**3.2 Grating figure**

A SEM image of the fabricated diffraction gratings is shown in Fig. 9. It demonstrates that the line edge roughness (LER) of the gratings is markedly low, indicating that the LER of the grating corrugations in the master mold is sufficiently suppressed and that the master patterns are precisely transferred to the substrate by the imprinting and the subsequent etching process. The depth of the diffraction gratings was measured to be approximately 15 nm by an atomic force microscope.

Fig. 9. SEM image of diffraction gratings.

#### **3.3 Reproducibility of the grating period**

The diffraction gratings with the period from 194 to 204 nm have been formed on a 2-in. InP wafer. The grating periods have been verified by measuring the diffraction angles of the transferred grating patterns. The wavelength of the incident light is 364.8 nm, and the 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 high reproducibility in transferring grating patterns.

#### **3.4 Device characteristics**

218 Recent Advances in Nanofabrication Techniques and Applications

Mold SiO2 70000 0.17 Substrate InP 60700 0.36 Liquid Elastomer 1000 0.49

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

Elastomer

A SEM image of the fabricated diffraction gratings is shown in Fig. 9. It demonstrates that the line edge roughness (LER) of the gratings is markedly low, indicating that the LER of the grating corrugations in the master mold is sufficiently suppressed and that the master patterns are precisely transferred to the substrate by the imprinting and the subsequent etching process. The depth of the diffraction gratings was measured to be approximately 15

The diffraction gratings with the period from 194 to 204 nm have been formed on a 2-in. InP wafer. The grating periods have been verified by measuring the diffraction angles of the transferred grating patterns. The wavelength of the incident light is 364.8 nm, and the

Table 2. Mechanical properties used for the stress simulation.

Mold

Substrate

[MPa] 1.05E+7 9.16E+6 7.87E+6 6.56E+6 5.25E+6 3.94E+6 2.62E+6 1.31E+6 0.00E+0

and a substrate.

(a)

**3.2 Grating figure** 

nm by an atomic force microscope.

Fig. 9. SEM image of diffraction gratings.

**3.3 Reproducibility of the grating period** 

Young's modulus [MPa] Poisson ratio

(b) [MPa]

1.05E+7 9.16E+6 7.87E+6 6.56E+6 5.25E+6 3.94E+6 2.62E+6 1.31E+6 0.00E+0 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 Fig. 12).

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 fabricated by utilizing the conventional EBL process.

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

Application of Nanoimprint Lithography to Distributed Feedback Laser Diodes 221

1.26 1.27 1.28 1.29 1.30 1.31

= 195 nm = 202 nm

Emission wavelength [um]

Fig. 13. Oscillation spectra of the DFB LDs with the grating period () of 195 nm and 202

shown in Fig. 14, which demonstrate that they show comparable variations in SMSR.

NIL

EBL

characteristics and their uniformities to ones by conventional EBL process.

We have compared the side-mode suppression ratio (SMSR) of phase-shifted LDs fabricated by NIL with those fabricated by EBL. SMSR is one of the parameters indicating the stability of the single-mode emission of DFB LDs. More than 300 LDs randomly sampled from respective 2-in. wafers have been evaluated. Histograms of SMSR for both types of LD are

These results indicate that DFB LDs fabricated here by utilizing NIL have comparable

1E-7

0.00

32

34

standard deviations of SMSR are almost the same, 2.0 and 1.8, respectively.

36

Fig. 14. Histograms of SMSR for phase-shifted LDs fabricated by NIL and by EBL. The

38

SMSR [dB]

40

42

44

46

0.05 0.10

0.15

0.20

Normalized frequency

0.25 0.30

0.35

1E-6

1E-5

1E-4

Output power [W]

nm.

1E-3

1E-2

1E-1

Fig. 11. Supplied current vs optical output and slope efficiency.

Figure 12 shows the oscillation spectrum of a phase-shifted DFB LD. The resolution of the wavelength is 0.02 nm. This demonstrates that the peak wavelength corresponds to the Bragg wavelength at the center of the stopband, indicating that the phase-shifted gratings function properly.

