Si薄膜フォトダイオードを用いた干渉計


Laser interferometry is a well-developed technique for the displacement measurement since it has high resolution and gives non-contact measuring methods. In this study, the new interferometer based on the standing wave detection is developed . Figure 1 shows the principle of the proposed interferometer. The laser beam is normally incident on the moving mirror and back reflected producing the standing wave. This thinnest interference fringe is measured by the photodiode whose active layer is thinner than the pitch of the standing wave. Based on the principle, an integrated interferometer is proposed as shown in Figure 2.

Figure 1: Principle of the interferometer detecting standing wave.

Figure 2: Integrated interferometer using transparent photodiode

In the fabrication, the initial Si-on-insulator wafer is made by the direct wafer bonding between Si and quartz. The photodiode area is around 1mm2. Figure 3 shows the fabricated photodiode. The characters on a paper can be seen through the photodiode substrate as shown in Figure 3. The photodiode is about 40nm thick. The overall transmission rate reaches 70% in power. Since the Si layer on glass is very thin, it is difficult to fabricate pn junction along its thickness. The pn junction is fabricated laterally. Figure 4 shows the magnified view of the photodiode. Two comb shaped regions which correspond to the pn junction fabricated laterally and periodically are seen to be a little different color. The depletion region extends like a snake between these regions for gathering the produced photocarrier as much as possible. The apparent sensitivity of the thin film photodiode is around 0.01 mA/W. This value is 4 order smaller the than that of the bulk Si photodiode. This is reasonable for the present design. The absorption rate is estimated to be 0.6% in power. (The transmission rate is almost decided by the reflection at Si-SiO2 and SiO2-air interfaces.)

Figure 5 is the obtained interference signal. The interference signals having 100 nA peak-to-peak amplitude and high contrast of 80% are obtained. The period is in good agreement with a half of the wavelength.

Figure 3: Fabricated transparent photodiode

Figure 4: Magnified view of the transparent photodiode .

Figure 5: Interference signal as a function of displacement.