Ion Beam Lithography: faster writing strategies for features between 150nm and 1um Brent P. Gila, Andes Trucco, David Hays Located in sunny Gainesville, FL (100 miles north of Disney World) https://nrf.aux.eng.ufl.edu/ 1
Ion beam lithography overview Patterning strategies are to separate features based upon size to minimize exposure time. Lithography processing gap: Affordable lithographic processes for average university facilities are contact aligners (and projection aligners) and SEM pattern generator attachments. These cover >1um and <1um respectively. Optical exposures are parallel and fast, electron beam exposures are serial and slow. Ion beam lithography can provide gap coverage between the 200nm and 1um feature range. This is a serial process like electron beam lithography. However the ion exposure required for the resists are 10 times lower, so the writing time is greatly reduced compared to electron beam lithography. 2
Raith ionline System Overview 100 mm laser interferometer stage 60nm stitch and overlay, 1nm min step NanoFIB TM ion column 15-40keV, 0.5pA-1nA, current stability <1%/hr Multi-species ion sources available Currently Au-Si 20 MHz, 16 bit digital pattern generator dedicated software for lithography and nano engineering Ion beam centric with high resolution small WD, compact optics Normal incidence precise and stable navigation High beam stability
Ion beam lithography overview Resists under go chain scission in e-beam lithography. Due to a large mass, ions create more chain scission events compared to the same dose of electrons. Typical exposures required for ion beam lithography are 2-10 uc/cm 2 compared to 100-300uC/cm2 for electron beam lithography. 30kV ion simulated in PMMA by SRIM Si+ Ga+ Au+ 4
Ion beam lithography overview Depth of resist exposure is dependent upon ion kv, but high kv does not lead to higher aspect ratio structures, only deeper exposures for thicker resists. Ion scattering in PMMA remains large. Si+ ion simulated in PMMA by SRIM 20kV 30kV 40kV 100nm resist 5
Ion beam lithography overview Simulations of incident ions only provide part of the total damage picture. When the recoiled atoms are include in the simulation, the cascade damage is much larger and thus the large minimum resolution for the ion beam exposure. 40kV Si+ 40kV Si+ H C O 6
Ion beam lithography overview Si ion beam exposure in 300nm PMMA has the same profile as the simulation. To ensure proper clearing dose and not implant/damage the substrate, must us a sacrificial stopping layer under the PMMA. Typical stopping layer employed is 200nm LOR. 7
Ga+ FBMS PMMA resist exposures 0.1mm/sec Patterning 50um PMMA over 300nm LOR with 6pA Ga+ 30kV, 0.25mm/sec 0.5mm/sec Increasing stage speed to lower exposure dose. Very easy to overexpose and crosslink. 1.0mm/sec
Si+ dose PMMA resist exposures 40kV, 2uC/cm2, 100nm PMMA on 200nm LOR 200nm design 400nm design 800nm design 600nm design 1000nm design 9
Si+ area dose PMMA resist exposures 2uC/cm2 4uC/cm2 6uC/cm2 8uC/cm2 10uC/cm2 15uC/cm2 10
Si+ area dose PMMA resist exposures Over exposures to PMMA do not lead to milling, but severe crosslinking. 65uC/cm 2 70uC/cm 2 11
Au+ area dose PMMA resist exposures 4uC/cm2 6uC/cm2 8uC/cm2 10uC/cm2 20uC/cm2 Onset of PMMA crosslinking noticed at 6uC/cm2. Complete crosslinking at 20uC/cm2. 12
Additional resist exposures Extend process to use conventional/commonly available resists. Use thinned nlof (an AZ negative resist) to create features. Si+ 40 kv, 2uC/cm 2, very low dose required to expose the resist. 13
Summary of ion beam lithography The use of ion beams to expose resist provides: 10x increase in write speed over electron beam exposures due to higher polymer chain scission (cascade damage) Correct exposure is ion dose and ion species dependent Use of standard resists and known processes (positive and negative) 3 rd party companies are now supporting attachments for FIBs Meets the need of patterning 200nm to 1000nm features at a fast rate. HIGH-THROUGHPUT FOCUSED-ION- BEAM LITHOGRAPHY USING DOUBLY CHARGED IONS; N. Garraud et al., presented at ENRI 2013 14