Fig. 12. Oscillation spectrum of the DFB LD.

Figure 13 shows the oscillation spectra of DFB LDs with the grating period of 195 nm and 202 nm, which are simultaneously fabricated on a wafer. This demonstrates that the peak wavelengths correspond to the respective Bragg wavelengths corresponding to each grating period.

0 20 40 60 80 100 Current [mA]

Figure 12 shows the oscillation spectrum of a phase-shifted DFB LD. The resolution of the wavelength is 0.02 nm. This demonstrates that the peak wavelength corresponds to the Bragg wavelength at the center of the stopband, indicating that the phase-shifted gratings

> 1489 1491 1493 1495 1497 1499 Wavelength [nm]

Figure 13 shows the oscillation spectra of DFB LDs with the grating period of 195 nm and 202 nm, which are simultaneously fabricated on a wafer. This demonstrates that the peak wavelengths correspond to the respective Bragg wavelengths corresponding to each

0.0

0.1

0.2

Slope efficiency [W/A]

0.3

0.4

0.5

0


Fig. 12. Oscillation spectrum of the DFB LD.

Output power [dB]

Fig. 11. Supplied current vs optical output and slope efficiency.

5

10

Output power [mW]

function properly.

grating period.

15

20

25

Emission wavelength [um]

Fig. 13. Oscillation spectra of the DFB LDs with the grating period () of 195 nm and 202 nm.

We have compared the side-mode suppression ratio (SMSR) of phase-shifted LDs fabricated by NIL with those fabricated by EBL. SMSR is one of the parameters indicating the stability of the single-mode emission of DFB LDs. More than 300 LDs randomly sampled from respective 2-in. wafers have been evaluated. Histograms of SMSR for both types of LD are shown in Fig. 14, which demonstrate that they show comparable variations in SMSR.

These results indicate that DFB LDs fabricated here by utilizing NIL have comparable characteristics and their uniformities to ones by conventional EBL process.

Fig. 14. Histograms of SMSR for phase-shifted LDs fabricated by NIL and by EBL. The standard deviations of SMSR are almost the same, 2.0 and 1.8, respectively.

Application of Nanoimprint Lithography to Distributed Feedback Laser Diodes 223

As described above, NIL is an effective and promising method for fabricating phase-shifted DFB LDs and is expected to have the advantage of mass-production capability in the near future. We conclude that NIL has high potential for fabricating DFB LDs, and we expect that NIL will be used for fabricating various optical devices consisting of nanostructures.

We would like to appreciate Dr. Taniguchi and his laboratory members at Tokyo University

Bailey, T.; Choi, B. J.; Colburn, M.; Meissl, M.; Shaya, S.; Ekerdt, J. G.; Sreenivasan, S. V. &

Chou, S. Y.; Krauss, P. R. & Renstrom, P. J. (1995). Imprint of sub-25 nm vias and trenches in

Fukuda, M. & Iwane, G. (1985). Degradation of active region in InGaAsP/InP buried heterostructure lasers, *J. Appl. Phys.*, Vol. 58, pp. 2932-2936, ISSN 0021-8979 Haisma, J.; Verheijen, M.; Heuvel, K. and Berg, J. (1996). Mold-assisted nanolithography: A

Kaden, C.; Griesinger, U.; Schweitzer, H.; Pilkuhn, M. H. & Stath N. (1992). Fabrication of

Kinoshita, H.; Yamauchi, A. & Sawai, M. (1999). High-resolution resist etching for

Kure, T.; Kawakami, H.; Tachi, S. & Enami, H. (1991). Low-Temperature Etching for

Matsuoka, T.; Nagai, H.; Noguchi, Y.; Suzuki, Y. & Kawaguchi, Y. (1984). Effect of the

Miller, M.; Doyle, G.; Stacey, N.; Xu, F.; Sreenivasan, S. V.; Watts, M. & LaBrake, D. L. (2005).

