This invention relates to a method for manufacturing semiconductor
devices, and more particularly to a method for manufacturing a Schottky electrode
on the surface of a GaAs (gallium arsenide) semiconductor substrate.
In a case where Schottky electrode material of Schottky diodes or
metal semiconductor field effect transistors (MESFETs) is deposited on the GaAs
substrate by a spattering method or vapor deposition method, a pre-processing
is effected to remove the natural oxide film on the surface of the semiconductor
substrate. A wet-etching process or NH&sub3; (ammonium) plasma process is effected
as the pre-processing, for example. The wet-etching process is effected by chemically
cleaning the semiconductor substrate using ammonium fluoride (NH&sub4;F), dilute
hydrofluoride acid (HF), hydrochloric acid (HC&litre;) or the like. However, in
this case, it is impossible to completely eliminate the natural oxide film on the
surface of the GaAs substrate. Further, as disclosed in papers pp. 18 to 23 (Shimada
et al.) of "27th SEMICONDUCTOR INTEGRATED CIRCUIT SYMPOSIUM", the natural oxide
film on the surface of the GaAs substrate can be completely removed by use of the
NH&sub3; plasma processing. In this case, however, if the GaAs substrate is exposed
to the atmosphere after the completion of the NH&sub3; plasma processing and before
the Schottky electrode material is deposited on the substrate, the natural oxide
film may be formed again on the surface of the substrate. As a result, a Schottky
electrode will be formed on the natural oxide film. In this way, if the natural
oxide film is formed between the Schottky electrode and the GaAs substrate, desired
Schottky junction cannot be attained. Therefore, the Schottky characteristics may
fluctuate, so that the n value (environmental factor) may be increased and/or
&phis;B (barrier height) may be decreased.
As described above, the conventional method has a problem that since
the surface of the substrate is exposed to the atmosphere after the natural oxide
film on the GaAs substrate is removed by the plasma processing and before the Schottky
electrode is formed on the substrate, desired Schottky junction cannot be obtained.
An object of this invention is to provide a method for manufacturing
semiconductor devices in which a Schottky electrode can be formed without forming
a natural oxide film between the Schottky electrode and the GaAs substrate, thus
attaining desired Schottky junction.
The above object can be attained by a method for manufacturing a
semiconductor device with a Schottky electrode, comprising the steps of (a) subjecting
the surface of a GaAs substrate to a spattering etching process in a spattering
processing chamber of a spattering device, and (b) depositing Schottky electrode
material by spattering on the surface of the substrate to form a Schottky electrode
in the same processing chamber without exposing the substrate to an atmosphere.
In this invention, since the spattering etching step and the spattering
deposition step are successively effected in the vaccum chamber, the Schottky electrode
can be formed directly on the surface of the GaAs substrate without forming the
natural oxide film thereon, and a semiconductor device such as a MESFET and Schottky
diode having desired Schottky junction can be obtained.
This invention can be more fully understood from the following detailed
description when taken in conjunction with the accompanying drawings, in which:
- Fig. 1 is a diagram schematically showing an example of a spattering device
which is used to effect a semiconductor device manufacturing method according
to one embodiment of this invention; and
- Figs. 2A to 2E are cross sectional views showing the manufacturing steps of
the semiconductor device according to one embodiment of this invention.
There will now be described a semiconductor device manufacturing
method according to one embodiment of this invention.
Fig. 1 schematically shows the construction of an example of a spattering
device which is used to effect a semiconductor device manufacturing method according
to one embodiment of this invention. The spattering device includes spattering
processing chamber 1 which is kept in a predetermined vaccum condition by a vaccum
pump (not shown). In processing chamber 1, lower electrode 2 and upper electrode
3 are disposed in opposition to and in parallel with each other, and shutter 4
which can be used to shield upper electrode 3 from the outer atmosphere is movably
mounted below upper electrode 3. GaAs substrate 10 to be spattered, as described
later, is held on lower electrode 2 by means of a holder (not shown), and target
5 formed of, for example, WSi (tungsten silicide) is held on the lower surface
of upper electrode 3. Lower electrode 2 is formed with a hollow portion to receive
cooling water from processing chamber 1, and gas inlet port 6 is provided on processing
chamber 1. A gas supply source (not shown) is arranged outside processing chamber
1. An inert gas such as Ar can be used therefor. Further, switch 9 which selectively
supplies high frequency power (for example, 13.56 MHz) from high frequency power
source 7 to lower electrode 2 or upper electrode 3 via impedance matching box
8 is provided outside processing chamber 1.
