Page 400 - Sami Franssila Introduction to Microfabrication

Microfabrication at Large 379

sensors, of course). Large square substrates mean that billion devices per year level, whereas even the most
the FPD industry cannot rely on CMOS for its tools, successful MEMS devices sell in tens of million pieces,
unlike most other microfabrication industries. and more complex ones considerably less, and annual
Solar cells are large-area devices like FPDs, but their volumes in the 10 000 range are common (corresponding
cost models are completely different: solar cells are the to 1% of monthly production of an IC megafab).
ultimate low-cost devices. Cost reduction starts at start- Microfluidic/BioMEMS devices have potentially large
ing wafer cost: one dollar would be typical for solar markets if they can be made cheap enough for disposable
grade silicon, an order of magnitude less than for IC applications such as point-of-care measurements in
grade wafer. Linewidths are relaxed, in the 10 to 100 µm health monitoring where $10 might be a reasonable
range. Microfabrication technologies are used for perfor- price, which translates to the cost of ca. $1 for the
mance, like PECVD nitride antireflective coatings, but silicon part.
traditional techniques such as screen-printing of conduc- Microsystems and nanotechnology are still in a
tive pastes are used for cost reduction. nascent state, and there are many contenders for
The microsystems industry is very fragmented com- main devices and device classes. Some of them will
pared to ICs or FPDs. Technologies differ: polysili- reach IC-like volumes and markets, some will remain
con surface micromachining, bulk silicon, DRIE single- niche applications, and most will never enter the
crystal silicon (bulk and SOI), thick poly surface micro- manufacturing stage. However, that is how evolution in
machining, LIGA and polymer imprint structures share technology imitates natural evolution: the more variation
the basic principles of microfabrication but differ in and experiments you conduct, the more likely it is that
many critical parts. Micropatterning, thin films and etch- some viable applications and technologies will emerge
ing are core concepts in all microsystems. Polysili- and will reproduce into many future generations.
con micromachining applies many of the features of
IC fabrication, such as reduction steppers for lithog-
raphy and plasma-etching for pattern delineation, and 39.7 EXERCISES
the number of photolithographic steps is quite simi-
lar to ICs: 10 masks is usual for polysilicon surface 1. What is the kg price of a CMOS wafer at the end of
micromechanics, whereas most other microsystems are the fabrication process?
made with four to six masks, and sometimes a sin- 2. What is the kg price (or carat price) of a thin-film
gle patterning step is enough, as for simple fluidic diamond if the PECVD capital cost is $500 000 and
systems. Microsystems use 100 and 150 mm wafer the running costs are $100 000/year? Take 10 nm/min
sizes, and for bulk micromechanics, scaling to 200 mm as deposition rate on a 150 mm wafer size in a single-
is not an option because wafer-thickness increases wafer system.
wastes area in through-wafer etching. Waveguide opti- 3. The solar cell cost can be lowered by direct writing
cal microsystems are fabricated on 200 mm wafers because the mask cost is eliminated. If laser direct
because the chips are large due to large radii of cur- writing for top metallization is done at a speed of
vature. 1 m/s and metal pitch is 200 µm (see Figure 24.1),
Integration of two technologies adds to process what is the throughput of such a direct write system?
complexity: roughly speaking, a 20% mask count 4. How many metres of wiring is there on a 0.13 µm
increase leads to ca. 20% cost increase. A surface technology CMOS wafer? What would be the
micromachined airbag accelerometer, integrated with throughput if direct writing at 1 m/s was used?
BiCMOS readout electronics, has been commercialized 5. What is the density of AFM tips that could be
2
and is being manufactured in significant volumes. In fabricated on a 1 cm area by the process described
many sensor applications, extremely advanced read- in Figure 39.1?
out ICs are required. Processing will then require an 6. Design a DRIE version of the AFM tip array of
advanced CMOS fabrication line, which is often overkill Figure 39.1 and calculate the tip areal density.
for the MEMS/sensor part. 7. Kilogram is defined as the mass of platinum–iridium
RF-MEMS devices are close to ICs in many respects: cylinder that is held at BIPM (Bureau International
they are mostly planar (or at least not highly 3D) de Poids et Mesures) in Sevres, near Paris. It has
devices, they are internal to the system (unlike sensors been suggested that a new standard should be made
and actuators) and reusable blocks and hierarchical of silicon because silicon is an extremely well-
design may be amenable to RF MEMS. There is characterized material. What uncertainties can you
a potentially large market for RF MEMS, in the name for a silicon kilogram standard piece?