Page 199 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

180 Chapter Sixteen

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c
UNPROGRAMMED:
c
2-Input AND
Not applicable __. 0-@ PROGRAMMED:
Figure 16-3. Adding desired antifuse links

Antifuse links are formed by creating a connection-called a alia-of
amorphous (noncrystalline) silicon between two layers of metalization. In its
unprogrammed state, the amorphous silicon is an insulator with a very high
resistance in excess of one billion ohms. The programming signal effectively
grows a link by changing the insulating amorphous silicon into conducting
polysilicon (Figure 16-4).

Polysilicon via
Amorphous (non-crystalline) I
silicon via

A 5ilicon dio
+-----Metal 4- Metal
w-- silicon substrate-

UNPROGRAMMED PROGRAMMED

Figure 16-4. Growing an antifuse link

The programmed links in the original antifuse technologies had resistance
values in the order of 1,000 ohms, but modern processes have reduced this value
to 50 ohms or lower. Devices based on antifuses are also one-time progrummable
(OTP), because once an antifuse link has been grown it cannot be removed.