Laser Processing for the Fabrication of MEMS Devices

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1 Laser Processing for the Fabrication of MEMS Devices Andrew S. Holmes Optical & Semiconductor Devices Group Department of Electrical & Electronic Engineering Imperial College, London SW7 2AZ, UK AILU Workshop, 22 September

2 Outline Motivation 3D and large area laser micromachining Laser transfer processes for MEMS Conclusions AILU Workshop, 22 September

$materials base: - Polymers - Biomaterials - Hard materials e.g. refractory metals, diamond, - Functional materials e.g. piezoceramics, shape memory alloys Enhancement of 3D capabilities - 3D (i.e. not just prismatic) structures on planar substrates - Processing of non-planar surfaces e.$

3 Laser Processing for MEMS/Microsystems MEMS (MicroElectroMechanical Systems) field traditionally dominated by silicon & silicon-related materials and processes Laser processing allows: Expansion of materials base: - Polymers - Biomaterials - Hard materials e.g. refractory metals, diamond, - Functional materials e.g. piezoceramics, shape memory alloys Enhancement of 3D capabilities - 3D (i.e. not just prismatic) structures on planar substrates - Processing of non-planar surfaces e.g. for trimming New options for manipulation / assembly / joining of parts Car crash bag accelerometer [Ford] AILU Workshop, 22 September

Laser Machining for MEMS/Microsystems Inkjet printer nozzle drilling

sidewall profiles Finer nozzle pitch; better finish Multi-level machining

reservoirs Ink jet nozzles with reservoir [Exitech Ltd, N Rizvi et al, ca

channels and/or metal electrodes and/or functional layers Laser can

4 Laser Machining for MEMS/Microsystems Inkjet printer nozzle drilling Laser processing allows: Tighter control over and greater flexibility in sidewall profiles Finer nozzle pitch; better finish Multi-level machining for e.g. reservoirs Ink jet nozzles with reservoir [Exitech Ltd, N Rizvi et al, ca 2000] Lab-on-chip technology Typically glass/polymer substrate with channels and/or metal electrodes and/or functional layers Laser can process all layers Dielectrophoretic conveyer [Univ. Wales / Genera Technol. Ltd / Exitech Ltd, R Pethig et al, 1998] AILU Workshop, 22 September

Laser Machining for MEMS/Microsystems RF MEMS Dielectric substrates

for via drilling and singulation MEMS sensors/actuators Patterning

devices/wafers with extreme topography RF switch arrays on glass for

Quartz MEMS accelerometers with laser machined gold metallisation

5 Laser Machining for MEMS/Microsystems RF MEMS Dielectric substrates offer low loss, but are difficult to machine Laser attractive option for via drilling and singulation MEMS sensors/actuators Patterning of tough (to process) materials Patterning of thin films on devices/wafers with extreme topography RF switch arrays on glass for adaptive antennas [Thales TRT/Exitech Ltd, T Dean et al, 2004] Quartz MEMS accelerometers with laser machined gold metallisation [Thales Avionics/Exitech Ltd, O Lefort et al, 2004] AILU Workshop, 22 September

6 Laser Machining Tools Excimer beam path (projection ablation) DPSS beam path (direct-write) Beam expander Iris Scanner (2-axis) Exitech M8000 System at Imperial College Attenuator Shutter Raw s.s. laser beam Objective Workpiece AILU Workshop, 22 September

3D Micromachining in Projection Mode Indexed Mask Projection (IMP) Static

multilevel structure or (in limit of many short exposures)

figure MEMS axial-flow turbine & generator for self-powered air-speed

7 3D Micromachining in Projection Mode Indexed Mask Projection (IMP) Static sample and beam; sequence of exposures with different masks stepped, multilevel structure or (in limit of many short exposures) quasicontinuous relief surface NB: projection optics omitted to simplify figure MEMS axial-flow turbine & generator for self-powered air-speed sensor, with laser machined SU8 polymer rotor (KrF excimer) AILU Workshop, 22 September

3D Micromachining in Projection Mode Half-tone Machining Pixellated binary mask with pixels sized below

(wh/p 2 ) 2 h p Attractive for small-area applications because less costly in terms of mask real estate

(debris issues) Polycarbonate micro-optical parts formed by half-tone ablation (KrF excimer) Potential MEMS