Next, an example of the method for manufacturing the semiconductor
device is explained with reference to Figs. 2A to 2E. First, as shown in Fig. 2A,
silicon ion (Si&spplus;) is ion-implanted into GaAs substrate 11 having main plane
of the Miller indices (100) in a condition that an acceleration voltage is set
at 150 keV and dose amount is set at 2.5 × 10¹² cm&supmin;². After
this, substrate 11 is subjected to heat treatment for 15 minutes at 820°C in an
atmosphere or arsine (AsH&sub3;) so as to anneal the doped silicon ion. As a result,
n-type impurity region 12 of low impurity concentration is formed in the surface
of substrate 11. The n-type GaAs substrate thus obtained is chemically cleaned
by use of NH&sub4;F (ammonium fluoride) to remove the natural oxide film on the
surface, although insufficiently. Then, GaAs substrate 10 is set in spattering
processing chamber 1, and the pressure in processing chamber 1 is lowered to 1
× 10&supmin;&sup4; Pa. After this, Ar (argon) gas is introduced as etchant
gas into processing chamber 1 until the inner pressure reaches approx. 0.8 Pa.
Then, high frequency power of approx. 1 kW is applied to lower electrode 2 in order
to effect spattering etching process for the substrate surface of about 5 minutes.
As a result of this, the substrate surface layer is partially removed as shown
by broken lines in Fig. 2A. In this case, in order to prevent target 5 from being
contaminated, shutter 4 is previously closed.
Next, N&sub2; (nitrogen) gas is introduced into processing chamber
1 to provide a partial pressure equal to 10% of the total gas pressure in processing
chamber 1, thus creating an atmosphere of (Ar+N&sub2;). Then, high frequency power
is applied to upper electrode 3 and shutter 4 is opened so as to effect the reactive
spattering process. This operation is effected until a nitride film of tungsten
silicide or WSi-Nx film 13 is deposited to a thickness of approx. 150 nm on the
substrate surface as shown in Fig. 2B. In this case, processing chamber 1 is held
in substantially vaccum condition while a process, from the step of spattering-etching
the substrate surface to the step of spattering-depositing the substrate surface,
is effected. In this way, since the substrate surface is not exposed to the atmosphere,
the natural oxide film will not be formed between WSi-Nx film 13 and the substrate
surface, permitting WSi-Nx film 13 to be formed directly on the substrate surface.
After this, the Schottky electrode of a Schottky diode of MESFET
is formed according to the ordinary process. That is, a photoresist pattern with
a 1 mm diameter corresponding to the size of the Schottky electrode is formed
on WSi-Nx film 13 by a photolithographic process, and then WSi-Nx film 13 is patterned
by a reactive ion etching method (RIE) or chemical dry etching method (CDE) with
the photoresist pattern used as a mask as shown in Fig. 2C. Then, PSG (phosphor
silicate glass) films 14 are formed as insulation coating films to a thickness
of approx. 400 nm on both surfaces of the substrate, and a heat treatment is effected
for 10 minutes at a temperature of about 800°C in order to restore the damage caused
in the substrate spattering process. After this, in order to form an ohmic electrode,
PSG film 14 is etched out, resist pattern 15 is formed as shown in Fig. 2D, and
a two-layered ohmic electrode layer such as Ni/AuGe film 16 having a lower layer
of AuGe (gol germanium) and an upper layer of Ni (nickel), for example, is formed.
Then, resist pattern 15 is removed, and a heat treatment is effected at a temperature
of 430°C in N&sub2; gas for three minutes to form an alloy. As a result, a GaAs
Schottky diode having Schottky electrode 13 and ohmic electrode 16 is obtained
as shown in Fig. 2E.
The Schottky diode formed to have the Schottky electrode is completed
as a discrete part after being subjected to the succeeding manufacturing steps.
Further, in order to lower the resistance of that part of n-type
low impurity concentration layer 12 which lies in the substrate surface under ohmic
electrode 16, it is possible to dope impurity into the part to form an n-type
high impurity concentration region.
In the above embodiment, WSi is used as target material, but it is
not limited to WSi. For example, other refractory metal such as W, WNx, Ti can
be used, and in this case, inert gas used in the spattering deposition process
will be selected according to the type of refractory metal used.
According to the manufacturing method of the above embodiment, since
the spattering etching step and the spattering deposition step are successively
effected in the vaccum chamber, the Schottky electrode can be formed directly
on the GaAs substrate surface without forming the natural oxide film therebetween
and MESFETs and Schottky diodes having desired Schottky junction can be obtained.
The characteristics of the Schottky diode manufactured according to the above embodiment
can be improved over those of the Schottky diode manufacturing without effecting
the spattering etching process, as shown in the following table. In the following
table, the average values of the diode characteristics measured at selected five
points in the semiconductor wafer are shown.
Schottky electrode material
leak current (nA) when reverse bias (VR=-10V) is applied
In the above table, composition ratio of W and Si in Schottky electrode
material WSi-N is 1 : 0.6. In this example, there occurs no damage due to etching,
and n value (environmental factor) and &phis;B (barrier height) are
not deteriorated, and the leak current occurring when the reverse bias voltage
(VR) is applied is suppressed to a minimum.
As described above, according to the semiconductor device manufacturing
method of this invention, a semiconductor device can be manufactured in which the
Schottky electrode can be formed by deposition on the GaAs substrate without forming
the natural oxide film therebetween and thus desired Schottky characteristics
can be obtained.