Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1997-04-23
2001-02-20
Nelms, David (Department: 2813)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S648000, C438S656000, C438S683000, C438S785000
Reexamination Certificate
active
06191021
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to low-resistance contacts on compound semiconductors.
BACKGROUND OF THE INVENTION
It is well known that low resistance ohmic contacts to GaAs are difficult to obtain due to a 0.8 eV Shottky barrier associated with the metal-GaAs interface. It is also known in the art that the metal-InGaAs interface produces a nearly zero Shottky barrier height and hence a low contact resistance. Ohmic contact may be made to GaAs by interposing an In
x
Ga
1−x
As layer, with x=0 at the GaAs interface and graded to x≈0.8 at the metal-InGaAs interface, between the GaAs layer and the metal contact. An advantage of this structure is that post-deposition alloying of the contact is not necessary.
In the fabrication of semiconductor circuits from Group III-V compounds, Au and Au-alloys are the most commonly employed metals for making electrical contact at the device level. Specifically, AuGe/Ni/Au, Ti/Pt/Au, and AuZn are a few of the metallization schemes that have been used to make contact to GaAs as well as to InGaAs. However, as demand for better device performance continues to increase, the need for a lower resistance contact scheme to InGaAs, in particular, has become apparent.
SUMMARY OF THE INVENTION
As has been stated hereinabove, the use of an interposed layer of InGaAs to form contact between a metal and GaAs is known in the art. However, the inventors hereof have found that Au-based metallization schemes, such as AuGe/Ni/Au, are susceptible to spiking. In efforts to overcome this problem, a refractory metal-based stack, such as Ti/Pt/Au has been used. This metallization, however, has been found to produce a contact that is unstable and generally high in contact resistance, apparently because the Ti reacted with the In. Indeed, another Ti-based metal, nitrided TiW, i.e. TiW sputtered in the presence of N
2
, was used in the belief that the N
2
would keep the Ti from reacting with the In in the InGaAs. This, however, suffered from poor adhesion and high compressive stress.
In further experimentation by the inventors, TiW was used as a contact on InGaAs. Surprisingly, this contact scheme has proven to have a low resistance and is stable. Surprisingly, the Ti in the TiW film appears to be sufficiently bound by the W to keep it from reacting with the In in the InGaAs layer.
In one form of the invention, a method is disclosed for forming an ohmic contact on a GaAs surface comprising the steps of depositing a layer of InGaAs over the GaAs surface, and depositing a layer of TiW on the layer of InGaAs, whereby a reliable and stable electrical contact is established. In another form of the invention, an ohmic contact to a GaAs surface is disclosed, the ohmic contact comprising a layer of InGaAs over the GaAs surface, and a layer of TiW on the layer of InGaAs. In still another form of the invention a bipolar transistor is disclosed. The transistor comprises a mesa, and the mesa comprises a first semiconductor layer, a layer of InGaAs atop the first semiconductor layer; and a layer of TiW atop the layer of InGaAs.
In addition to its advantage in contact resistance, TiW has been proven to provide superior adhesion as compared to nitrided TiW possibly because TiW exhibits less undercutting when etched with common etchants than does nitrided TiW. Also, as disclosed in co-assigned U.S. Pat. No. 5,055,908, TiW possesses the advantage of selectable stress, i.e. the stress of the TiW film (over a broad range from compressive to tensile) is found to be dependent upon sputter deposition pressure. Nitrided TiW, on the other hand, has been found to produce only a compressively stressed film. The ability to select stress in metallic films is known to be important in producing reliable contacts between dissimilar materials under conditions such as temperature cycling.
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Delaney Joseph B.
Fuller Clyde R.
Nagle Thomas E.
Berry Renee
Nelms David
Ogonowsky Brian D.
Skjerven Morrill & MacPherson LLP
Stewart Daniel P.
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