8 3D Micromachining in Projection Mode Half-tone Machining Pixellated binary mask with pixels sized below resolution of projection optics w Mask behaves as (spatially) variable attenuator with local transmission: T = (wh/p 2 ) 2 h p Attractive for small-area applications because less costly in terms of mask real estate Ultimately less efficient than IMP because fluence levels very close to threshold are unusable in practice (debris issues) Polycarbonate micro-optical parts formed by half-tone ablation (KrF excimer) Potential MEMS applications in microfluidics & microchemistry e.g. channels, mixers, filters and micro-optics AILU Workshop, 22 September

exposure site sees all apertures in turn Different apertures illuminated by different parts of beam SIS microlens array [Exitech

9 Large Area Machining Synchronised Image Scanning (SIS) Similar to IMP but with indexed workpiece; suitable for large area arrays Letter box beam illuminates array of mask apertures on fixed pitch s; workpiece translated by distance s after every pulse Each exposure site sees all apertures in turn Different apertures illuminated by different parts of beam SIS microlens array [Exitech Ltd] - Reduces projection lens telecentricity errors - Reduces distortions caused by non-uniform illumination AILU Workshop, 22 September

array formed in polycarbonate by HT-SIS 50 μm Central region of each microlens has line roughness Ra <10 nm Similar results

10 SIS with Half-tone SIS with normal binary masks leaves unwanted stepping in the final surface This can be virtually eliminated by incorporating half-tone into the SIS mask array, giving surfaces with exceptionally low roughness: Microlens array formed in polycarbonate by HT-SIS 50 μm Central region of each microlens has line roughness Ra <10 nm Similar results can be achieved by laser polishing after binary mask SIS, but this requires a second process step AILU Workshop, 22 September

requirement for iterative process that includes prototyping Useful for structures that are deep (compared to

11 Mask Design Tools Typical laser microfabrication route using Excimer laser: Simulation CAD/CAM Mask design Mask projection - static Mask projection -dynamic Parts Iteration Simulation can assist in mask design, reducing requirement for iterative process that includes prototyping Useful for structures that are deep (compared to focal depth) and/or contain steeply inclined surfaces e.g. turbine rotor shown earlier AILU Workshop, 22 September

Workpiece can be ignored when calculating fluence distribution Etch depth per pulse ( μm) 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.

12 Simulation of Multi-pulse Ablation Mask pattern Fluence calculation Surface propagation n done? y Machined surface Laser params Optics params Modelling assumptions: Ablation events due to successive pulses do not overlap in time Etching characteristics of material remain stable over time Workpiece can be ignored when calculating fluence distribution Etch depth per pulse ( μm) Ablation curve Angular dependence Experimental data (halftone mask) Beer's law fit Polynomial fit (5th order) Fluence (mj/sq.cm) # pulses Typical material input data (for SU8) Experimental data Angle dep. model (after P.E. Dyer) In spite of gross assumptions, this approach can make good predictions even for surfaces with extreme topography AILU Workshop, 22 September

Prediction of sidewall taper/undercut Early

13 Prediction of sidewall taper/undercut Early 193 nm results for laminated dry film photoresist: F = 0.25 J/cm 2, N = 200 F = 0.25 J/cm 2, N = 400 F = 0.25 J/cm 2, N = 600 F = 0.15 J/cm 2, N = 600 F = 0.25 J/cm 2, N = 600 F = 0.50 J/cm 2, N = 600 AILU Workshop, 22 September

significantly improves simulation accuracy for deeper structures, with good predictions even in regions with extreme gradient Improved

14 Simulation of deep Half-tone Ablation Plots compare experimental profiles with simulations using (a) angular dependence model normally assumed in literature (dashed line) and (b) empirical angle-dependent etch function (solid line) Use of empirical angular dependence significantly improves simulation accuracy for deeper structures, with good predictions even in regions with extreme gradient Improved angular dependence model, recently proposed by P.E. Dyer (and pending publication), will reduce the need for empirical data AILU Workshop, 22 September

al, 2000] Direct-write 2-level spiral inductor on polyimide A similar approach may be used for assembly of MEMS devices from parts fabricated on

15 Laser Transfer Processes Processes such as LIFT (Laser Induced Forward Transfer) and MAPLE (Matrix Assisted Pulsed Laser Evaporation) are well-established methods for selective transfer of materials from one wafer to another: MAPLE-DW process [Potomac Photonics Inc / NRL, S A Mathews et al, 2000] Direct-write 2-level spiral inductor on polyimide A similar approach may be used for assembly of MEMS devices from parts fabricated on separate wafers. This can be useful where a fully monolithic approach is difficult or impossible due to yield or process compatibility issues AILU Workshop, 22 September

