Browse Prior Art Database

Bit Switch with Equalizing Device for High Density DRAMs

IP.com Disclosure Number: IPCOM000109339D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 2 page(s) / 88K

Publishing Venue

IBM

Related People

Katayama, Y: AUTHOR [+4]

Abstract

Disclosed is a bit-switch (BSW) with an equalizing device (EQ) for high-density DRAMs. The BSW and EQ are designed separately and LOCOS isolation is used for each layout design in the conventional DRAMs. But a large imbalance in the diffusion capacitance may be caused by misalignment of polysilicon and diffusion masks, or by process deviation. For higher density DRAMs, a small layout design is also essential for these pitch-limited circuits.

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Bit Switch with Equalizing Device for High Density DRAMs

       Disclosed is a bit-switch (BSW) with an equalizing device
(EQ) for high-density DRAMs.  The BSW and EQ are designed separately
and LOCOS isolation is used for each layout design in the
conventional DRAMs.  But a large imbalance in the diffusion
capacitance may be caused by misalignment of polysilicon and
diffusion masks, or by process deviation. For higher density DRAMs, a
small layout design is also essential for these pitch-limited
circuits.

      Fig. 1 shows the layout of the invention and Fig. 2 its
schematic.  BSWs and EQs are designed next to each other, and a pair
of bit-line contacts is shared by a BSW and an EQ.  BSWs and EQs work
not only as transfer gates but also as isolators.   Because BSWs and
EQs are never turned on simultaneously, BSWs can be used as isolators
when EQs are turned on, and similarly, EQs can be used as isolators
when BSWs are turned on.  By combining EQs with BSWs, the size of the
layout area  is reduced to half of that in the conventional scheme.
The  diffusion capacitance of these circuits connected to the
bit-lines is also halved.  Moreover, a diffusion area is  hardly
affected by misalignment of the masks, because it is enclosed in
polysilicon.  Polysilicon isolation also has the advantage of causing
less imbalance in the diffusion capacitance due to process deviation
than LOCOS isolation.