Photonic Crystal Patterns Definition and Dry-Etching Processes…36

At first, the QPC pattern is designed by using CAD tools. And then we load this file into Nanometer Pattern Generation System (NPGS) system. Then photonic crystal patterns were defined by JEOL JSM-6500F electron-beam lithography (EBL) system. The EBL system is a field-emission scanning electron microscope, which employs a schmasky type fields-emission gun for the electron source and state-of-the-art computer technology for high-resolution image observation. Before EBL, an A5 polymethylmethacrylate (PMMA) resist layer is spin-coated on the wafer after previous dielectric deposition process. The PMMA thickness is 300 nm.

After defining QPC patterns by EBL system, the pattern will be transferred by the following processes. In transferring patterns, Oxford Instruments Plasma Technology Plasma lab 100 inductively coupled plasma / reactive ion etching (ICP/RIE) system is used. At first, the sample is etched by O2 plasma in order to clean the residual PMMA in air holes. And then the Si3N4 hard mask is etched by CHF3/O2 mixed gas in RIE mode dry etching. The Si3N4

etching environment recipes are 150W RF power and 55 mTorr at 20℃ with CHF3 and O2 gas flow rate of 5 sccm and 50 sccm, respectively. The etching rate in CHF3/O2 mixed gas is about 1.5 nm/s in average and the selectivity etching ratio to PMMA is 8. After transferring the pattern into Si3N4 layer, we use O2 plasma to remove the survival PMMA layer. And then the pattern transforming into InP/InGaAsP MQWs layer is achieved by H2/CH4/Cl2 mixed gas in ICP mode dry etching. The MQWs etching environment recipes are 73W RF power, 1000W ICP power, and 4 mTorr at 150℃ with H2、CH4 and Cl2 gas flow pressure of 0.8, 0.4, and 0.3 mTorr, respectively. The etching rate in H2/CH4/Cl2 mixed gas is about 5.5 nm/s in average and the selectivity etching ratio to Si3N4 is 6. After a serious of dry etches process mentioned above, the PC patterns have already been transferred onto the QWs.

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3.4.3. Construct of Central Post Structure

In order to form the central-post structure, the InP substrate below the MQWs should be removed and leave only a post under the center of our microcavity. This can be achieve by immersion the sample into a mixture solution with HCl : H2O = 3 : 1 at 2.5 for ℃ about 1 minutes, and then the solution goes through the drilled holes to form a small post. This process also removes 60 nm InP cap layer and smoothes the surface and the sidewall of the air holes.

In general photonic crystal patterns, for example, triangular lattice photonic crystal, the CAD design must includes windows to break the InP etching stop plane and achieve membrane structure [29]. However, the etching stop plane can be easily broken due to the lattice structure of 12-fold QPC and the undercut will form. As a result, it is unnecessary to define the windows in our CAD file. Fig. 3.10 shows the SEM picture of our sample after wet etch process. We find that the undercut of outer region of QPC is not formed caused by smaller air-hole in the outer region due to proximity effect during EBL process. Also, in the same sample with larger r/a ratio, the membrane structure has been formed under the same wet-etching time. This also indicates the sensitivity of wet etching time in different r/a ratio pattern.

Figu

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Figure 3.12: (a)The top view SEM picture of fabricated sample. A 12-fold QPC microcavity with central post. (b) The parallelogram shape of central post. (c) A fabricated 12-fold QPC microcavity membrane with crashed outer region due to the over dosage of EBL is supported by the central post.

Fig. 3.12 (a) shows the top-view SEM picture of fabricated 12-fold QPC microcavity with central post. The parallelogram shape of central post is observed from the SEM picture in Fig. 3.12 (b). This asymmetric shape is caused by the anisotropic etching rate of InP material.In Fig. 3.12 (c), a fabricated 12-fold QPC microcavity membrane with crashed outer region due to the over dosage of EBL is supported by the central post. This structure is very fragile because the absence of outer connections. To solve this problem, we design the other two CAD files and the first one is show in Fig. 3.13. This design will produce contacted bridge to connect membrane and wafer to form a stronger central post structure. Fig. 3.14 shows the top-view SEM picture of fabricated device with contacted bridge between QPC membrane and the wafer. Moreover, we can shift the larger air holes away from the other air hole to make the contacted bridge stronger.

(a) (b)

(c)

Figu

with

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Figure 3.17: The (a) top-view and (b) side-view SEM picture of 12-fold QPC microcavity with central post. The diameter of top and root of the central post are estimated to be 1.19μm and 0.776μm. And the gap between membrane and substrate is estimate to be 1.5μm.

From the top view SEM picture, the circular shadow can be used to estimate the diameter of central post. From Fig. 3.17 (a), the circular shadow is estimate to be 0.84μm in diameter.

From the direct measurement in side-view SEM picture in Fig. 3.17 (b), the diameter of central post is estimated to be 1.19μm. As a result, there is about 30 % inaccuracy when we estimate post size from the circular shadow.

However, when we reduce the radius of central post, the circular shadow will disappear.

This does not mean the post is no longer under the cavity. The shadow is caused by the different charge distribution in the cavity region and the interface with the central post.

However, when the post size reduces, this charge distribution difference will become difficult to observe, i.e. the shadow is difficult to observe and judge the existence of the central post.

Thus, to confirm the existence of the central post, the side-view or tilted-view SEM pictures are necessary.

(a) (b)

1.19μm

0.776μm 1.5μm

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As shown in Fig. 3.18 (b), there is no shadow observed in the microcavity region, but the post can be clearly observed from a tilted-view SEM picture. The smaller post size can only be estimated from the tilted-view SEM picture. We can first measure the angle of the post. By the diameter estimation at arbitrary point, we can calculate the diameter at the interface of the central post.

Figure 3.18: The tilted-view SEM picture of 12-fold QPC microcavity with the central post.

(a) The circular shadow can be clearly observed in the microcavity region when the diameter of the central post is large. (b) However, the circular shadow cannot be observed in the microcavity region when the diameter of the central post is small.