Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into... – Diffusing a dopant
Reexamination Certificate
2001-05-02
2003-08-12
Lebentritt, Michael S. (Department: 2824)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
Diffusing a dopant
C438S149000, C438S167000, C438S305000, C438S315000, C438S319000, C438S571000, C438S597000, C438S622000, C438S623000, C438S670000, C438S692000, C438S696000, C438S624000, C438S951000
Reexamination Certificate
active
06605519
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for thin film lift-off of metals using a laterally extended or “T” shaped etching (or etch) mask. In particular, a controllable lateral extension can be formed on the etching mask after an etching process has taken place. The etching mask with the lateral extension can be used for the deposition of a non-contiguous thin film of metal (or other material) on a substrate. The method of the present invention can be part of a process step during the fabrication of electronic devices such as integrated circuits.
2. Related Art
Integrated circuits, such as transistors (field effect transistors (FETs), high electron mobility transistors (HEMTs), metal-semiconductor field effect transistors (MESFETs), heterostructure bipolar transistors (HBTs), etc.), diodes, lasers and other circuits using patterned, aligned, and/or non-contiguous metals, can be formed on substrates by a variety of conventional processes.
For example, in a conventional subtractive etching process, a complete or continuous layer of metal is deposited on a substrate. The layer of metal then receives a coating of photoresist that is patterned lithographically using exposure and development techniques. The patterned photoresist serves as a protective material during the metal etching step where the unprotected metal is removed. The undesired metal is removed or etched away by conventional wet chemical etching processes, ion milling, and/or dry etching processes.
Another conventional fabrication method involves creating patterned thin metal layers on a substrate by utilizing overhanging structures. The overhanging structures define areas intended to remain free of metal during the metal deposition step. These protective overhanging structures receive a coating of metal that can be lifted off with metal remaining only in the areas unprotected by the overhanging structure.
For example, in one conventional method, shown in
FIGS. 1A-1E
, a patterned photoresist and dielectric can be utilized. In
FIG. 1A
, a photoresist
2
is deposited on a dielectric layer
8
, which is deposited on a metal
10
, which is deposited on a substrate
12
. In
FIG. 1B
, the photoresist is exposed to radiation through a conventional photo-mask, exposing regions
4
and
6
. Following development or removal of the exposed regions, the patterned structure
16
remains in
FIG. 1C. A
selective etchant is then used to remove the dielectric layer
8
in the uncovered regions. The dielectric can be “over” etched to reduce the dimensions of the dielectric such that the lateral dimension is less than the patterned photoresist
16
(i.e. the dimension x is less than the dimension x′). Another selective etchant can then be used to remove portions of the metal layer
10
where the dielectric material acts as an etching mask.
The above process creates a metal area
20
that can be used as part of a device, e.g., as a gate in a transistor structure. After this etch, a second metal deposition can be performed over the entire structure (see layers
22
,
24
,
26
) as shown in FIG.
1
F. Due to the presence of the overhanging structure
16
, metals (
22
and
24
) are deposited on either side of layer
20
and are spaced away from
20
, creating non-contiguous metal layers. The photoresist and dielectric can then be removed and the metal
26
atop the photoresist is lifted off. The final arrangement of metals are shown in FIG.
1
G.
There are several problems with the aforementioned conventional process. The lateral dimension of the overhang is difficult to control to the degree required by conventional semiconductor manufacturing standards. The problem is compounded since the dielectric area under the patterned photoresist is not the same as the initial photo-mask dimensions. Thus, for example, the width of the line (x) shown in
FIG. 1C
in the unexposed area is not the equal to the width of the line (x′) defined by structure
18
in FIG.
1
D. The width of the metal line
20
is the result of an overetch that is typically difficult to control. This discrepancy is problematic since the dimensions of metal lines are critical in semiconductor fabrication technology.
In addition, there are various device fabrication designs that require portions of the substrate to be etched prior to the deposition of metal on either side of a patterned structure. The use of conventional etching processes to etch away these areas of the substrate can be problematic. For example, many chemical solutions, primarily acids and bases, that are used to etch semiconductor materials, remove the material preferentially along crystallographic directions. This lack of symmetry in the etch rate and profile in different directions in the etched material is undesirable. Most of the substrate materials used in the semiconductor industry are single crystal materials, which often preferentially etch at different rates along different crystal planes (e.g. silicon, gallium arsenide, indium phosphide, cadmium telluride, indium antimonide, plus all related combinations of group II and VI elements, and combinations of group III and V elements).
Two such example structures are shown in
FIGS. 2A and 2B
. These figures show overhanging photoresist layers
30
and
32
, disposed on etched substrates
31
and
33
, respectively. These figures illustrate the resulting features
34
and
35
with sloped substrate sidewalls. These sloped substrate sidewall profiles can be unsatisfactory when uniform patterning is desired on all features in all orientations. Similar unsatisfactory crystallographic profiles are often generated during dry etching when the process is primarily chemical in nature. The crystallographic nature of etching can lead to undesirable undercut that can limit the minimum feature sizes that can be created.
Non-crystallographic wet and dry chemical processes are also available for the material removal or etching step. In this case, material may be removed leaving a curved undercut profile such as that shown in FIG.
3
. The degree of undercut is difficult to control as there is often a difference between the vertical and horizontal etching rates. A common result is a small “foot” or extended region at the base of the etched structure. The substrate in semiconductor devices is frequently comprised of multiple layers of epitaxial material, some of which are etched during device fabrication. At this “foot”, epitaxial layers are exposed and there is a strong possibility of inducing a electrical short between layers when metal is deposited in these regions.
For example, as shown in
FIG. 3
, a semiconductor material can include vertically stacked epitaxial layers
41
,
42
, and
43
deposited on a substrate
40
. During isotropic etching of layer
43
(which can be performed to create an emitter region in a HBT type device), recessed or undercut side walls
44
and
45
are formed with an extending “foot” at points
48
and
49
. During deposition of contacts
46
and
47
, the possibility of a short (e.g., between base contacts
46
and
47
and emitter
43
) is very high. An example of such a conventional method of fabricating a heterostructure bipolar transistor using isotropic etching is described in U.S. Pat. No. 5,804,487. Thus, an isotropic dry etch capable of producing an undercut typically forms the described sidewall profile that resembles a scallop or half circle. This type of profile is sufficient to satisfy the requirements for metal lift-off but is often inadequate for separating the deposited metal from the active portion of the etched substrate or device.
SUMMARY OF THE INVENTION
Thus, what is needed is a more controllable method to create an overhanging structure in order to form non-contiguous thin metals on a substrate, where the spacing of contacts can be controlled and the likelihood of electrical shorts can be reduced or eliminated.
In view of the foregoing, it would be desirable to provide a method for thin film lift-off of metals using a laterally extended e
Foley & Lardner
Lebentritt Michael S.
Luhrs Michael K.
Unaxis USA Inc.
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