Methods for LDMOS and other MOS transistors with hybrid contact
A lateral DMOS transistor structure includes a substrate of a first dopant polarity, a body region of the first dopant polarity, a source region, a drift region of a second dopant polarity, a drain region, a channel region, a gate structure over the channel region, a hybrid contact implant, of the second dopant polarity, in the source region, and a respective metal contact on or within each of the source region, gate structure, and drain region. The hybrid contact implant and the metal contact together form a hybrid contact defining first, second, and third electrical junctions. The first junction is a Schottky junction formed vertically between the source metal contact and the body. The second junction is an ohmic junction formed laterally between the source metal contact and the hybrid contact implant. The third junction is a rectifying PN junction between the hybrid contact implant and the channel region.
1. A method of fabricating a power transistor structure, comprising the steps of:
(a) providing a substrate of a first dopant polarity, wherein the substrate includes a body region that is also of the first dopant polarity;
(b) forming a drift region, of a second dopant polarity, on or within the substrate;
(c) forming a gate structure by oxide growth, polysilicon deposition and polysilicon etch over a portion of the body region and a portion of the drift region;
(d) implanting a dopant of the second dopant polarity into the body region which is aligned to the gate structure, the implanted dopant defining a hybrid contact implant that has a first depth;
(e) forming a gate sidewall spacer structure such that a first portion of the hybrid contact implant is disposed beneath the gate sidewall spacer structure and a second portion of the hybrid contact implant is exposed adjacent to the gate sidewall spacer structure;
(f) diffusing a dopant of the first dopant polarity into the body region to form a body tap;
(g) applying a photoresist coating to the body region, including the body tap, but leaving the gate structure and drift region entirely or nearly entirely exposed;
(h) diffusing a dopant of the second dopant polarity into the gate structure and into the drift region to form a drain region;
(i) depositing a blanket layer of metal;
(j) annealing the metal of the blanket layer to react with exposed silicon/polysilicon on the body tap, the drain region, the gate structure and the second portion of the hybrid contact implant to form a silicide, wherein the silicide formed on the second portion of the hybrid contact implant has a second depth, and wherein the second depth is greater than the first depth; and
(k) removing unreacted portions of the blanket layer of metal, thereby resulting in metal contacts to the body tap, the drain region, the gate structure, and the hybrid contact implant;
(l) whereby the silicide, the first portion of the hybrid contact implant, and the second portion of the hybrid contact implant together form a hybrid contact, which constitutes a source region, and wherein a channel region is established between the source region and the drift region; and
(m) whereby the resulting hybrid contact defines first, second, and third electrical junctions, wherein the first electrical junction is a Schottky junction formed vertically between the silicide and the substrate, wherein the second electrical junction is an ohmic junction formed laterally between the silicide and the first portion of the hybrid contact implant, and wherein the third electrical junction is a rectifying PN junction between the first portion of the hybrid contact implant and the channel region.
2. The method of claim 1 , wherein the method further comprises a step, prior to the implanting step, of applying a first photoresist coating to the drift region and a portion of the body region, and wherein the implanting step is carried out using the first photoresist coating and a portion of the gate structure as a mask.
3. The method of claim 1 , wherein the step of applying a photoresist coating to the body region, including the body tap, prevents a dopant of the second dopant polarity from being implanted into the body region during the step of diffusing a dopant of the second dopant polarity.
4. The method of claim 1 , wherein depositing metal on the second, exposed, portion of the hybrid contact implant and annealing the metal deposited thereon causes the formation of an interfacial dopant layer between the silicide and the body region.