Ex parte IKEGAMI

Board of Patent Appeals and Interferences

Sep 4, 1998

08190388 (B.P.A.I. Sep. 4, 1998)

Application for patent filed February 2, 1994.1
1
THIS OPINION WAS NOT WRITTEN FOR PUBLICATION
The opinion in support of the decision being entered today
(1) was not written for publication in a law journal and
(2) is not binding precedent of the Board.
Paper No. 18
UNITED STATES PATENT AND TRADEMARK OFFICE
__________
BEFORE THE BOARD OF PATENT APPEALS
AND INTERFERENCES
__________
Ex parte MASAAKI IKEGAMI
__________
Appeal No. 96-1271
Application 08/190,3881
__________
ON BRIEF
__________
Before HAIRSTON, KRASS, and BARRETT, Administrative Patent
Judges.
KRASS, Administrative Patent Judge.
DECISION ON APPEAL
This is a decision on appeal from the final rejection of
claims 1, 3 through 7 and 9 through 11, all of the claims
remaining in the application.
Appeal No. 96-1271
Application No. 08/190,388
2
The invention pertains to a semiconductor device
including a bipolar transistor. More particularly, a base
contact impurity region of the same conductivity type as the
base region, but having a higher impurity concentration than
the base region, is formed. The base contact impurity region
is at a depth shallower than a depth of the emitter impurity
region and the base contact impurity region is combined with a
metal silicide layer including one metal silicide selected
from the group consisting of titanium silicide, hafnium
silicide, vanadium silicide, tantalum silicide, molybdenum
silicide and tungsten silicide. Forming the higher impurity
concentration in the base contact impurity region is said to
prevent the increase in the base contact resistance which
occurs in conventional semiconductor devices during a silicide
reaction. Since the base contact impurity region is of a
higher impurity concentration, flow of impurities out of the
base contact impurity region during titanium silicide
formation, for example, does not result in an impurity
concentration in the base contact impurity region of such a
low concentration that base contact resistance would increase.
Appeal No. 96-1271
Application No. 08/190,388
3
Representative independent claim 1 is reproduced as
follows:
1. A semiconductor device, comprising:
a collector impurity region of a first conductivity-type;
a base impurity region of a second conductivity-type
having a first impurity concentration formed in a
predetermined region on the main surface of said collector
impurity region;
an emitter impurity region of the first conductivity-type
having a first depth formed in a predetermined region on the
main surface of said based [sic, base] impurity region; and
a base contact impurity region of the second
conductivity-type having a second impurity concentration
higher than said first impurity concentration and a second
depth shallower than said first depth formed on the main
surface of said based [sic, base] impurity region spaced apart
by a predetermined distance from said emitter impurity region,
said base contact impurity region having a metal silicide
layer formed thereon, said metal silicide layer including one
metal silicide selected from the group consisting of TiSi ,x
HfSi , VSi , TaSi , MoSi , and WSi .x x x x x
The examiner relies on the following references:
Lechaton et al. 4,752,817 Jun. 21, 1988
(Lechaton)
Welch et al. 4,980,738 Dec. 25, 1990
(Welch)
Prior art figs. 28 through 31 and 34
Appeal No. 96-1271
Application No. 08/190,388
4
Claims 1, 3 through 7 and 9 through 11 stand rejected
under 35 U.S.C. § 103. As evidence of obviousness, the
examiner cites Lechaton and Welch with regard to claims 1, 3,
4, 6 and 7, adding prior art Figures 28-31 with regard to
claims 9 through 11 and adding prior art Figure 34 to the
Lechaton/Welch combination with regard to claim 5.
Reference is made to the briefs and answer for the
respective positions of appellant and the examiner.
OPINION
We reverse.
With regard to the independent claims, the examiner
applies Lechaton, specifically referring to Figure 4 therein.
The semiconductor device of Figure 4 of Lechaton is identified
by the examiner as having a collector impurity region 12, a
base impurity region 22, an emitter impurity region 30 and a
base contact impurity region (annular region) 34, all as
claimed. With this much, appellant agrees.
Appellant also agrees with the examiner that while
Lechaton discloses a platinum or palladium silicide layer 36,
Appeal No. 96-1271
Application No. 08/190,388
5
it does not teach or suggest a metal silicide selected from
the group listed in the claims.
The examiner cites Welch which teaches a titanium
silicide layer 68 used in a bipolar transistor device. The
examiner then concludes that it would have been obvious to use
the particular silicide material (i.e., titanium silicide) of
Welch in the Lechaton device “since the prior art specifically
