Phase-sensitive scattering polarimeter for characterization of surface treatment of titanium dental implant

散散射射相相位位偏偏光光儀儀於於鈦鈦金金屬屬人人工工牙牙根根表表面面處處理理製製程程特特性性之之檢檢測測 Tsung-Chih Yu

尤崇智

Metal Industries Research & Development Centre tcyu@mail.mirdc.org.tw

Shiou-An Tsai 蔡修安

National Cheng Kung Universityn18991152@mail.ncku.edu.tw

Yu-Lung Lo 羅裕龍

National Cheng Kung Universityloyl@mail.ncku.edu.tw

Abstract

We have characterized micro-rough surfaces of titanium dental implant materials by measuring the phase of the heterodyne signal corresponding to the scattering angles. The proposing scattering polarimeter combining the phase-modulation technique has the capability of characterizing the sand-blasted titanium surfaces with different acid-etching time more rapidly than the stylus profiler and SEM. The detected phase of heterodyne signal can be expressed as a function of the ellipsometric parameters (Ψ, ∆). The experimental results show that the polarization properties strongly depend on the surface topography, the system can easily characterize the sand-blasted titanium surfaces with different acid-etching time that cannot be easily distinguished by stylus profiler or SEM. Hence, the proposing method has the ability of surface quality inspection for the in-line surface-treatment process control.

keywords：Common-path heterodyne interferometry; Polariscope; Ellipsometry; Modulated; Scattering; Surface Roughness 1. Introduction

In the past decades, the number of dental implant procedures has increased steadily worldwide, reaching about one million dental implantations per year. The clinical success of oral implants is related to their early osseointegration. Geometry and surface topography are crucial for the short- and long-term success of dental implants. These parameters are associated with delicate surgical techniques, a prerequisite for a successful early clinical outcome [1]. The anchorage of osseointegrated implants primarily relies upon mechanical interlock. The surface structure of implants modulates the bone response and interfacial shear strength between implants and bone [2]. Acid etching, blasting and etching, and chemical modification are commonly used surface treatments of many modern implant systems [3]. Beside the macro-structure created by sand blasting or plasma spray techniques, the microstructure created by the etching process has a significant influence on the interfacial shear strength between titanium (Ti) implants and bone. Titanium dental implants with similar macrostructure had significantly greater removal torque values when they also had an acid etched microstructure [4]. The effects of grit blasting and acid etching on the topography of titanium implant surfaces have previously been characterized using scanning electron microscope (SEM) [5], stylus profilometry [6], optical techniques such as white light interferometry and confocal laser scanning microscopy [7], and atomic force microscopy (AFM) [8]. However, the instruments or techniques mentioned above are not suitable for the surface quality inspection on the mass production line because of their insufficient inspectig speed.

The characterization of rough surfaces by light scattering has remained an active research field ,and light scattering has been shown to be a powerful diagnostic technique to characterize optical surface qualities [9-11]. Although the use of the real space imaging techniques, such as AFM and SEM, allow the surface morphology to be probed directly,

indirect methods based on the light scattering from the sample surface still keep relevant advantages such as the contactless methodology and the capability to detect nanotopographic surface features.

The aim of this study was to rapidly characterize the surfaces of titanium dental implant materials. We employ scattering polarimeter using a phase-modulation technique to measure the phase of the heterodyne signal as a function of the scattering angle. The experimental results show that the polarization properties strongly depend on the surface micro-topography, the system can easily characterize the sand-blasted titanium surfaces with different acid-etching time that cannot be easily distinguished by stylus profiler and SEM. The proposing method has the capability of surface quality inspection for the in-line surface-treatment process control.

2. Principles of Operation In ellipsometry, p- and s-polarized waves are irradiated onto a sample at the Brewster

angle, and the optical constants and film thickness of the sample is measured from the change in the polarization state by light reflection or transmission. In ellipsometry measurement, the polarization states of incident and reflected light waves are described by the coordinates of p- and s-polarizations. Thus, upon light reflection on a sample, p- and s-polarizations show different changes in amplitude and phase. Ellipsometry measures the two values (Ψ, Δ) that express the amplitude ratio and phase difference between p- and s-polarizations, respectively. Therefore, the variation of light reflection with p and s-polarizaions is measured as the change in polarization state.

