Anneal-free process for forming weak collector

Details Anneal-free process for forming weak collector Anneal-free process for forming weak collector

2000-05-05

2001-06-05

Elms, Richard (Department: 2824)

Semiconductor device manufacturing: process

Making regenerative-type switching device

Having field effect structure

C438S268000, C438S270000, C438S273000, C438S527000, C438S529000, C438S589000

Reexamination Certificate

active

06242288

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to Insulated Gate Bipolar Transistors (IGBTs) and more specifically relates to an IGBT having a transparent collector formed by a shallow implant in which an anneal step to activate the implant is avoided.
BACKGROUND OF THE INVENTION
IGBTs employing a weak collector are well known. Such devices, using a non-punch through technology, and using ultra-thin float zone wafers rather than more expensive wafers with an epitaxially formed silicon layer for device junctions and buffer zones for example, are described in a paper 0-7803-3106-0/96; 1996 I.E.E.E., entitled NPT-IGBT-Optimizing for Manufacturability, in the names of Darryl Burns et al.
As described in that paper, high voltage nonpunch through IGBTs (NPT-IGBTs) offer reasonable on state voltages, high short-circuit ruggedness, and minimal turn-off losses without heavy metal or E-beam lifetime killing. In addition, they have reduced cost as compared to the more conventional epitaxial IGBT because they are fabricated on low-cost bulk (float zone) silicon substrates and do not use thick, expensive epitaxial layers. The final thickness of float zone wafers for non-punch through IGBTs ranges from 80 microns for 600 volt devices to 250 microns for 1700 volt devices. Even thinner wafers are needed for even lower breakdown voltage. Such wafers are fragile and subject to breakage during processing. Typically, the wafer will be about 80 microns thick for a 600 volt breakdown and 185 microns thick for a 1200 volt breakdown.
The known NPT-IGBT uses a simple, shallow low concentration backside P type implant (a “weak” or “transparent” collector) to form an emitter with low efficiency, thereby providing fast turn-off time. (In contrast, the conventional epitaxial IGBT uses an N
+
epitaxial buffer layer and lifetime killing to obtain fast turn-off time.) A collector contact including a first aluminum layer is then sintered into the bottom of the silicon wafer. A post metal deposition anneal is then needed to enable interaction (activation) between the aluminum and the backside silicon.
This subsequent anneal after metallizing to activate the junction requires repeated handling of the ultra-thin wafers, and wafer breakage can occur during these process steps despite great care in the wafer handling.
It is desirable to reduce the number of times the wafer must be handled during its processing to reduce the loss of wafers during the manufacturing process.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the invention, it has been found that the anneal step following the formation of the sputtered back contact can be eliminated if, prior to the formation of the back contact, the wafer is heated to 300° C. to 400° C. for 30 to 60 seconds in vacuum. It is believed that this preheat step drives absorbed water molecules and other volatile impurities from the silicon back surface, enabling and enhancing the interaction between the metal and the P type ion implanted silicon back surface. This heating step can take place in the back metal sputtering “tool” or apparatus and is followed by the sputtering of one or more contact metals, for example, sequentially applied layers of pure aluminum, titanium, nickel-vanadium and silver (Al, Ti, NiV, Ag,). The preheat makes the subsequent prior art anneal step unnecessary to create the anode junction and reduces the exposure of the fragile wafer to breakage during the anneal step.


REFERENCES:
patent: 4505946 (1985-03-01), Suzuki et al.
patent: 5136349 (1992-08-01), Yilmaz et al.
patent: 5728607 (1998-03-01), Shibib
patent: 5814546 (1998-09-01), Hermansson
patent: 6043126 (2000-03-01), Kinzer

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