High resolution electron beam lithography with HSQ resist
HSQ or Flowable Oxide (FOx), is used as a low-k dielectric for back-end metal interlayer isolation and has excellent planarization and gap filling capabilities. We use this negative tone electron beam resist for many applications owing to the following advantages that it offers:
- HSQ can be applied using standard spin coating techniques,
- High resolution, several studies have used HSQ to demonstrate the resolution limits of EBL demonstrating structures down to 1-2 nm in size on thin membrane substrates,
- Patterned structures show excellent line edge roughness (LER),
- Features good resistance to plasma etching and can be used directly as an etch mask for pattern transfer.
A challenge with EBL/HSQ process is its low repeatability with time and obtaining desired structures across large areas, both related to high condensation rate of HSQ even in vacuum. Moreover, the HSQ developer is a relatively concentrated strong base that is not always compatible with the underlying surface, typical example would be Ge. Hence we have developed HSQ process flows that are compatible with a variety of challenging substrates and surfaces such as Ge, NiSi, TiN, etc. Another challenge with HSQ patterning is the development of high-density (sub-20 nm pitch) structures due to high contribution of electron proximity effects. We have developed s HSQ exposure work flows for incident beam energies ranging from 10 kV – 100 kV using two different EBL systems e.g. RAITH e-line and Elionix ELS-G100.
HSQ, EBL, high resolution, high density, Si, Ge, NiSi, TiN, SiN, 10kV, 30kV, 100kV
- 4 and 6 inch wafers
- Various substrates
- GDSI file with the pattern design
- Nano for Quantum Technologies
- Disruptive Devices
Key Enabling Capability
One of the key requirements in lithography is the development of extreme density (sub-20nm pitch) patterned line structures and their pattern transfer via RIE. This is an extremely challenging task, with a major promise coming from the future EUV for mass fabrication. While EUV area is still far from practice, EBL offers the possibility to test resists and etching recipes by developing prototype structures and devices. However only few research groups have demonstrated EBL process flows for sub-20nm pitch features worldwide, and to the best of our knowledge, there is no other nano-fabrication facility capable of routinely fabricating such dense patterns on large areas. While, we are capable of routinely fabricating widely spaced 7-8nm line width structures, the repeatable patterning of extreme density patterns (Figs. 3.a and 3.b) showed to be very challenging. Our best results (obtained with good repeatability) now are at 25nm pitch structures (Fig. 3.c) with line widths of 12nm and line spacing of 13nm.