Tsuji, Y.; Yanagisawa, M.; Yoshinaga, H.; Inoue, N. & Nomaguchi, T. (2011). Highly uniform

nanoimprint lithography, *Jpn. J. Appl. Phys.* 50, 06GK06, ISSN 1347-4065 Yanagisawa, M.; Tsuji, Y.; Yoshinaga, H.; Kono, N. & Hiratsuka, K. (2009). Application of

polymers, *Appl. Phys. Lett.,* Vol. 67, pp. 3114-3116, ISSN 1077-3118

Willson, C. G. (2000). Step and flash imprint lithography: Template surface treatment and defect analysis, *J. Vac. Sci. Technol.,* B 18, pp. 3572-3577, ISSN 1071-

process for reliable pattern replication, *J. Vac. Sci. Technol.,* B 14, pp. 4124-4128, ISSN

nonconventional distributed feedback lasers with variable grating periods and phase shifts by electron beam lithography, *J. Vac. Sci. Technol.,* B 10, pp. 2970-2973,

quartermicron lithography using O2/N2 supermagnetron plasma, *J. Vac. Sci.* 

Deep-Submicron Trilayer Resist, *Jpn. J. Appl. Phys.,* 30, pp. 1562-1566, ISSN

grating phase at the cleaved facet on DFB laser properties, *Jpn. J. Appl. Phys.* 23, pp.

Fabrication of nanometer sized features on non-flat substrates using a nano-imprint

fabrication of diffraction gratings for distributed feedback laser diodes by

Nanoimprint Lithography to Fabrication of Distributed Feedback Laser Diodes*, Jpn.* 

**5. Acknowledgment** 

**6. References** 

1023

1071-1023

1347-4065

L138-L140, ISSN 1347-4065

ISSN 1071-1023

of Science for cooperative works and fruitful discussion.

*Technol.,* B 17, pp. 109-112, ISSN 1071-1023

lithography process, *Proc. SPIE 5751,* pp. 994-1002

*J. Appl. Phys.*, Vol. 48, 06FH11, ISSN 1347-4065

#### **3.5 Reliability**

We have investigated long-term reliability of fabricated DFB LDs. Figure 15 shows the timedependent change in the threshold current of phase-shifted LDs with the output power of 10 mW at the ambient temperature of 358 K. The number of samples is 18. The change in the threshold current after 5000 hours is less than ±1%, indicating that the phase-shifted DFB LDs fabricated in this study have high stability in lasing characteristics.

Fig. 15. Time-dependent change in the operation current of the DFB LDs with the output power of 10 mW at the ambient temperature of 358 K.

#### **4. Conclusion**

We have successfully demonstrated fabrication of phase-shifted DFB LDs by utilizing NIL process, which have comparable characteristics and their uniformities to ones fabricated by conventional EBL process. Fabricated DFB LDs have shown high stability of characteristics in long-term reliability test. We have also demonstrated the feasibility of the VARI-mold, which can be used for the fabrication of DFB LDs with various wavelengths, indicating that we can drastically reduce the cost of molds in our mass production phase in the near future. Considering the results above, we conclude that NIL is a promising candidate of the production technique for phase-shifted DFB LDs featuring low cost and high throughput.

NIL is expected to be used as a fabrication process for many applications. However, there are still some difficulties with its use as a mass-production process, for example, defects and low throughput in patterned media, defects and poor alignment accuracy in semiconductor lithography, and the necessary of increasing field size and throughput in displays. Although those difficulties may be common to the fabrication of DFB LDs, they would not be insurmountable problems. Even if defects in the imprinted pattern influence the yield of LDs, failed chips could be easily rejected because each dye is as small as approximately 300 m. There is no need for a larger field size than a 3-in.-diameter circle, because the diameter of compound semiconductor substrates used for LDs is 3 inches or smaller. Regarding alignment accuracy, an error of up to approximately 5 m is acceptable. Even if throughput is limited to less than one wafer per hour, NIL would still have a higher throughput than EBL.

As described above, NIL is an effective and promising method for fabricating phase-shifted DFB LDs and is expected to have the advantage of mass-production capability in the near future. We conclude that NIL has high potential for fabricating DFB LDs, and we expect that NIL will be used for fabricating various optical devices consisting of nanostructures.
