Additive Pattern Transfer

Additive pattern transfer involves the addition of metal films through a resist mask onto a substrate; the photoresist layer is then removed or lifted-off. Successful additive pattern transfer is not possible with ultra-thin resist. To achieve lift-off the resist should have a slightly undercut profile, and the resist thickness should be significantly greater than the metal being deposited. A trilayer scheme has been developed which improves the resist profile for lift off processes. The trilayer scheme consists of a top imaging layer (the ultra-thin photoresist), a barrier layer that acts as a hard etch mask (SiOx) and an anti-reflection coating (ARC) bottom layer. Fig. 17.9 illustrates the trilayer additive pattern transfer technique that has been developed for ENFOL. In this process a 130 nm thick film of a commercial ARC (XLT, Brewer Science) is first spun on the silicon substrates. This is then baked on a hot plate for 20 sec at 95°C followed by a one-minute bake at 170°C. Following this a silicon oxide SiOx barrier layer is thermally evaporated to a thickness of 16 nm, and the final ultra-thin photoresist imaging layer is then spun to give a uniform thickness of 45 nm. The sample is now ready for ENFOL exposure and development. Fig. 17.10(a) shows an atomic force microscope (AFM) image of a 270-nm period grating exposed into the imaging photoresist layer of a trilayer sample. As before, a clear sub-diffraction-limited pattern is evident, although there is a significant degree of LER. This AFM image of the photoresist prior to any pattern transfer process indicates that the LER is related to the resist not to any subsequent RIE. After exposure and development of this imaging layer, the substrate is subjected to two RIE steps, to dry etch the SiOx and the ARC respectively. The RIE recipes are given in Table 17.1. Fig. 17.10(b) shows a cross-sectional view of a 270 nm period grating after these RIE steps have been performed, and the transfer of the pattern into the full trilayer thickness is evident. The resist now has sufficient surface relief depth to allow lift-off of relatively thick films, however the required undercut sidewall profile of the etched trenches cannot be resolved with the AFM. This can only be determined from the success of the resultant lift-off. Any number of different materials could be deposited in the lift-off stage, and for this work a 30 nm NiCr film was then deposited by thermal evaporation and lifted off in MF320 developer. Fig. 17.10(c) shows an AFM image of the final 270 nm period NiCr gratings obtained with this trilayer pattern transfer technique. No degradation in pattern quality has resulted from the pattern transfer and lift-off processes, so this should be capable of being used for finer-period structures if the LER issues in the ENFOL exposure can be resolved.

Trilayer Lift Off
Fig. 17.9. Schematic diagram of the lift-off process used with trilayer additive pattern transfer scheme. The anti-reflection coating layer is 130 nm thick, SiOx layer is 16 nm thick and the NiCr is 30 nm thick
Table 17.1. RIE recipes for SiOx and ARC pattern transfer processes

Gas

Etch rate

Pressure

Flow rate

Power

DC self

Temp

(nm/min)

(mT)

(sccm)

density (W/cm2)

bias (V)

(°C)

CHF3

20

3

15

0.45

506

22

O2

90

6

20

0.45

489

22

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