SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography
Section 2.7 Resists: 2.7.4 Multilayer Systems
2.7.4 Multilayer Systems
188.8.131.52 Low/high molecular weight PMMA
Multilayer resist systems are useful for several purposes: when an enhanced undercut is needed for lifting off metal, when rough surface structure requires planarization, and when a thin imaging (top) layer is needed for high resolution. Figure 2.28 showed the simplest bilayer technique, where a high molecular weight PMMA is spun on top of a low molecular weight PMMA. The low weight PMMA is more sensitive than the top layer, so the resist develops with an enhanced undercut. At high energies (>20 kV), thin PMMA (<0.5 m) will not normally develop an undercut profile; the best resist profile will be perpendicular to the substrate. The moderate undercut from this technique is useful when liftoff is required from densely packed features.
The two-layer PMMA technique was patented in 1976 by Moreau and Ting  and was later improved by Mackie and Beaumont  by the use of a weak solvent (xylene) for the top layer of PMMA. Use of a weak solvent prevents intermixing of the two layers. A further refinement of the technique  substituted MIBK, a solvent of intermediate strength, for the xylene. PMMA of various molecular weights dissolved in MIBK can now be purchased commercially. 
EXAMPLE PROCESS: LIFTOFF OF THIN METAL WITH PMMA BILAYER
- Clean wafer, on the spinner, by spraying with acetone, then isopropanol. Spin dry.
- Spin 495 K MW PMMA, 2% (in any solvent) 4 krpm for 30 s., for a thickness ~50 nm.
- Bake at 170-180C for 1 h.
- Spin 950 K MW PMMA, 2% in MIBK 4krpm for 30 s., for a thickness ~50 nm.
- Bake at 170-180C for 1 h.
- Expose at 50 kV, 350 to 450 C/cm2.
- Develop in MIBK:IPA, 1:3 for 1 min. Rinse in IPA, blow dry.
- Optionally, remove surface oxide of GaAs with 10 s dip in NH4OH : H2O (1:15). Blow dry.
- Evaporate 15 nm of Au:Pd (3:2) alloy, 210-6 Torr, base pressure, 0.5 nm/s.
- Lift off by soaking in methylene chloride. Optionally, finish with mild ultrasonic agitation.
A larger undercut resist profile is often needed for lifting off thicker metal layers. One of the first bilayer systems was developed by Hatzakis.  In this technique a high sensitivity copolymer of methyl methacrylate and methacrylic acid [P(MMA-MAA)]  is spun on top of PMMA. The exposed copolymer is soluble in polar solvents such as alcohols and ethers but insoluble in nonpolar solvents such as chlorobenzene. A developer such as ethoxyethanol/iso-propanol is used on the top (imaging) layer, stopping at the PMMA. Next, a strong solvent such as chlorobenzene or toluene is used on the bottom layer. This technique has been used to fabricate 1 um memory arrays with thick gate metalizations.
A more common use of P(MMA-MAA) is as the bottom layer, with PMMA on top. In this case the higher speed of the copolymer is traded for the higher resolution of PMMA.  For simplicity a single developer is used -- the nonpolar solvent working on the PMMA and the polar solvent developing the copolymer. Effective developer combinations include ethylene glycol monoethyl ether : methanol (3:7) and MIBK:IPA (1:1). The undercut of this process is so large that it can be used to form free-standing bridges of PMMA, a technique developed by Dolan  and used extensively for the fabrication of very small superconducting tunnel junctions. Other shadowing and "step edge" techniques for fabricating small lines and junctions are covered in the chapter by Howard and Prober.  The polymer PMGI (polydimethylglutarimide) is used for the same purpose as P(MAA-MAA). [178-179]
184.108.40.206 Trilayer systems
Bilayer techniques using P(MMA-MAA) or PMGI work well because the polar/nonpolar combination avoids intermixing of the layers. Almost any two polymers can be combined in a multilayer if they are separated by a barrier such as Ti, SiO2, aluminum, or germanium,   forming a so-called trilayer resist. After the top layer is exposed and developed, the pattern is transferred to the interlayer by RIE in CF4 (or by Cl2 in the case of aluminum). The interlayer serves as an excellent mask for RIE in oxygen. The straight etch profile available from oxygen RIE allows the fabrication of densely packed, high aspect ratio resist profiles. Such resist profiles can then be used for liftoff or for further etching into the substrate.
|FIGURE 2.30 (a) Resist cross-section (PMMA on P(MMA-MAA) on PMMA) for the lift-off of a "T" shaped gate. (b) Metal gate lifted off on GaAs. (Courtesy of R. C. Tiberio et al. ) |
If we start with Hatzakis's bilayer scheme (PMMA on the bottom and copolymer on the top) and then add another top layer of PMMA, we have a structure that can be developed into a mushroom shape,  as shown in Fig. 2.30. In this technique a heavy dose is given to the central line and a lighter dose to the sides. Mutually exclusive developers are used to form the "T-gate" shape, and a thick layer of metal is lifted off. This technique is widely used to form MESFET gates with low capacitance and low leakage (from the small contact area) and low resistance (from the large metal cross-section).
Next Sub-Section: 2.7.5 Inorganic and Contamination Resists
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