The (Ψ, Δ) measured from ellipsometry are defined from the ratio of the amplitude reflection coefficients for p- and s-polarizations：

ρ⎛ ⎞ ⎛ ⎞

= Ψ Δ = ⎜ ⎟ ⎜⎜ ⎟ ⎝ ⎠⎝ ⎠ tan exp( ) p rp rs

ip iss

r E EiE Er ⎟ (1)

Therefore, Ψ represents the angle determined from the amplitude ratio between reflected p- and s-polarizations, while Δ expresses the phase difference between p- and s-polarizations. If we use polar coordinates to represent the amplitude reflection coefficients, we can get：

δ δΨ = Δ = −tan ,prp rs

s

rr

(2)

The basic schematic diagram of the system is illustrated in Fig.1, as can be seen that the He-Ne Laser with 632.8 nm wavelength is launched into the measurement system containing a polarizer, electric-optic modulator (EOM), a the quarter-wave plate (QWP), scattered by the sample, and an analyzer before being incident on the photo-detector. In the optical configuration of proposing scattering polarimeter the polarizer is adjusted to 0° and the QWP and the analyzer are adjusted to 0° and -45°, respectively. The scattered light is filteedr signal received by the photo-detector could be locked the phase using the lock-in amplifier. It should be noticed that the origination in all optical components are aligned based on the slow axis of the EOM.

The light vector emerging from the configuration in Fig. 1 is given by：

( ) ( ) ( ) ( ) ( )

[ ] ω

ω ω

ω ω

= − ° ⋅ Ψ Δ ⋅ ° ⋅ − ° ⋅ ° ⋅

⎡ ⎤⎢ ⎥⎡ ⎤ ⎡ ⎤ ⎡ ⎤

= ⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎢ ⎥−⎣ ⎦ ⎣ ⎦ ⎣ ⎦⎢ ⎥⎢ ⎥⎣ ⎦

00

45 , 0 45 0

t tcos sin1 -1 1 01 2 2 2 -1 1 0 t t 0sin cos

2 2

in

i t

E A S Q EO P E

i ESample e

i i

(3)

where 0 is the amplitude of the incident electric field, P(0°) represents the Jones matrix of the polarizer aligned with x-axis, Q(0°) represents the Jones matrix of the quarter-wave plate, whose slow axis is aligned with x-axis, S(Ψ, Δ) represents the Jones matrix of the sample. Furthermore, EO(-45°) represents the Jones matrix of the EOM driven by a saw tooth voltage waveform with an angular frequency ω and its slow axis is oriented at -45° related to the x-axis, and A(-45°) represents the Jones matrix of the analyzer whose transmission axis forms an angle -45° with the x-axis.

E

As a result, the intensity of the detected signal is given by :

ω ωω

= Ψ Δ − Ψ

= + Φ1

1

(1+sin2 cos sin cos2 cos ) + sin( )

dc

dc

I I t tI R t

(4)

where = 2

0 4dcI E is the dc component of the output intensity, and is the intensity of the input light. R represents the amplitude, and

2

0EΦ represents the phase. We can the phaseΦ :

( )−Φ = − Ψ Δ1tan cot 2 sec (5)

He‐Ne laser PC

polarizer

EOM

sample

Power Amplifier

LIAQWP

Function Generator

Fig.1 Schematic diagram of phase-sensitive scattering polarimeter

3. Experimental Setup and Results

The schematic illustration of the experimental setup used in this study is shown in Fig. 1. In Fig. 1, the polarizer is adjusted to 0° and the quarter-wave plate are adjusted to 0°, and the analyzer is adjusted to -45°, respectively. The sample stage is rotated at the angle which is equivalent to the incident angle. In the system, the He-Ne laser (SL 02/2, SIOS Co.) was used as a light source. The frequency of the saw tooth signal from a function generator that applied to the EOM was 1 kHz. The experimental setup includes two Sheet polarizers (Sigma Koki, Model: SPF-30C-32), one quarter wave plate (Sigma Koki, Model: WPQ-6328-4M). The laser beam passes sequentially through a polarizer, and electro-optic (EO) modulator, the quarter-wave plate, scattered by the sample, and an analyzer before being incident on the photo-detector. And the phase of detected signal can be therefore extracted by the lock-in

amplifier (SRS, Model: SR-830). It should be notice that the incident light is perpendicular to the sample stage before

measuring the sample to calibrate the optical path and the inclination of the sample stage. Furthermore, we should assure the light reflected from the sample stage matches the incident laser spot absolutely. This is an important calibration step for the measurement.

