Wet etch apparatus

Details Wet etch apparatus Wet etch apparatus

1999-01-26

2001-06-12

Mills, Gregory (Department: 1763)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

With microwave gas energizing means

C216S083000, C438S745000

Reexamination Certificate

active

06245191

ABSTRACT:

BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to methods of etching by use of maskless techniques. In particular the present invention relates to methods of etching films under conditions that cause substantially vertical etch side walls into the film. More particularly, the present invention relates to methods of infusion and effusion of an etch solution droplet as it rests upon the film to be etched.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which includes active or operable portions of semiconductor devices. The term substrate assembly is intended herein to mean a substrate having one or more layers or structures formed thereon. As such, the substrate assembly may be, by way of example and not by way of limitation, a doped silicon semiconductor substrate typical of a semiconductor device.
In the microelectronics industry, films are required for such structural operations as gate oxide layers, etch barriers, polysilicon layers, and epitaxial layers.
FIG. 1
depicts the wetting of an etching solution droplet
10
on a film
20
. In the droplet, an angle known as &thgr; or the contact angle, forms between the plane of the solid surface to be wetted and a tangent to the perimeter of the liquid contacting the solid surface. In describing the forces at a solid-liquid-gas triple point interface
12
, three surface tensions must balance in a static state. The surface tension between the solid and the gas, &ggr;
sg
, is usually very small relative to the other surface tensions. In
FIG. 1
the surface tension of the solid and gas, &ggr;
sg
, is depicted as a vector
14
at solid-liquid-gas triple point interface
12
. The surface tension of the solid and liquid, &ggr;
sl
, is depicted as a vector
16
at solid-liquid-gas triple point interface
12
. The surface tension of the liquid and the gas, &ggr;
lg
, is depicted as a vector
18
that forms an angle, &thgr; with the solid surface. A force balance around solid-liquid-gas triple point interface
12
reveals that
&ggr;
sg
−&ggr;
Sl
=&ggr;
lg
cos &thgr;.  (1)
This expression can be rearranged to be solved for the contact angle &thgr; as
cos &thgr;=(&ggr;
sg
−&ggr;
sl
)/&ggr;
lg
.  (2)
FIG. 2
illustrates the interplay between surface tension of the liquid in the gas and surface tension of the solid in the liquid where the surface tension of the solid is held constant. If the surface tension of the liquid in the gas is high, an obtuse angle, &thgr;
1
is formed. If the surface tension of the solid in the liquid exactly equals the surface tension of the solid in the gas then the contact angle is a right angle, &thgr;
2
and the surface of the solid is neutral to hydrophobicity or hydrophilicity. If the surface tension of the liquid in the gas is low enough, an acute angle &thgr;
3
is formed and the surface of the solid is hydrophilic to the liquid. Equation 2 does not apply when complete wetting occurs such that &thgr;
3
is zero degrees and &ggr;
sg
>&ggr;
sl
+&ggr;
lg
, and does not apply when there is no wetting at all such that &thgr;
1
is 180 degrees and &ggr;
sl
>&ggr;
sg
+&ggr;
lg
.
During etch of a thin film in preparation for thin film measurement, there exists an etchant surface tension problem in the prior art that results in a poor step etch.
FIG. 3
illustrates an etching solution progression for an etching solution droplet
10
, positioned upon film
20
which is upon substrate
30
. The etching solution progression is depicted for droplet
10
as viewed in four sequential instances, from left to right. In the first instance depicted at the left, etching solution droplet
10
is seen at the instant etching solution droplet
10
makes contact with film
20
. Etching solution droplet
10
is characterized by a right contact angle &thgr;. Upon the commencement of etching, the chemical makeup of etching solution droplet
10
changes where chemical action of etching alters the chemical makeup of etching solution droplet
10
. As film
20
dissolves into etching solution droplet
10
, the chemical makeup of etching solution droplet
10
becomes changed by dilution of etch products of film
20
into etching solution droplet
10
.
As the chemical makeup of etching solution droplet
10
changes, one parameter that changes is the surface tension of etching solution droplet
10
. Where surface tension drops, wetting increases and contact angle &thgr; decreases. As contact angle &thgr; decreases, wetting causes the original footprint of etching solution droplet
10
to increase as seen by the progression of illustrations in FIG.
3
. As the original footprint increases, the etch also continues to penetrate film
20
.
FIG. 4
illustrates a good step etch of film
20
in which an etch between the upper surface
24
of film
20
and the exposed upper surface
26
of the substrate
30
is accomplished with minimal sloping of etched walls
22
. Although some sloping may be inevitable, it is desirable to achieve minimal sloping as illustrated in
FIG. 4
by the distance D
1
. The prior art problem of maskless diagnostic etching is illustrated in
FIG. 5
in which an etch between upper surface
24
of film
20
and exposed upper surface
26
of substrate
30
depicts substantial sloping of etched walls
22
. The distance between unetched upper surface
24
of film
20
and the last remaining portion of film
20
is illustrated by the distance D
2
. The result of a decreasing surface tension of etching solution droplet
10
is a poor step etch of film
20
that is characterized by substantial sloping of etched walls
22
in film
20
.
Any attempt to measure the thickness of film
20
is hindered by substantial sloping of etched walls
22
. Substantial sloping of etched walls
22
causes a non-discrete step between upper surface
24
of film
20
and exposed upper surface
26
of substrate
30
. Where a poor etch step has occurred in film
20
, it is difficult to measure the thickness of film
20
with existing measurement devices and techniques such as with a stylus micrometer. In contrast, a sharp etch step produced by an anisotropic etch provides a more discrete difference in height between the upper surface
24
of film
20
and exposed upper surface
26
of substrate
30
.
What is needed is an etching solution droplet that maintains its optimum surface tension qualities and thereby provides an etch that is uniform in surface tension and chemistry throughout the etch.
In connection with an etching solution that will maintain its optimum wetting qualities, what is also needed is a method of using an etching solution that will not continue its etching action once the surface to be etched is etched down to an etch stop.
SUMMARY OF THE INVENTION
The present invention is directed toward wet etch methods of anisotropically etching a substantially vertical etched step in a film. Methods of forming a substantially vertical etched step include depositing an etch solution droplet upon the surface of the film to be etched and monitoring at least one characteristic parameter of the etching solution droplet. Control of the etching solution droplet is carried out in response to observed changes to monitored parameters of the etching solution droplet. Control is carried out through infusion and effusion of the etching solution droplet in order to replenish the chemical reactants in the etching solution droplet. Control is also carried out by partially lifting or compressing the etching solution droplet. Replenishing the etching solution droplet, while keeping the droplet of a uniform size, maintains a uniform etching chemistry as the etching solution droplet would otherwise constantly change in its chemistry as it etches material from the surface being etched. A change in the etching solution droplet chemistry is due to progressive dilution of etched materials into the droplet and is also due to depletion of etch reactants from the original chemical makeup o

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