teaches that such a silicide layer on a heavily doped external
base region results in reduced contact resistance” [final
rejection-page 2].
It is the examiner’s substitution of titanium silicide
for the platinum or palladium silicide layer in Lechaton to
which appellant vehemently objects. It is appellant’s
position that there would have been nothing to lead the
artisan to make the substitution, all evidence of record
appearing to “teach away” from such a substitution. We agree
with appellant.
As pointed out by appellant, at page 13 of the principal
brief, “titanium silicide and palladium silicide are not mere
equivalents which could be readily substituted for each
Appeal No. 96-1271
Application No. 08/190,388
VLSI Technology, S.M. Sze; McGraw Hill; 1983; Chapter2
9, pages 347-350.
6
other.” As shown in Publication 1, Table XXI, at page 95, in
Exhibit A (as well as the other publications forming part of
Exhibit A), attached to the principal brief, when forming
titanium silicide, the main diffuser is silicon whereas in
forming palladium silicide or platinum silicide, the main
diffuser is palladium or platinum. Thus, appellant has
established, by objective evidence, that palladium or platinum
silicide clearly does not exhibit the same properties as
titanium silicide. The examiner has not countered this
argument with any objective evidence to the contrary.
Accordingly, it is our view that the preponderance of the
evidence favors appellant’s position of nonobviousness of the
instant claimed subject matter.
The examiner takes the position that Lechaton suggests
the use of a silicide to provide reduced contact resistance.
Yet, the examiner fails to point to anything in Lechaton which
would lead to such a suggestion. Furthermore, as pointed out
by appellant, at page 4 of the reply brief, and identifying a
textbook reference as support thereof, contact resistance is2
Appeal No. 96-1271
Application No. 08/190,388
7
determined by more than merely the use of a silicide. Rather,
the contact resistance is shown to be proportional to a
barrier height at an interface between the metal and the
silicon. As pointed out by appellant, at pages 4-5 of the
reply brief, since the value of the barrier height varies with
the type of material employed, “the contact resistance is not
necessarily reduced by using a silicide as alleged by the
Examiner.”
Moreover, as pointed out by appellant, at page 8 of the
reply brief, and as supported by the objective evidence
provided by Publication No. (4) attached to the principal
brief in Exhibit A, the “snowplow effect” of palladium or
platinum silicide appears to stop the movement of implanted
impurities (e.g., arsenic). Therefore, since palladium and
platinum silicide layers appear to prevent the diffusion of
implanted impurities into the metal silicide layer, the
impurity concentration at the interface region is not lowered.
Thus, unlike the result achieved by the instant claimed
invention, the artisan would not have expected the
concentration of the impurity region in Lechaton to drop after
Appeal No. 96-1271
Application No. 08/190,388
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or due to the silicide reaction. It is only appellant who
teaches the use of specific metal silicides, e.g., titanium
silicide, as a layer over a base contact impurity region of
such high impurity concentration that the flow of impurities
out of the base contact impurity region during the titanium
silicide formation does not result in an impurity
concentration in the base contact impurity region of such a
low concentration that base contact resistance would increase.
While titanium silicides, for example, were well known,
contrary to the examiner’s position, we find no reason that
would have led the artisan to employ the titanium silicide
layer of Welch in place of the palladium/platinum silicide
layer of Lechaton.
We also note that the examiner’s attempt, at page 6 of
the answer, to somehow discount the specific claim limitation
of the particular group of metal silicide layer used, based on
the apparent reasoning that the instant claims are product-by-
process claims, is not well founded. Quite clearly, the
recited metal silicide layer, consisting of a certain group of
metal silicides, is a structural limitation and cannot be
Appeal No. 96-1271
Application No. 08/190,388
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ignored as a mere process step within a claim otherwise
reciting a product.
The examiner’s decision rejecting claims 1, 3 through 7
and 9 through 11 under 35 U.S.C. § 103 is reversed.
REVERSED
Kenneth W. Hairston )
Administrative Patent Judge )
)
)
)
Errol A. Krass ) BOARD OF
PATENT
Administrative Patent Judge ) APPEALS AND
) INTERFERENCES
)
)
Lee E. Barrett )
Administrative Patent Judge )
tdc
Appeal No. 96-1271
Application No. 08/190,388
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