Page 258 - Sami Franssila Introduction to Microfabrication

24

Process Integration

Process integration is the task of putting together individ- Design rules:
ual process steps to create functional devices. This neces-
sitates interfacing device design and processing, knowl- • What is the minimum width allowed for lines?
edge of process capability and device operation, under- • How closely can you place structures?
standing materials interactions and being prepared for • How much area should be allowed for misalign-
equipment limitations – all aspects of microfabrication. ment tolerances?
Process integration is about questions such as
the following: Mask considerations:

Wafer selection: • Which photomasks are critical, which are non-
critical?
• Should n-type or p-type wafers be used? • Does etch undercutting need to be compensated on
• Can epitaxial or SOI wafers contribute to device the mask?
performance? • How much area should be reserved for test chips and
• Are mechanical wafer specifications important, or how much for device chips?
electrical, or both?
Order of process steps:
Materials compatibility:
• Does the stress relief anneal affect structures already
fabricated?
• Are the interfaces stable at process temperatures?
• Will the thermal expansion coefficient mismatches • Can any steps be done after thin membrane formation?
create stresses? • Should front-side processing be completed before
• Do the metals withstand the wet cleaning solutions? backside processing?
Reliability:
Process-device interactions:
• Do current densities in wiring need to be limited?
• How do thermal treatments add to diffusion profiles? • How do stresses build up when more layers are
• Is etch profile critical? deposited?
• How does lithographic linewidth variation affect
• What is the breakdown voltage of thin oxides?
device performance?
Equipment and process capability: 24.1 PROCESS INTEGRATION ASPECTS
OF A SOLAR-CELL PROCESS
• How much of the underlayer is lost during overetching?
• What is the step coverage of sputtered films in The simple solar-cell process described in Figure 24.1
contact holes? features some important interactions between pro-
• Can thick stacks of bonded wafers be inserted cess steps that arise when complete processes are
into tools? put together.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)