Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Having heterojunction
Reexamination Certificate
2001-05-08
2004-06-08
Lebentritt, Michael S. (Department: 2824)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Having heterojunction
C438S059000, C438S170000, C438S197000, C438S235000, C438S258000, C438S309000, C438S312000, C438S313000, C438S320000, C438S326000, C438S694000
Reexamination Certificate
active
06746928
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More specifically, the invention is in the field of fabrication of various layers related to HBT transistors in semiconductor devices.
2. Related Art
A technique often employed in the fabrication process for semiconductor devices involves creating an opening in a certain region of various layers or the substrate in the semiconductor device in order to deposit or grow other materials in the opening. For example, one of the steps in constructing a silicon germanium heterojunction bipolar transistor (“SiGe HBT”) requires opening a targeted region in a layer of oxide and growing a film of SiGe in the opening to serve as the transistor base. Conventional methods for creating such an opening have proven less than satisfactory.
FIG. 1
depicts structure
10
which is used to illustrate problems associated with depositing a relatively thick film over the semiconductor substrate and opening a certain region in the film. In this example, creation of an opening is needed so that SiGe can be grown in the opening to complete fabrication of an NPN HBT transistor. Certain details and features have been left out of
FIG. 1
which are apparent to a person of ordinary skill in the art. Structure
10
includes components of a field effect transistor (“FET”), in this instance a P-channel field effect transistor (“PFET”), and a NPN HBT that are still under construction.
As seen in
FIG. 1
, structure
10
includes collector
12
for a SiGe NPN HBT. Collector
12
has top surface
14
. Buried layer
16
, which is composed of N+type material, is formed in silicon substrate
17
in a manner known in the art. Collector sinker
18
, also composed of N+type material, is fonned by diffusion of heavily concentrated dopants from the surface of collector sinker
1
8
down to buried layer
16
. Deep trench structures
22
and field oxide structures
24
,
25
,
26
, and
28
, composed of silicon dioxide (“SiO
2
”) material. are formed in a manner known in the art. Deep trench structures
22
and field oxide structures
24
,
25
,
26
, and
28
provide electrical isolation from other devices on silicon substrate
17
in a manner known in the art.
Structure
10
of
FIG. 1
also includes features and components of a CMOS device, such as a P-channel field effect transistor, or PFET, on the same wafer as the NPN HBT. Structure
10
includes N well
32
for a PFET. Structure
10
further includes source
34
and drain
35
. Structure
10
also includes gate oxide
36
and gate
38
, which in this example is polycrystalline silicon. Both gate oxide
36
and gate
38
are formed in a manner known in the art.
It is additionally seen in
FIG. 1
that thick conformal layer
42
is deposited on the semiconductor surface, and additional conformal overcoat layer
44
is deposited on thick conformal layer
42
. Conformal layer
42
can be a dielectric material known to those skilled in the art and may be, for example, SiO
2
or silicon nitride. Overcoat layer
44
can consist of, for example, polycrystalline silicon. Opening
45
with width
48
is the result of an etching process that etches thick conformal layer
42
and overcoat layer
44
selectively down to top surface
14
. Etching can be done by a method known in the art such as a hydrogen fluoride (“HF”) wet etch. Some problems with this approach are that HF is an isotropic etcher, i.e. it etches in all direections, and further a lengthy etch time is required to etch through thick conformal layer
42
. These problems cause undercutting in thick conformal layer
42
. These problems and the resulting undercutting they produce leave width
48
of opening
45
uncertain, and most often, larger than intended.
Continuing with
FIG. 1
, the uncertainty in size of width
48
of opening
45
create by the undercutting of thick conformal layer
42
means that any subsequent film deposited in opening
45
will be of indeterminate dimensions. An example would be the epitaxial growth of SiGe film
46
in opening
45
to serve as the base for an HBT. Undercuts
47
result in lack of control of critical physical dimensions in the SiGe HBT. such lack of control in turn results in undesirable electrical properties. For example, for certain applications, the base-collector parasitic capacitance can be undesirably increased. As another example, in certain applications control over precise dimensions of the SiGe HBT base can be impaired. Whenever there is imprecision in building the SiGe HBT. performance is compromised. Thus, when undercutting occurs from the etching step such that the dimensions of width
48
of opening
45
are uncertain the SiGe HBT will not perform optimally. Another problem caused by the undercutting is the difficulty in trying to implant, for example, dopants in certain regions of the deposited film that are obscured by the undercut.
Creating an opening in a relatively thin layer of material does not present the level of undercutting associated with creating an opening in a relatively thick layer. For example, when an opening is needed in a thin layer of a-dielectric film, the etch time necessary to etch through the relatively thin dielectric layer is comparatively brief, and the short etch time keeps undercutting to a minimum. Unfortunately, a different problem arises with thin layers.
FIG. 2
shows the difficulties encountered with having to create an opening in a thin conformal film. Structure
20
of
FIG. 2
is similar to structure
10
of FIG.
1
and has the same device components that make up structure
10
in FIG.
1
. In addition, thin conformal film
52
is deposited over silicon substrate
17
and its various regions such as the field oxide regions. and forms a thin conformal film. Conformal overcoat
54
is deposited on thin conformal film
52
. Opening
55
with width
58
has been etched through overcoat
54
and thin conformal film
52
, down to top surface
14
in a manner known in the art.
The etch time necessary to etch through thin conformal film
52
is relatively short, and minimal undercutting occurs. The significant reduction in undercutting means that the dimensions of width
58
of opening
55
can be controlled with greater precision, and any subsequent material deposited in opening
55
will have dimensions that are better controlled.
FIG. 2
further has SiGe film
56
grown over thin conformal layer
52
and conformal overcoat
54
. It is seen that SiGe film
56
also grows in opening
55
where it assumes width
58
. In the present example, control of the dimensions of the grown SiGe layer is critical to the performance of the HBT. Because undercutting of conformal layer
52
is minimal, the dimensions of SiGe film
56
grown in and close to opening
55
can be controlled much more precisely. When creating an opening in a relatively thin film, the uncertainty in the dimensions of subsequent materials deposited or grown in the opening created in a thick layer is avoided.
While creating an opening in thin conformal layer
52
causes minimal undercutting,
FIG. 2
shows that other problems develop with this process. Because of the relative thinness of thin conformal film
52
where it deposits on gate
38
, the subsequent depositing of conformal overcoat
54
at gate
38
produces a sharp edge or cusp at the upper coerners of gate
38
. It is seen in
FIG. 2
that the: growth of SiGe film
56
on gate
38
is non-conformal and produces a non-ideal property of deposition. This non-ideal property of deposition is also referred to as non-isotropic deposition. In the present example, the non-isotropic deposition of SiGe leads to a phenomenon called “breadloafing”. Breadloafing occurs where the non-conformal film, in this case SiGe film
56
, wraps around the sharp edges of conformal overcoat
54
at the upper corners of gate
38
, and extends beyond the upper corners of gate
38
, producing a profile resembling a slice of bread.
Continuing with
FIG. 2
, overhangs
57
characterize br
Racanelli Marco
Schuegraf Klaus F.
Farjami & Farjami LLP
Lebentritt Michael S.
Luhrs Michael K.
Newport Fab LLC
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