Characterization of Polysilicon Thin Films for MEMS Applications

DAN O. MACODIYO*1, HITOSHI SOYAMA

2 AND KAZUO HAYASHI

1

1Institute of Fluid Science, Tohoku University, 2-2-1 Katahira, Aoba-ku,

Sendai 980-8577, Japan 2Department of Nanomechanics, Tohoku University, 6-6-01 Aoba, Aramaki,

Aoba-ku, Sendai 980-8579, Japan

Email: dan@mm.mech.tohoku.ac.jp

Abstract: The microstructure of thin polycrystalline films formed by molecular beam epitaxy (MBE) has been

studied by transmission electron microscopy (TEM). Beneficial compressive residual stress was introduced by

cavitation impacts. Surface morphology was characterized using atomic force microscopy (AFM). The films show

two distinct microstructural regimes at the low and high end with an intermediate transition region.

Homogeneous nucleation and growth in the bulk of the films were observed. Numerous planar twins are

observed on the lower end while the polysilicon films has a highly textured columnar microstructure. The

textured grains had rough edges. Both homogeneous nucleation and growth in the bulk of the films and

heterogeneous nucleation and growth in the film-substrate interface are observed. The main defects have been

identified as dislocations and microtwins. The locations of the dislocations are also discussed.

Keywords: Polysilicon film, Molecular beam epitaxy, Cavitation, Transmission electron microscopy, Defects

1. Introduction

The performance of polysilicon films in micro-

electro-mechanical systems (MEMS) depends on

residual stress and stress gradient, which are adversely

affected by the microstructure. The microstructure is

governed by the properties of the films and the

deposition conditions. Thus, it is necessary to

characterize the microstructure in a bid to meet the

requirements of MEMS systems. Integrated circuit

fabrication technologies requires the definition of small

geometries, precise dimension control, design flexibility,

interfacing with microelectronics, repeatability,

reliability, high yield and low cost. During

microfabrication of semiconductor devices, defects do

occur at micro-scale. These defects may be detrimental

to the performance of device and/or even cause device

failure. The advances in understanding of processes

such as gettering have changed views on impurity

requirements. Defects if properly controlled, can be

used to improve the electrical characteristics of a

silicon-based device [1,2].

When a bubble collapse near another bubble, a wall,

free surface, in a static or dynamic pressure gradient, it

suffers instability that drives the shape to be

nonspherical [3]. The collapsing nonspherical bubbles

develops a microjet which causes plastic deformation

on the wall. The impact of cavitation bubbles, herein

referred to as cavitation impacts, can be used to

generate beneficial residual stress. The authors have

already demonstrated the use of cavitation impacts for

effective gettering of silicon wafers [1,2].

In this study, structural analysis has been performed

using high resolution transmission electron microscopy

(HRTEM). The main defects in the polysilicon films are

dislocation and microtwins. The dislocation sources are

located within the volume of the grain, at the grain

boundary and at free surface as shown in Fig 1. The

effects of cavitation impacts on the grain boundary and

free surface are illustrated on the plan-view HRTEM

Proceedings of the 10th WSEAS International Conference on SYSTEMS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp14-19)

Si2H6

Control valve

Gases

Tray

Wafer

Vacuum pumps

Loading chamber

Heater

Vacuum pumps

have been discussed. The cross-sectional HRTEM view

illustrates fine layer of amorphous and two distinct

microstructural regimes on polysilicon. The growth-

death rate zone at the poly-Si/SiO2 interface highly

which depends on the deposition conditions, had small-

size mixture of textured-columnar crystals. Surface

characterization using the atomic force microscopy

showed that the surface finish on the back of the

polysilicon film was smoothened out by the cavitation

impacts.

Fig. 1 Dislocation sources in polycrystalline silicon

2 EXPERIMENTAL TECHNIQUES

2.1 Sample preparation

2.1.1 Growing polysilicon using a gas-source MBE

Figure 2 shows schematic illustration of the gas-

source molecular beam epitaxy system used for

growing of poly-Si/SiO2 samples. The CZ Si(100)

substrate wafer was chemically cleaned. The sample

was put on the tray and heated to 800 oC for 1 hour to

remove native oxide. Subsequently, a 10 nm silicon

buffer was deposited, followed by heating at 700 oC for

3 hours in disilane (Si2H6) gas at a flow rate of 2.5 sccm.

The pressure during growth was 10-4 Torr.

2.1.2 Introduction of compressive stress

Figure 3 shows the test section of the cavitating jet

apparatus used for the introduction of beneficial

compressive residual stress. The surface of silicon

wafer was masked with a tape and placed onto

specimen holding device and then immersed in DI

water. The test liquid, DI water, was injected onto the

surface of the specimen through a nozzle, diameter 0.8

mm, with an upstream and downstream pressure of 2.5

MPa and 0.1 MPa, respectively. The standoff distance

was 17 mm. The standoff distance sd, is defined as the

distance between the upstream corner of the nozzle

throat and the surface of the specimen under test. Hence,

the optimum standoff distance sopt is determined

qualitatively by an erosion test in which the standoff

distance is varied [4]. Details of the cavitating jet in

submerged condition are found in references [5-9].

Upon leaving the nozzle, a cavitating jet was formed.

The collapsing of the cavitation bubbles causes shock

wave and/or microjets on the specimen surface thereby

causing suitable plastic deformation. The cavitation

impacts was controlled by adjusting hydraulic

parameters such as injection pressure of the cavitating

jet and standoff distance. The cavitating jet was

traversed in the x-direction using an auxiliary leadscrew

controlled by a motor thereby allowing uniform

exposure on the specimen. Thus, the exposure time per

unit length t is expressed as the ratio of the number of

b

Grain

boundaries

Ledge

Dislocation

loop

Grain

Ledge

Grain

Slip

plane

Dislocation

Fig. 2 Schematic illustration of gas-source molecular

beam epitaxy system (AirWater VCE S2020)

Proceedings of the 10th WSEAS International Conference on SYSTEMS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp14-19)

scans n to the scanning speed v.

