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Processes to Reduce and Control the P Type Doping Concentration at the Boundary Between Recesed Oxide and Active Device Regions

IP.com Disclosure Number: IPCOM000045821D
Original Publication Date: 1983-Apr-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 6 page(s) / 65K

Publishing Venue

IBM

Related People

De La Moneda, FH.: AUTHOR

Abstract

In the conventional fabrication of recessed thick silicon dioxide, commonly used to isolate MOSFETs and N/+/ diffusions, the boron concentration thereunder is increased by means of ion implantation of boron. There results high threshold voltage for the recessed silicon dioxide without having to grow it extremely thick. After the implantation, several heat cycles grow thick and thin silicon dioxide. Consequently, the implanted boron under the thick silicon dioxide spreads into non recessed silicon dioxide regions wherein MOSFET's and N diffusions are formed. This situation is illustrated by the cross-section of Fig. 1 along the width direction of a MOSFET and an N/+/ diffusion.

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Processes to Reduce and Control the P Type Doping Concentration at the Boundary Between Recesed Oxide and Active Device Regions

In the conventional fabrication of recessed thick silicon dioxide, commonly used to isolate MOSFETs and N/+/ diffusions, the boron concentration thereunder is increased by means of ion implantation of boron. There results high threshold voltage for the recessed silicon dioxide without having to grow it extremely thick. After the implantation, several heat cycles grow thick and thin silicon dioxide. Consequently, the implanted boron under the thick silicon dioxide spreads into non recessed silicon dioxide regions wherein MOSFET's and N diffusions are formed. This situation is illustrated by the cross-section of Fig. 1 along the width direction of a MOSFET and an N/+/ diffusion.

In Fig. 1, it is noticed that the P-type doping concentration surrounding the perimeter of the N/+/ diffusion has been increased by the spread of the boron implanted under the recessed silicon dioxide isolation (ROI). This leads to increased perimeter junction capacitance (Cp), by a factor of 2 or 3 and lowered junction breakdown voltage, threshold voltage then goes up with narrowing channel width as the extent of the boron penetration becomes comparable with it. Over a distribution of MOSFET devices, this effect increases the threshold voltage sigma since boron spread does not track channel width. This effect is distinct from the well-known rise in threshold voltage when the total depleted bulk charge adjacent to the channel edges is large relative to the bulk charge under the channel. This second width efffect on threshold voltage is probably intensified by increased boron doping around the channel edges since the adjacent depleted bulk charge is proportional to the (boron doping)(see original).K

In addition to these adverse electrical effects, conventional growth of recessed silicon dioxide also produces the "bird's beak" structure which sets the minimum width of the isolating field regions in a VLSI chip at a higher level than required by electrical or photo graphic considerations. Such minimum width can be reduced by growing less thick silicon dioxide. However, if the thick silicon doxide threshold voltage, is to remain unchanged, the circuits have to be designed to absorb higher Cp, lower BV, and higher active device threshold voltage all resulting from having to increase boron dosage to compensate for the less thick field silicon dioxide.

In what follows, four fabrication sequences are described to offset the field boron implant away from the perimeter of the active device regions so that the boron concentration at the common boundary of field and active regions can be independently set at a level different from that inside the field regions. The first process sequence yields a semi-recessed field oxide structure, while the others yield fully recessed field oxide structures. Measures are taken in all processes to prevent the...