In this chapter, the fabrication instruments will be introduced. We will show how to fabricate the photonic crystal lasers on a PDMS substrate. The preparation of the PDMS substrate will also be introduced in the following content.
2-1 Introduction to the Fabrication Instruments
2-1-1 The Scanning Electron Microscope (SEM) System
For the micro-scale photonic crystal devices, it is important to check the geometry parameters in the fabrication process. We can not only assure whether the device is successfully fabricated but also can perform the simulations with these parameters to further understand the data we recorded. The scanning electron microscope is the key instrument for the macro fabrication process. It acts as the eyes for us to check the device step by step in the process flow. Figure 2-1.1 shows the scanning electron microscope system. The system model is Inspect F from FEI Company.
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Figure 2-1.1 Picture of the scanning electron microscope system
2-1-2 Plasma Enhanced Chemical Vapor Deposition(PECVD) System
We have to deposit silicon nitride (SiNx) layer as a hard mask layer on the epitaxial wafer for the following fabrication process. We use the Oxford plasmalab 80 plus to deposit 240 nm thickness SiNx on the epitaxial wafer with SiH4/NH3/N2
mixture gases. The SiNx thin film quality strongly depends on the NH3/ SiH4gas ratio.
The SiNx thin film deposited with NH3 addition would be easy for chemical etching process. Actually, the SiNx layer can also be deposited at absence of NH3 gas. In this way, the resistance to InP dry etching will be much higher, but the SiNx layer cannot be removed completely by the BOE wet etching. It is a trade off because the high resistance to dry etching is good for the InP etching but it is hard for the pattern on PMMA to be transferred to SiNx layer. As a result, we tried different recipes by changing NH3/ SiH4 ratio and find the acceptable recipe for the fabrication procedures.
Finally, the equal amount of NH3 and SiH4 gases is used for the SiNx deposition. We deposit SiNx layer with 8 sccm SiH4, 8 sccm NH3, 250 sccm N2. The RF power is 20 w, the temperature is 200˚C, and the chamber pressure is 1000 mtorr .
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Figure 2-1.2 Picture of the plasma enhanced chemical vapor deposition system
2-1-3 E-beam Lithography Process
Because the patterns are in macro scale, the electron-beam lithography is used instead of traditional optical lithography method. We use the polymethylmethacrylate (PMMA) as the e-beam resist. The PMMA is a standard positive electron beam resists with 950,000 molecular weight resins in chlorobenzene either anisole. The ratio of PMMA to the anisole is 5:100, which is called A5. The more the PMMA dissolve into anisole the bigger thickness for the same spin speed of coating.
We spin coat the PMMA on the top of the SiNx-deposited wafer by a spin coater with two steps. The rotation speed is 1000 revolution per minute (rpm) for ten seconds in first step. The PMMA would be uniform on the wafer after step 1. The step 2 starts instantly with 3500 rpm for 3 minutes to get the thinner thickness. After coating the PMMA, some of the solvent will vaporize by the 180 ̊C baking for 2 minutes. The PMMA would be harder after backing. The thickness of the PMMA would be around 300 nm after these steps.
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We use the e-beam lithography process to define the patterns with PMMA on the SiNx deposited wafer. After e-beam exposure, the exposed area would be developed with a mixture solution of isopropanol (IPA) and MIBK with 3:1 ratio for 70 seconds.
The ratio of MIBK and isopropanol affects the resolution and sensitivity. The lower MIBK to IPA ratio is, the higher resolution is; but the sensitivity is lower. After the develop process, we use the pure isopropanol to remove the developer, and immerse the sample in deionized water. Finally, we dry the sample by nitrogen gas.
We can check whether the patterns are successfully developed by OM system.
Figure 2-1.3 shows a SEM image of the cross section view to the air hole patterns after the develop process. The vertical profile of the hole shapes indicates that the e-beam dosage is appropriate for the pattern and the develop process is successful.
Figure 2-1.3 Cross section view of the air hole patterns after the develop process.
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2-1-4 ICP/RIE System
In the fabrication procedures, the patterns on PMMA are transferred to the SiNx
and further to InGaAsP layer by dry etching process. The anisotropic etching of dry etching is very common in the semiconductor industry. The key process of the dry etching is the plasma formation. The plasma is a state of matter similar as gases in which part of the particles are ionized. With the well selection of gas types, the active plasma would react with the exposed area and forms the gas-type products which would be pumped out of the system. This kind of reaction is chemical etching process.
Sometimes we need some high weight ions to bombard the reactant surface in order to increase the etching rate. Combination of sputtering and chemical etching processes significantly increases etching rate in a specific direction. It may caused by the higher local temperature of the bombardment area to increase the chemical etching rate. This kind of the etching is called reactive ion etch (RIE) process. If we want to increase the plasma density, we can make the electrons accelerated in angular direction. The RF power is inductivity couples to plasmas, which is called inductive couple plasma (ICP). In this way, we can achieve high plasma density at low temperature. Besides, it allows independent control of ion flux and ion energy, which is feasible for us to control the etching condition.
Our dry etching system is ICP/RIE system. The brand model is Oxford Plasmalab system 100. CHF3, O2, N2 and Cl2 are provided for dry etching. We can tune recipes by varying the gas flow, chamber pressure, temperature, RF power and ICP power. The high density plasmas of this system are very useful for fabricating the sub-micron structures with controllable selectivity, sooth morphology, vertical profile and high etching rate.
