Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Responsive to non-optical – non-electrical signal
Reexamination Certificate
1999-12-22
2001-08-07
Smith, Matthew (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Responsive to non-optical, non-electrical signal
C438S166000
Reexamination Certificate
active
06271549
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Related Art
The present invention relates to a semiconductor fabrication process for obtaining a metal silicide layer with a low resistance and uniform profile, and an apparatus therefor.
2. Description of the Invention
The self-aligned silicide process is a well-known fabrication process of MOS type field-effect transistors (MOSFET) whereby the surfaces of gate, source and drain are processed into suicides compatible to each other in a self-controlled manner, and at the same time reduction of the transverse resistance of a suicide layer (across-layer resistance) is achieved (see, for example, Japanese Patent Laid-Open No. 9-69497). This conventional process for fabricating semiconductors will be described with reference to FIGS.
1
(
a
) to
1
(
d
) which present the cross-sections of the product in order of fabrication procedures.
The first procedure is shown in FIG.
1
(
a
). An N-well
402
is formed on a silicone substrate
401
by a conventional method. In the next step, a field oxide film
403
is formed by selective oxidation. On the active region surrounded by the field oxide films
403
are allowed to grow a gate insulating film
404
made of, for example, a silicon oxide film, and a polycrystalline silicon film to serve as a gate electrode
405
in order. In a further step, the polycrystalline silicon is doped with phosphor by a conventional method so that its electric resistance may be reduced. In a still further step, the polycrystalline silicon is pattern-etched by conventional methods such as photolithography and dry-etching, to form a gate electrode
405
. In a still further step, N-type layers
406
doped with low-concentration impurities (N-type impurity layers) and P-type layers
407
doped with low-concentration impurities (p-type impurity layers) are formed. In a still further step, side-wall spacers
408
made of a silicon oxide film or silicon nitride film are formed on the lateral aspects of gate electrode
405
using known CVD and etching technologies.
The next procedure is shown in FIG.
1
(
b
). Source/drain regions consisting of N-type impurity layers
406
and source/drain regions consisting of P-type impurity layers
407
are formed by photolithography and ion injection. Through those steps, an N-type source/drain region
409
and P-type source/drain region
410
having an LDD structure are formed. In the next step, naturally formed oxide films (not shown here) formed on the surfaces of polycrystalline silicon serving as gate electrode
405
and of silicone substrate
401
are removed. In a further step, cobalt, that is, a metal having a high melting point is allowed to deposit on the substrate
401
by sputtering using a magnetron sputtering apparatus while the substrate
401
is being heated at 200-500° C. (for example, 450° C.), thereby forming a cobalt film
411
thereupon. During this operation, the cobalt film
411
reacts with the gate electrode
405
and source/drain regions
409
and
410
, to form a dicobalt monosilicide (Co
2
Si) layer
412
there.
A further procedure is shown in the left-hand side of FIG.
1
(
c
). The silicon substrate
401
is subjected to a rapid heating treatment (to be referred to as “RTA” hereinafter) which consists of heating the substrate to 500° C. or higher in nitrogen atmosphere so that the gate electrode
405
and source/drain regions
409
and
410
react with the dicobalt monosilicide layer to produce a cobalt monosilicide (CoSi) or cobalt disilicide (CoSi
2
) layer. During this operation, part of cobalt films
411
in contact with the field oxide film
403
and side-wall spacers
408
is also oxidized.
A still further procedure is shown in the left-hand side of FIG.
1
(
d
). The silicon substrate
401
is immersed into an aqueous solution of hydrochloric acid and hydrogen peroxide for selective wet etching, so that only unreacted cobalt films and cobalt films partly oxidized may be removed. In the next step, the substrate is subjected to RTA consisting of heating to a temperature higher than that of previous RTA, thereby forming a cobalt disilicide layer
414
FIG. 2
gives the flat view of a conventional sputtering apparatus. Operation of this apparatus will be described below with reference to FIG.
2
.
As shown in
FIG. 2
, the currently commonly used sputtering apparatus has a multi-chamber structure in which a number of sputtering chambers
101
to
104
, a load lock chamber
11
and a separate chamber
12
are combined. The sputtering chambers
101
to
104
are film forming chambers to achieve independent deposition of different materials. The load lock chamber
11
and separate chamber
12
form together a channel for transportation (those chambers are called together as “transportation chambers” hereinafter) through which a silicone substrate can be moved in and out of any one of sputtering chambers
101
to
104
while the substrate is kept in a vacuum. The load lock chamber
11
connects the separate chamber
12
with outside. The separate chamber
12
is connected to each of sputtering chambers
101
to
104
and to load lock chamber
11
. For this reason, a silicone substrate
18
can move from one sputtering chamber to another as shorn by the interrupted line
181
of
FIG. 2
, while being safely protected by the load lock chamber
11
and separate chamber
12
against the exposure to the air.
The separate chamber
12
has a gate valve; the load lock chamber
11
has an air inlet valve; and the silicone substrate is fixed on a wafer holding/carrying arm.
Fabrication of an MOSFET transistor was undertaker using an apparatus as above. A substrate which had been given a gate electrode
405
made of polycrystalline silicon doped with boron ion at a high concentration of about 3×10
15
atoms/cm
2
had a cobalt film
411
formed thereupon with a conventional sputtering apparatus as shown in the left-hand side of FIG.
1
(
c
). Then, after the surface of cobalt film
411
was turned into a cobalt silicide layer via RTA, the across-layer resistance of gate electrode
405
was measured. The across-layer resistance there was about 10 /xx.
The next procedure is shown in the right-hand side of FIG.
1
(
c
). The silicon substrate
401
which have had N-type source/drain regions doped with arsenic ion at a high concentration of about 5×10
15
atoms/cm
2
had a cobalt film
411
formed thereupon by the conventional sputtering apparatus. The next step is shown in the right-hand side or FIG.
1
(
d
). The surface of cobalt film
411
was turned into a cobalt silicide layer via RTA; and cobalt films
411
deposited on the side-wall spacers
408
and others were removed by wet etching. Subsequent to this step, foreign matters looking like elevated silicon mass appeared over N-type source/drain regions
409
.
The foreign matter
415
may arise through the following mechanism. Firstly, while a cobalt film
411
is being formed by high temperature sputtering, dicobalt monosilicide formed on N-type source/drain regions
409
is oxidized. During this reaction, dicobalt monosilicide is split into cobalt oxides and silica or silicon dioxide, of which only cobalt oxides are removed by subsequent wet-etching. Thus remaining silica or silicon dioxide spreads and deposits over the surface of silicon substrate
401
to form elevations there which appear as a foreign matter
415
.
With the conventional semiconductor fabrication process and apparatus therefor, particularly when it is applied for depositing a metal with a high melting point on the surface of a wafer to form a film with a layer of silicide of that metal thereupon by sputtering at a high temperature, the across-layer resistance of the film becomes high. This has been a problem.
It the same process is applied to a silicon substrate which has N-type source/drain regions doped with arsenic ion at a high concentration of 5×10
15
atoms/cm
2
, foreign matters develop over the surface of silicide layer, to degrade the latter. This has been another problem.
These problems become increasingly serious as
Lee Calvin
NEC Corporation
Smith Matthew
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