer curvature method is also
applicable to amorphous thin films.
Although the effect of sputter partial pressure has been performed earlier (Menon
et al. 2011; Singh et al. 2015) however, the influence of oxygen partial pressure on
as-deposited ZnO thin film’s structural, optical, and mechanical properties has never
been investigated in detail. In the present work, we report on the role of oxygen partial
pressure (R O2 ) and stress measurement of ZnO thin film using the wafer curvature
method. By analysis, it was found that stress variation depends on the oxygen partial
pressure. At low R O2 , oxygen defects are prominent and responsible for in-plane
residual stress. Experimentation has been carried out to obtain minimal stress ZnO
film, which is highly desirable for the fabrication of ZnO based MEMS devices.
2 Experimental Procedure
Single crystal Si (100) wafers were cleaned by a standard semiconductor cleaning
process. Wafers were boiled in organic solvents such as trichloroethylene, acetone,
and methanol and followed by rinsing with deionized (DI) water. Organic residues
were removed in piranha (H 2 SO 4 :H 2 O 2 = 5:1) solution. After Piranha dip, thin layer
of oxide was grown over the Si surface and it was removed in 2% HF solution and
then wafers were rinsed with DI water. Dried wafers were loaded in oxidation furnace
for ~1 μm thick thermal silicon oxide (SiO 2 ) growth. The SiO 2 layer works as an
isolation layer between underlying Si substrate and ZnO film. Reactive magnetron
sputtering system was used for the growth of ZnO film. The base vacuum of sputtering
chamber was achieved at ~2×10
−6 Torr. The oxygen partial pressure ratio (R O2 ) is
defined as R O2 = P O2 /(P O2 + P Ar ) × 100%, where P Ar and P O2 are argon and oxygen
partial pressure, respectively. This ratio was varied in 30–60% range by controlling
Ar + O 2 gas flow.
The ZnO film thickness was ~0.8 μm and retained the same in all the samples.
The industrial sputtering process was established up to 6-in. diameter wafer and
optimal processing parameters are listed in Table 1. Stress was measured using dual
laser switching technology. Crystalline structure and surface morphology of ZnO
thin film were investigated by Bruker X-ray diffractometer (XRD) and atomic force
microscopy (AFM). To measure the optical band gap, ZnO film was deposited on
SiO 2 /glass substrate and transmittance spectra was recorded using high sensitivity
3600 UV-VIS-NIR Spectrophotometer (wavelength range 200–1100 nm). Raman
497
to develop cracks or fracture in multilayer stack due to the thermal coefficient of
expansion (TCE) mismatch. Therefore, an industrial process is required to deposit
the ZnO thin film at room temperature (Iborra et al. 2004; Engelmark et al. 2000).
Usually, thin-film stress is estimated by X-ray diffraction technique (Conchon et al.
2010) and this process is only applicable to crystalline films. X-ray diffraction is an
indirect method and only provides local information about a small area of a sample.
Nevertheless, wafer curvature method is a direct measurement technique and provides
overall information about the measuring wafer. The resulting information is more
precise and based on bow of the wafer. Moreover, the wafer curvature method is also
applicable to amorphous thin films.
Although the effect of sputter partial pressure has been performed earlier (Menon
et al. 2011; Singh et al. 2015) however, the influence of oxygen partial pressure on
as-deposited ZnO thin film’s structural, optical, and mechanical properties has never
been investigated in detail. In the present work, we report on the role of oxygen partial
pressure (R O2 ) and stress measurement of ZnO thin film using the wafer curvature
method. By analysis, it was found that stress variation depends on the oxygen partial
pressure. At low R O2 , oxygen defects are prominent and responsible for in-plane
residual stress. Experimentation has been carried out to obtain minimal stress ZnO
film, which is highly desirable for the fabrication of ZnO based MEMS devices.
2 Experimental Procedure
Single crystal Si (100) wafers were cleaned by a standard semiconductor cleaning
process. Wafers were boiled in organic solvents such as trichloroethylene, acetone,
and methanol and followed by rinsing with deionized (DI) water. Organic residues
were removed in piranha (H 2 SO 4 :H 2 O 2 = 5:1) solution. After Piranha dip, thin layer
of oxide was grown over the Si surface and it was removed in 2% HF solution and
then wafers were rinsed with DI water. Dried wafers were loaded in oxidation furnace
for ~1 μm thick thermal silicon oxide (SiO 2 ) growth. The SiO 2 layer works as an
isolation layer between underlying Si substrate and ZnO film. Reactive magnetron
sputtering system was used for the growth of ZnO film. The base vacuum of sputtering
chamber was achieved at ~2×10
−6 Torr. The oxygen partial pressure ratio (R O2 ) is
defined as R O2 = P O2 /(P O2 + P Ar ) × 100%, where P Ar and P O2 are argon and oxygen
partial pressure, respectively. This ratio was varied in 30–60% range by controlling
Ar + O 2 gas flow.
The ZnO film thickness was ~0.8 μm and retained the same in all the samples.
The industrial sputtering process was established up to 6-in. diameter wafer and
optimal processing parameters are listed in Table 1. Stress was measured using dual
laser switching technology. Crystalline structure and surface morphology of ZnO
thin film were investigated by Bruker X-ray diffractometer (XRD) and atomic force
microscopy (AFM). To measure the optical band gap, ZnO film was deposited on
SiO 2 /glass substrate and transmittance spectra was recorded using high sensitivity
3600 UV-VIS-NIR Spectrophotometer (wavelength range 200–1100 nm). Raman