**3. Results**

When we use NIL for fabricating DFB LDs, we have paid attention to mechanical damage in epitaxial layers by imprint pressure, because compound semiconductor crystals used for LDs are easily damaged by mechanical stress, leading to severe deterioration in long-term reliability of device characteristics (Fukuda et al., 1985). In this study, photoluminescence (PL) method have been used for evaluation of the crystal damage, and simple 2-D simulation is conducted as qualitative analysis of distribution of the PL intensity.

Furthermore, we have also focused on the accuracy of the grating period, because it dominantly influences the emission wavelength which is the essential parameter of LD characteristics. The emission wavelength directly impacts on the yield of DFB LDs, so its repeatability is required to be sufficiently high for volume production. We have verified the accuracy of the grating period by measuring diffraction angles described below.

Finally, we have fabricated quarter-wavelength shifted DFB LDs by utilizing the new fabrication process combining NIL and the conventional LD process, and evaluated their characteristics including long-term reliability.

#### **3.1 Influence of imprint pressure**

216 Recent Advances in Nanofabrication Techniques and Applications

We have evaluated the linewidth uniformity of the corrugations within 6 wafers. Figure 6 is

the histogram of the linewidth, indicating the standard deviation is less than 4 nm.

Fig. 5. SEM image of the diffraction grating after the penetration etching.

characteristics including long-term reliability.

Frequency

**3. Results** 

30

36

42

48

simulation is conducted as qualitative analysis of distribution of the PL intensity.

accuracy of the grating period by measuring diffraction angles described below.

54

Fig. 6. Histogram of the corrugation linewidth within 6 wafers after the penetration etching.

When we use NIL for fabricating DFB LDs, we have paid attention to mechanical damage in epitaxial layers by imprint pressure, because compound semiconductor crystals used for LDs are easily damaged by mechanical stress, leading to severe deterioration in long-term reliability of device characteristics (Fukuda et al., 1985). In this study, photoluminescence (PL) method have been used for evaluation of the crystal damage, and simple 2-D

Furthermore, we have also focused on the accuracy of the grating period, because it dominantly influences the emission wavelength which is the essential parameter of LD characteristics. The emission wavelength directly impacts on the yield of DFB LDs, so its repeatability is required to be sufficiently high for volume production. We have verified the

Finally, we have fabricated quarter-wavelength shifted DFB LDs by utilizing the new fabrication process combining NIL and the conventional LD process, and evaluated their

60

Linewidth [nm]

66

72

78

**500 nm** 

84

90

#### **3.1.1 Photoluminescence intensity**

In order to evaluate the mechanical damage in semiconductor crystal induced by imprinting pressure, we have investigated deterioration of PL intensities from the epitaxial layers. We prepared two indium-phosphide substrates with epitaxial layers and a blank (with no patterns) mold. We compared photoluminescence intensities between the two samples: imprinted with UV-curable resin between the mold and the substrate, and imprinted without resin. The field size of the blank mold used here is 10 mm x 10 mm. The evaluation results of PL intensities are shown in Fig. 7. Field size of the blank mold used here is 10 mm x 10 mm. Imprinting pressure is 0.8 MPa for both samples. The sample without resin shows evident deterioration of PL intensities indicated in dark (green-like) colors in Fig. 7. The deteriorations of intensity are found mainly in the edge of the imprinted area. This means that the imprinting pressure concentrates near the edge of the mold. On the other hand, no evident deterioration is found in the sample with resin. These results indicate that the resin functions such as a cushion to prevent severe damage in epitaxial layers by imprinting pressure.

Fig. 7. PL intensities of the epitaxial layers after imprints. The field size of the imprint is 10 mm x 10 mm.

#### **3.1.2 Simulation**

We have calculated how resin between a mold and a substrate influences on mechanical stress induced in the substrate. We used a simple model for the finite element method (FEM) as described in Table 2 and Fig. 8. In this study, UV-curable resin is to be considered as liquid having nonlinear viscoelasticity; however, elastomer is substituted for resin because we do not have an adequate tool for dynamic simulation of viscoelastic material. Thickness and the Young's modulus of the elastomer are assumed of 50 nm and 1000 MPa, respectively. Imprinting pressure is 1 x 107 MPa. The elastomer and the other materials are connected with common FEM nodes at the boundaries.

Figure 8 shows two-dimensional distribution of von Mises stress in molds and substrates. For the sample without elastomer, mechanical stress is concentrated in periphery of the edge of the mold [Fig. 8(a)]. On the other hand, when the elastomer is supposed between the mold and the substrate, concentration of stress on the surface of the substrate is clearly suppressed [Fig. 8(b)]. These results are qualitatively consistent with the evaluation using PL described in above section.

Application of Nanoimprint Lithography to Distributed Feedback Laser Diodes 219

measurement error of the grating period is estimated less than 0.03 nm. Figure 10 shows a correlation between the grating periods of the mold and those of the transferred patterns. The difference between both of the values is less than 1%, and the wafer-to-wafer variation of the periods is less than 0.2 nm through the 6 wafers. This shows that the NIL process has

We have evaluated DFB LDs fabricated by our novel process in order to verify the basic characteristics and their uniformity in 2-in. wafer. The nominal wavelength of the measured LDs is 1490 nm, and the corresponding grating period is approximately 232 nm (excluding

Figure 11 shows the dependence of the optical output and the slope efficiency on the supplied current for a typical phase-shifted DFB LD fabricated in this study. The threshold current and the slope efficiency at room temperature were measured to be 8 mA and 0.28 W/A, respectively, which are comparable to those of typical phase-shifted DFB LDs

high reproducibility in transferring grating patterns.

fabricated by utilizing the conventional EBL process.

192

192

194

196

Fig. 10. Correlation between the designed (horizontal) and the measured (vertical) grating

198

Grating period [nm] (mold)

200

202

204

206

194 196

198

Grating period [nm]

(transferred)

200

202 204

206

**3.4 Device characteristics** 

Fig. 12).

periods.


Table 2. Mechanical properties used for the stress simulation.

Fig. 8. Distribution of von Mises stress without (a) and with (b) elastomer between a mold and a substrate.