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Figure 2-1.4 Picture of the ICP/RIE System
The defined patterns are transferred to SiNx layer by RIE at 20℃with CHF3：50 (sccm), O2：5 (sccm), RF power：150 (W) and pressure：55 (mtorr). Figure 2-1.5 and Figure 2-1.6 shows the SiNx etching profile the line with 200 nm line width and holes with 200 nm in diameter. The vertical profile is obtained by controlling the etching time. After removing the residual PMMA, the pattern was transferred into the InGaAsP MQWs layers by ICP etching at 160℃ with N2:6 (sccm), Cl2:5 (sccm), RF power:100(W) , ICP power:400(W) and pressure: 10 mtorr. Figure 2-1.7 shows the InP etching profile of the line with 200 nm line width and holes with 200 nm in diameter. The selectivity is about 9 for the photonic crystal patterns.
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Figure 2-1.5 Cross section view of SiNx etching profiles.
Figure 2-1.6 Cross section view of SiNx etching profiles (a) line with 200 nm linewidth (b) holes with 200 nm in diameter.
Figure 2-1.7 Cross section view of InP etching profiles (a) line with 200 nm linewidth (b) holes with 200 nm in diameter.
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2-1-5 Wafer Bonding to PDMS Substrate
A commercial polydimethylsiloxane (PDMS) material (Sylgard 184, supplied by Dow Corning) serves as a flexible polymer here. PDMS is optically clear and considered to be inert, non-toxic and non-flammable. It acts as an elastic solid, similar to rubber. The PDMS substrate is prepared by mixing the Sylgard184A and Sylgard184B solutions with a 4:1 ratio of volumes. As the ratio becomes larger, the PDMS will be softer. That is to say, the ratio of the A and B solutions determines the hardness of the PDMS substrate. It takes about 1day that the mixed solution can be stiffening at 25℃. The PDMS thickness is controlled by the volume of the PDMS glue we added. The refractive index of the PDMS is about 1.43, which can support good light confinement in the vertical laser structure.
The InP-etched devices can be put upside down and bonded directly on a well-prepared PDMS substrate. After we bonding the device to the PDMS substrate, the InP substrate will be removed by dilute HCl solutions (HCl: H2O=3:1) at room temperature for 40 minutes. The PDMS can afford strong acid and base; therefore it would not be destroyed in the wet etching process.
Figure 2-1.8 Picture of the Sylgard184A
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2-2 Fabrication Steps for Flexible Photonic Crystal Laser
The triangular-lattice photonic crystal structures are fabricated on a polydimethylsiloxane (PDMS) substrate. The illustration of the laser structure is shown in Figure 2-2.1. The 240 nm thick InGaAsP active layer consisted of four 10-nm-thick strained InGaAsP quantum wells (QWs). The epitaxial structure of InGaAsP/InP MQWs for the device is shown in Figure 2-2.2. It has been confirmed that the photo-luminance spectrum (PL) of the QWs is centered at 1550 nm and the full-width half-maximum (FWHM) is about 200 nm. The PL spectrum is shown in Figure 2-2.3
Figure 2-2.1 Illustration of the triangular lattice photonic crystals on a PDMS substrate.
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Figure 2-2.2 The epitaxial structure of InGaAsP QWs
Figure 2-2.3 The PL spectrum of InGaAsP/InP MQWs
The PDMS substrate with a 500 um thickness was served as a flexible platform here. The refractive index of the PDMS was about 1.43, which was low enough to support good optical confinement in the vertical direction of the lasers.
There are seven steps to fabricate the flexible devices. First, we deposited silicon-nitride (SiNx) on the epitaxial wafer as a hard mask, and spin-coated polymethylmethacrylate (PMMA) resist on it. Then, the triangular-lattice photonic
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crystals were patterned on PMMA by electron-beam lithography. The lattice constant varied from 370 nm to 430 nm. Followed by RIE and ICP dry-etching processes, we transferred the patterns to the SiNx layer with CHF3/O2 mixture gases at 20℃ and further to the QWs layer with N2/H2 mixture gases at 160℃. After that, we bonded the QWs layer to the well-prepared PDMS substrate. Finally, the photonic crystal structure on the PDMS substrate was obtained by selective wet etching with the HCl diluted solution to remove the InP substrate. Figure 2-2.4 is the picture of the fabricated structure on a PDMS substrate. Figure 2-2.5 shows the scanning electron microscope (SEM) image of a fabricated triangular-lattice photonic crystal band-edge laser on a PDMS substrate.
Figure 2-2.4 Picture of the fabricated structure on a PDMS substrate
Figure 2-2.5 Magnified SEM image of triangular lattice photonic crystals
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The fabrication flow chart is shown below step by step.
SiNx deposition with PECVD system
PMMA spin-coating on the SiNx hard mask
Pattern defined with the electron-beam lithography system
Step1
Step2
Step3
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Pattern transferred to SiNx layer with ICP/RIE etching
Pattern transferred to InGaAsP layer with ICP/RIE etching
Bonding the structure upside-down on the PDMS substrate
Step4
Step5
Step6
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HCl wet etching to remove the InP substrate.
Finally, we get the fabricated devices.
2-3 Conclusion
In conclusion, we introduce the fabrication procedures for the photonic crystal lasers on a flexible PDMS substrate. Then, we introduce the fabrication instruments and the recipes we used. With the well-tuned recipe, the devices are successfully fabricated and the SEM image of the device is also shown in this chapter.
Step7
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