Repeatability and control in nanoimprint lithography Sarah Felix 4/14/08 EE C235 - Nanoscale Fabrication

Objective Look at how lithography system components influence performance of nanoimprint lithography (NIL)

Process overview

Parameters and cycle time Translation speed, v t Speed to contact template, v press Contact force, F c Hold time, t hold Exposure time, t expose Release speed, v release Alignment gap, d

Key sources of variation Template fabrication Physical contact –Resist accumulation –Degradation of anti-sticking layer –Uniformity or resist for spin-coated layers Positioning and alignment methods –Overlay and pattern transfer –Force and parallelism

Repeatability study (2004) Resist spin-coated over entire wafer Over 1000 imprints without cleaning template Measured resist thickness Residual resist is most important –Don’t want too much overetching –Depends on initial resist thickness –+/- 8 nm variations (~ 2  ) from local fluctuations –Systematic tilting error from passive compensation Residual resist thickness M. Otto et al., Microelectronic Engineering (2004),

Repeatability study (2007) Resist spin-coated over wafer Over 100 imprints without cleaning template Measure feature sizes to characterize critical dimension (CD) control 3  variation < 9nm They conclude no gradual variations observed from adhesion, accumulations, etc.? Not so sure… M. Otto et al., Microelectronic Engineering 84 (2007),

Stage control needs X,Y,  Z –Overlay and pattern transfer –Minimize lateral motion during contact Z,  X,  Y –Parallel surface contact –Uniform contact force X Y ZZ Z YY xx

Passive vs. active compensation Passive, compliant mechanisms control force across wafer Various configurations possible Disadvantages –Systematic errors –Variation in flexures –Causes lateral error if rotation axis not in plane of template template not at center of wafer Not adequate for multi-step, multi-layer NIL H. Lan et al., Microelectronic Engineering 84 (2007),

Types of stages Linear motors for coarse control: high bandwidth, reduced nonlinearities Fine control options –Flexure-based with piezo actuators Reduced size, inertia, and friction Poor dynamics due to coupling –Air bearing Non-contact, so no backlash or stick-slip friction Better dynamics, but lower stiffness and doesn’t handle dynamic loads well –Mag-lev Resolves above issues, but high cost, requires complex control and has some physical limitations H. Lan et al., Microelectronic Engineering 84 (2007),

Sensing for feedback Considerations: Linearity, Resolution, Stability, Bandwidth, Cost Laser interferometer typically used for position control Types of gap sensing for force control –Capacitive –Interferometry –Ellipsometry –Spectral reflectometry H. Lan et al., Microelectronic Engineering 84 (2007),

Conclusions Step-and-repeat NIL is promising for high throughput Critical dimensions are stable in the face of repeated physical contact Positioning is most significant challenge for multi-step, multi-layer implementation Sophisticated feedback control needs to be developed (e.g. Berkeley)

Questions?