(1)

2.1 Structural characterization

The surface of the specimen treated by cavitation

impact was observed using atomic force microscope.

The threshold deformation and surface roughness of

active device side was 3.99 nm and 2.09 nm,

respectively [10]. In this study, the backside of thin

film was examined.

For the plan-view TEM observation, the specimen

was shaped to 5 mm square using a micro-cutter and

then cleaned using ethyl alcohol (CH3CH2OH). The

sample was fixed to a glass using epoxy resin and

then mechanically polished to thickness t = 20 µm.

The specimen was ion-milled at low angle 12o, 3 kV

and argon flow 0.3 cm3 until perforation. For the

cross-sectional view TEM observations, the samples

were thinned and then glued with deposited films

facing each and the ion-milled. The HTREM

observations were performed using JEOL operated at

300 kV.

3 RESULTS

3.1 Surface characterization of poly-Si/SiO2

using AFM

The surface was characterized using the

Environscope atomic force microscope equipped with

the Nanoscope IV Controller, manufactured by Veeco

(Digital Instruments). Figure 4 shows the AFM images

of the specimen before and after exposing cavitation

impacts. The surface roughness increased due to

cavitation impacts while the deformation on the back-

side of the polysilicon thin film decreased as shown in

Table 1. This shows that the roughness due to

deposition was smoothened out by the cavitation

impacts.

Table 1. Values of surface roughness and deformation

before and after cavitation impacts

Before cavitation

impacts

After cavitation

impacts

Surface

roughness

Ra

Deformation

nm

Surface

roughness

Ra

Deformation

nm

5 610 127 10

3 429 113 37

7 258 153 30

v

nt =

Fig. 3 Test section of cavitation jet apparatus

s

Flow

Overflow Wafer

p

Specimen

holder

Fig. 4 AFM images for surface roughness and

deformation on the back-side of polysilicon film.

(a) Before cavitation impact

(b) After cavitation impact

Proceedings of the 10th WSEAS International Conference on SYSTEMS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp14-19)

3.2 Microstructural observation of poly-

Si/SiO2 using HRTEM

Figure 5 shows plan-view HRTEM image indicating

the textured-columnar structure. Parallel microtwins

can be seen along the grain boundary. They are also

associated with dislocation and stacking faults. The slip

planes are evidence where two parallel grains are

inclined. Dislocation sources are also present in ledges.

When the dislocation is at free surface but in the

vertical position an edge dislocation is formed while a

screw dislocation results for the case of horizontal

position of the loop.

Figure 6 shows the cross-sectional bright- and dark

field view of polysilicon film image with g=[220]

reflectance. The microstructure of the polysilicon has

two definite structural sizes. The upper part has large

textural-columnar grains and tends to be near parallel.

The lower part, death-growth zone, has small grains

size 20 to 50 nm. The stacking faults (SF) are illustrated.

The ledge can be seen as a kink between the grain G1

and grain G2. The diffraction pattern around the

microtwins show speckle of bright lights outside the

silicon annular ring with high bright spot in the middle

(see insert in Fig. 6(a)). Half and full dislocation loops

were observed (see Fig. 6(b))

100 nm

Ledge

Parallel microtwins

along the grain

boundary

Fig. 5 Plan-view Poly-Si/SiO2TEM showing

the crystallites

(b) Dark field

Poly-Si SiO2

Si

g

SF SF

Dislocation

loop

Microtwins

Poly-Si SiO2 Si

g

SF

Death-growth

zone

Ledge

G1

G2

(a) Bright field

Fig. 6 Cross-sectional view Poly-Si/SiO2 image with g=[220] reflectance

illustrating stacking faults

Proceedings of the 10th WSEAS International Conference on SYSTEMS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp14-19)

4 CONCLUSIONS

The microstructure of polysilicon thin films grown

by MBE and exposed to cavitation impacts has been

studied.

Based on the experimental evidence, the following

conclusions may be drawn:

1. Dislocation sources can be located within the

volume of the grain, at the grain boundary and at

free surface. Dislocation sources located at the

grain boundary generates a dislocation loop. The

size of the dislocation for the specimen treated by

cavitation impacts depends on the grain size and

cavitating parameters. The surface morphology

demonstrates that the rough surface of the as-

deposited gas was smoothened by the cavitation

impact.

2. The films show two distinct microstructural

regimes at the low and high end with an

intermediate transition region. Homogeneous

nucleation and growth in the bulk of the films

were observed. Cavitation impacts decrease the

gap between two intra-grains. Both homogeneous

nucleation and growth in the bulk of the films and

heterogeneous nucleation and growth in the film-

substrate interface are observed. Deformation

streaks and twins were seen on the large columnar

crystals.

3. The death-growth zone at which there is a trade-

off in crystal size is approximately 50 nm in length.

Surface dislocation loops either vertical or

horizontal direction leading to edge and screw-

dislocation were also observed.

Acknowledgments

This work was supported by the Japan Society

for the Promotion of Science (JSPS) under the Grant

No. 16004335. The authors would like to thank Prof.

Noritaka Usami for the assistance on growing

polysilicon films using MBE facility and Mr. Y.

Hayasaka of the IMR, Tohoku University, for his

assistance in TEM observations.

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Proceedings of the 10th WSEAS International Conference on SYSTEMS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp14-19)

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Proceedings of the 10th WSEAS International Conference on SYSTEMS, Vouliagmeni, Athens, Greece, July 10-12, 2006 (pp14-19)