We used the five Ti plates with the newly-formulated sand-blasted larg-grit acid-etched (NFSLA) treatment with gold plated surface (for SEM photography) as samples. The NFSLA samples acid-etched for 10 minutes, 20 minutes, 30 minutes, 40 minutes, and 50 minutes are named as SLA-10, SLA-20, SLA30, SLA-40, and SLA50 respectively. In the case, the SEM micrographs of NFSLA Ti plates are shown in Fig.2 ~ Fig.6. Thus, we measure the phase Φ by using the optical configuration proposed in Fig. 1. We could obtain the phases (see Fig. 7) from the lock-in amplifier corresponding to the scattering angles from 20° to 50° in the incident angle θ i = 55°.

10,000X 5,000X 2,000X 500X

Fig. 2 SEM micrograph of Ti plate with NFSLA treatment (acid-treated for 10 min)

10,000X 5,000X 2,000X 500X

Fig. 3 SEM micrograph of Ti plate with NFSLA treatment (acid-treated for 20 min)

10,000X 5,000X 2,000X 500X

Fig. 4 SEM micrograph of Ti plate with SLA treatment (acid-treated 30 min)

10,000X 5,000X 2,000X 500X

Fig. 5 SEM micrograph of Ti plate with NFSLA treatment (acid-treated for 40 min)

10,000X 5,000X 2,000X 500X

Fig. 6 SEM micrograph of Ti plate with NFSLA treatment (acid-treated for 50 min) In Fig.7, the experimental results show that the phase Φ curves of the Ti plates with SLA

treatment almost matched together. Therefore, the reiteration of the phase Φ curve of Ti plates with SLA treatment and the feasibility of the proposed optical model could be verified.

20 25 30 35 40 45 5060

80

100

120

140

160

180

Scattering angle (deg)

Pha

se (d

eg)

SLA10 & SLA20 & SLA30 & SLA40 & SLA50

SLA-10SLA-20SLA-30SLA-40SLA-50

Fig. 7 The phase Φ of Ti plates with newly-formulated SLA-10~SLA-50 treatments

The Ti plates with the with the NFSLA treatment without gold plated surface were used as

samples as well, and named as SLA-10-1, SLA-20-1, SLA30-1, SLA-40-1, and SLA50-1. The phases acquired by the lock-in amplifier corresponding to the scattering angles from 20° to 50° in the incident angle θi = 55° are shown in Fig. 8 ~ Fig. 13.

20 25 30 35 40 45 5060

80

100

120

140

160

180

Scattering angle (deg)

Pha

se (d

eg)

SLA10

SLA-10SLA-10-1

Fig. 8 The phase Φ of Ti plates with newly-formulated SLA-10 & SLA10-1(without gold plated

surface) treatments

20 25 30 35 40 45 5060

80

100

120

140

160

180

Scattering angle (deg)

Pha

se (d

eg)

SLA20

SLA-20SLA-20-1

Fig. 9 The phase Φ of Ti plates with newly-formulated SLA-20 & SLA20-1(without gold plated surface) treatments

20 25 30 35 40 45 5060

80

100

120

140

160

180

Scattering angle (deg)

Pha

se (d

eg)

SLA30

SLA-30SLA-30-1

Fig.10 The phase Φ of Ti plates with newly-formulated SLA-30 & SLA30-1(without gold plated

surface) treatments

20 25 30 35 40 45 5060

80

100

120

140

160

180

Scattering angle (deg)

Pha

se (d

eg)

SLA40

SLA-40SLA-40-1

Fig. 11 The phase Φ of Ti plates with newly-formulated SLA-40 & SLA40-1(without gold plated

surface) treatments

20 25 30 35 40 45 5060

80

100

120

140

160

180

Scattering angle (deg)

Pha

se (d

eg)

SLA50

SLA-50SLA-50-1

Fig. 12 The phase Φ of Ti plates with newly-formulated SLA-50 & SLA50-1(without gold plated

surface) treatments

20 25 30 35 40 45 5060

80

100

120

140

160

180

Scattering angle (deg)

Pha

se (d

eg)

