Page 394 - Sami Franssila Introduction to Microfabrication

39

Microfabrication at Large

Integration of different technologies is a mega trend photoactive siloxanes that can be patterned like resists
all over microfabrication. Analog–digital (mixed sig- but with oxide-like properties, or iridium and ruthenium
nal) ICs integrate resistors and capacitors with digital with good interface properties with high-k dielectric
MOS or bipolar transistors; BiCMOS integrates bipo- BaSrTiO 3 .
lars and CMOS; and microprocessors integrate more and
more SRAM memory (which, in fact, takes up most 39.1.1 Silicon carbide and diamond
of the silicon area in processors). MEMS, microelec-
tromechanical systems, integrate mechanical and elec- Both silicon carbide (SiC) and diamond are wide
trical functions. Microsensors for mechanical, optical, bandgap semiconductors, and transistors, diodes, thyris-
chemical and magnetic quantities most often produce tors and other semiconductor devices can be made on
an electrical output signal that opens up possibilities to them. Wide bandgap equals low noise and/or high oper-
process, store and transmit those signals with microelec- ating temperature. Single-crystal SiC wafers are avail-
tronics, which may be integrated on the same chip. able in sizes of up to 2 in., with price tags of ca. $1000
Microfabricated devices have a number of benefits for a prime quality wafer. Crystalline SiC comes in
compared to classic or macroscopic devices: small size, many polytypes: 3C-SiC, 4H-SiC and 6H-SiC, which
low-cost, high speed (of electron transit time across are slightly different with respect to physical, mechani-
bipolar base, or of microreactor thermal ramp time), cal and electrical properties. Diamond wafers are avail-
low-power consumption (and low-reagent consumption able with 1 cm diameter, but most diamond devices
in chemical microsystems) and high device-packing den- fabricated so far have been processed on gemstones.
sity (of DRAM memory cells or attached DNA strands) Double heteroepitaxy of diamond shows promise: on
all relate to the exceptional possibilities offered by a sapphire starting wafer, a layer of epitaxial irid-
microfabrication. One of the special benefits of micro- ium is grown, and a single-crystal diamond is grown
fabrication is the completely different cost structure on iridium.
compared to macroworld manufacturing. Material usage In the thin-film form, SiC and diamond are deposited
is minuscule and almost any material can be used if by CVD, and the basic features of their deposition
it can be micromachined, because material price is not are not unlike oxide or polysilicon deposition. For
a limiting factor. We will next discuss some novel example, boron addition to PECVD chamber during
materials that are being introduced in microfabrica- deposition leads to p-type doped diamond. In the thin-
tion. film form, diamond costs about the same as other
thin film materials: capital cost and operating costs
are similar for (PE)CVD reactors, and methane (CH 4 )
39.1 NEW MATERIALS
source gas is even cheaper than silane (purity levels
New materials are being introduced regularly for func- strongly affect prices). However, as always with thin
tionality, ease of fabrication, better compatibility or films, the resulting materials differ from bulk materials.
just curiosity. Recent demonstrations include negative Instead of diamond, people prefer to talk about diamond-
thermal-expansion coefficient material ZrW 2 0 8 , pho- like carbon, DLC.
topatternable electrically conducting polymer (by silver Microfabrication applications for diamond/DLC and
nanoparticle inclusion) and similar magnetic material, SiC are based on their very special combination of

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