16 Laser Transfer of Parts Release layer Pulsed UV radiation Pulsed UV radiation Carrier Target Confined ablation of release layer through (UV-transparent) carrier Expanding ablation products impart momentum to released part Several applications to date - Assembly of hybrid MEMS first demonstration (Imperial College, 1998) - Transfer of metal/solder bumps for flip-chip assembly (Imperial College/Celestica) - Release of IC dies from blue tape (Philips/Univ. Twente) - Assembly of electronic circuits (NRL/Princeton Univ./ Catholic Univ. of America) AILU Workshop, 22 September

UV-LIGA (electroforming photoresist molds)

17 Assembly of Hybrid MEMS First demonstration of MEMS assembly by laser-driven transfer was for electrostatic micromotors: Parts made by UV-LIGA (electroforming photoresist molds) 12 μm-thick, 1 mm-dia rotor on polyimide release layer 4-level stator Alignment Laser-driven release O 2 plasma cleaning 1 mm AILU Workshop, 22 September

18 Recent Applications in RF MEMS RF MEMS devices such as switches and varactors can benefit from incorporation of ferroelectric materials with high and/or tuneable dielectric constant Such materials are typically grown and/or annealed at high temperatures that are incompatible with other MEMS processing e.g. above 500 C One solution is to transfer ferroelectric film from growth substrate to device substrate by laser transfer. Film is bonded to device substrate and then released from growth substrate: Patterned metal dep. FE film e.g. sol-gel PZT Metal seed layer Release layer UV-transparent growth substrate Bonding e.g. thermosonic Plasma/laser etch Metal for bonding and structural support of film Laser release Stripping of release and seed layers AILU Workshop, 22 September

Laser Transfer of PZT First Results TS bonding + laser release Ni/Au Silica PZT (0.

transferred is removed by front side ablation PZT can be transferred by single KrF laser pulse in fluence range

$(in contrast, films transferred without bond metal tend to fracture) Delamination can occur at the PZT/Ti interface$

19 Laser Transfer of PZT First Results TS bonding + laser release Ni/Au Silica PZT (0.5 μm) Ti/Pt ITO Silicon Transferred PZT is damagefree except near edges PZT/Ti/Pt/ITO surrounding pad to be transferred is removed by front side ablation PZT can be transferred by single KrF laser pulse in fluence range mj/cm 2 Bond metal provides structural support during laser transfer and prevents mechanical damage to film (in contrast, films transferred without bond metal tend to fracture) Delamination can occur at the PZT/Ti interface rather than silica/ito; unexpected but not necessarily undesirable (if reproducible) This work is a collaboration with Cranfield University AILU Workshop, 22 September

scheme for integrating ferroelectric layers by laser transfer: 50 0 0 10 20 30 40 50

20 Laser transfer of PZT Target Device Currently fabricating zipping varactors with sputtered silica dielectric: Capacitance C [ff] Possible scheme for integrating ferroelectric layers by laser transfer: Bias Voltage [V] TS bond & Laser release TS bond & Laser release AILU Workshop, 22 September

21 Summary Many potential applications for laser processing in MEMS manufacture, of which only two discussed here: - 3D machining in projection mode - Laser transfer of parts/layers Several approaches available for 3D structuring, all of which can achieve good profile accuracy with correct mask & process design Half-tone masking attractive for small-area applications because less costly in terms of mask area; other methods preferred for large areas Laser transfer can overcome yield and process incompatibility issues that may preclude monolithic processing. Currently we are developing this approach for integration of functional materials into RF MEMS devices AILU Workshop, 22 September

22 Acknowledgements This work was funded by the UK Engineering & Physical Sciences Research Council (EPSRC) Laser micromachining results were obtained using laser facilities kindly provided by Exitech Ltd / Oerlikon Optics UK Ltd James Pedder (now with M-Solv Ltd) carried out most of the laser machining work as part of an Industrial CASE Studentship with Imperial College and Exitech Ltd Bob Hergert conducted the recent PZT laser transfer trials using material provided by Cranfield University AILU Workshop, 22 September

Pulsed Laser Ablation of Polymers for Display Applications

Pulsed Laser Ablation of Polymers for Display Applications James E.A Pedder 1, Andrew S. Holmes 2, Heather J. Booth 1 1 Oerlikon Optics UK Ltd, Oxford Industrial Estate, Yarnton, Oxford, OX5 1QU, UK 2