SLA10 & SLA20 & SLA30 & SLA40 & SLA50

SLA-10SLA-20SLA-30SLA-40SLA-50

Fig. 13 The phase Φ of Ti plates with newly-formulated SLA(without gold plated surface)

treatments The experimental results have shown that the phase Φ curves of the Ti plates with

newly-formulated SLA without gold plated surface almost matched together. The reiteration of the phase Φ curve of Ti plates with SLA treatment and the feasibility of the proposed optical system could be verified. The polarimetric scattering system measure the curve of phase-scattering angle of the scattering heterodyne signal. The topography of the NFSLA w/ and w/o gold plated is similar, but it can be observed from Fig.8- Fig 12 that phase curves of the NFSLA samples w/ and w/o gold plated have apparent difference. It can be explained that polarization properties, like the detected phase, of the scattering light are not only sensitive to the surface topography but the surface material. The proposing system has shown the capability of high-speed characterization of micro-topographic surfaces. With further improvement, the system can be integrated for the in-line process characterization of the dental implant surface treatment.

The six dental implants with the newly-formulated SLA (NFSLA) treatment (for SEM photography) were also used as samples. The NFSLA dental implants acid-etched for 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, and 60 minutes are named as Implant 10, Implant 20, Implant 30, Implant 40, Implant 50, and Implant 60 respectively. In the case, the SEM micrographs of NFSLA dental implants are shown in Fig.14 ~ Fig.19. Thus, we measure the phase Φ by using the optical configuration proposed in Fig. 1. The phases acquired by the lock-in amplifier corresponding to the scattering angles from 50° to 80° in the incident angle θ i = 55° are shown in Fig. 20.

10,000X 5,000X 2,000X 500X Fig. 14 SEM micrograph of dental implant with NFSLA treatment (acid-treated 10 min)

10,000X 5,000X 2,000X 500X Fig. 15 SEM micrograph of dental implant with NFSLA treatment (acid-treated 20 min)

10,000X 5,000X 2,000X 500X Fig. 16 SEM micrograph of dental implant with NFSLA treatment (acid-treated 30 min)

10,000X 5,000X 2,000X 500X Fig. 17 SEM micrograph of dental implant with NFSLA treatment (acid-treated 40 min)

10,000X 5,000X 2,000X 500X Fig. 18 SEM micrograph of dental implant with NFSLA treatment (acid-treated 50 min)

10,000X 5,000X 2,000X 500X Fig. 19 SEM micrograph of dental implant with NFSLA treatment (acid-treated 60 min)

50 55 60 65 70 75 80

-150

-100

-50

0

50

100

150

Scattering angle (deg)

Pha

se (d

eg)

Implant10 & Implant30 & Implant30 & Implant40 & Implanth50 & Implanth60

Implant10Implant20Implant30Implant40Implant50Implant60

Fig. 20 The phase Φ of all dental implants with NFSLA treatments

4. Conclusions To investigate the relationship polarization states of the reflected or scattered light from

microtopographic surface, the phase of the heterodyne signal based on the scattering polarimeter are measured as function of scattering angles. The presence of environmental disturbances cannot affect the measurement results via the use of a common-path interferometric configuration. Experimental results show that the polarization properties of scattered light strongly depend on the surface topography. The experimental results have shown that the phase Φ curves of the dental implants with newly-formulated SLA almost matched together but not matched well within above cases. It can be explained that polarization properties, like the detected phase, of the scattering light are not only sensitive to the surface topography but the geometric configuration of the surface material. The reiteration of the phase Φ curve of dental implants with newly-formulated SLA treatment and the feasibility of the proposed optical system could be verified. The polarimetric scattering system measured the curves of phase vs. scattering angle of the scattering heterodyne signal. The proposing system has shown the capability of high-speed characterization of micro-topographic surfaces. With further improvement, the system can be integrated for the in-line process characterization of the dental implant surface treatment.

The performance of the measuring system was verified in the experiments. In addition, the measurement by the SEM must be in the vacuum treatment which needs the most of measuring time. The cost and the measuring method of the SEM equipment are both the reasons that the SEM cannot be applied for the in-line process control. Therefore, the modulated scattering polarimeter will be a very sensitive and powerful tool suitable for the in-process surface quality characterization of medical devices, optical elements, and other surface-treated components.

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