A THICK POLYSILICON THREE-STATE FUSE
Original Publication Date: 1983-Mar-01
Included in the Prior Art Database: 2001-Oct-09
This technique provides a factor of two increases in the amount of information that can be pro- grammed in a single link. Also the technique can be used to reduce the link resistance which in turn provides a greater delta between on and off impedance.
TM\ , MOTOROLA Technical Developments Volume 3 March 1983
A THICK POLYSILICON THREE.STATE FUSE
By Jim Doyle
This technique provides a factor of two increases in the amount of inform&ion that can be pro- grammed in a single link. Also the technique can be used to reduce the link resistance which in turn provides a greater delta between on and off impedance.
A polysilicon fuseable link can be selectively programmed in a 3.state manner. This effect exists when polysilicon of approximately 50OO"A is doped to a uniform resistivity .00250-cm @OWlsquare).
This phenomenon occurs when the link is selectively shorted with the aluminum metallization or blown open due to excessive power dissipation. This option can be determined by an external dropping resistor. The dropping resistor reduces the amount of peak power that can be applied to the link when it approaches the melting point. At the melting point, the links' resistance drops toa lowervalue, thus the current is limited by any series resistance. if the current is sufficient to melt the metal but insufficient
to melt the polysiiicon, the link shorts. The shorted resistance of this link Is then less than 1 (about an
order of magnitude greater than an equivalent length of aluminum metallization).
The thermal conductivity of silicon dioxide is not sufficient to dissipate all the energy from a link if a high current pulse is applied. If the melting point of the polysilicon link (1415°C) is reached in a short interval ( (1~ second), the link will typically open. If on the other hand, a dropping resistor is placed in series with the link, the peak energy requirements necessary to melt the link are not achieved, instead the metal at the contacts melts (645°C) and flows across the link.
By injecting a current after the links have been programmed, an output voltage can be used to determine if the link was programmed to a high or low or not programmed at all. Another advantage of a
3.stage programming technique is that the 0 level has very low series resistance which makes the on/off difference greater. Normally in the offstate, some leakage exists; therefore a large on/off im- pedance difference insures a greater operating range and is less process sensitive.
The possible states which can be obtained by programming are:
I. The first state is obtained when a non-current limited potential of 15 volts is initially applied to the 3 link f...