Isolation region forming methods

Details Isolation region forming methods Isolation region forming methods

1998-09-03

2002-04-16

Chaudhuri, Olik (Department: 2814)

Semiconductor device manufacturing: process

Formation of electrically isolated lateral semiconductive...

Grooved and refilled with deposited dielectric material

C438S701000, C438S713000

Reexamination Certificate

active

06372601

ABSTRACT:

TECHNICAL FIELD
The invention pertains to methods of forming isolation regions and can have particular application to methods of forming shallow trench isolation regions.
BACKGROUND OF THE INVENTION
In modern semiconductor device applications, numerous individual devices are packed onto a single small area of a semiconductor substrate. Many of these individuals devices need to be electrically isolated from one another. One method of accomplishing such isolation is to form a trenched isolation region between adjacent devices. Such trenched isolation region will generally comprise a trench or cavity formed within the substrate and filled with an insulative material, such as, for example, silicon dioxide. Trench isolation regions are commonly divided into three categories: shallow trenches (trenches less than about one micron deep); moderate depth trenches (trenches of about one to about three microns deep); and deep trenches (trenches greater than about three microns deep).
Prior art methods for forming trench structures are described with reference to
FIGS. 1-12
. Referring to
FIG. 1
, a semiconductor wafer fragment
10
is shown at a preliminary stage of a prior art processing sequence. Wafer fragment
10
comprises a semiconductive material
12
upon which is formed a layer of oxide
14
, a layer of nitride
16
, and a patterned layer of photoresist
18
. Semiconductive material
12
commonly comprises monocrystalline silicon which is lightly doped with a conductivity-enhancing dopant. To aid in interpretation of the claims that follow, the term “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
Oxide layer
14
typically comprises silicon dioxide, and nitride layer
16
typically comprises silicon nitride. Nitride layer
16
is generally from about 400 Angstroms thick to about 920 Angstroms thick.
Referring to
FIG. 2
, patterned photoresist layer
18
is used as a mask for an etching process. The etch is typically conducted utilizing dry plasma conditions and CH
2
F
2
/CF
4
chemistry. Such etching effectively etches both silicon nitride layer
16
and pad oxide layer
14
to form openings
20
extending therethrough. Openings
20
comprise peripheries defined by nitride sidewalls
17
and oxide sidewalls
15
. The etching stops upon reaching silicon substrate
12
.
Referring to
FIG. 3
, a second etch is conducted to extend openings
20
into silicon substrate
12
. The second etch is commonly referred to as a “trench initiation etch.” The trench initiation etch is typically a timed dry plasma etch utilizing CF
4
/HBr, and typically extends openings
20
to less than or equal to about 500 Angstroms into substrate
12
. A purpose of the trench initiation etch can be to clean an exposed surface of silicon substrate
12
within openings
20
(i.e., to remove defects and polymer material) prior to final trenching into substrate
12
. Another purpose of the trench initiation etch can be to form polymer over exposed sidewall edges
15
and
17
of oxide layer
14
and nitride layer
16
, respectively. Such polymer can alleviate erosion of sidewall edges
15
and
17
during subsequent etching of substrate
12
.
Referring to
FIG. 4
, a third etch is conducted to extend openings
20
further into substrate
12
and thereby form trenches within substrate
12
. Extended openings
20
comprise a periphery
22
defined by substrate
12
. The third etch typically utilizes an etchant consisting entirely of HBr, and is typically a timed etch. The timing of the etch is adjusted to form trenches within substrate
12
to a desired depth. For instance, if openings
20
are to be shallow trenches, the third etch will be timed to extend openings
20
to a depth of less than or equal to about one micron.
Referring to
FIG. 5
, photoresist layer
18
(
FIG. 4
) is removed and a first oxide layer
24
is thermally grown within openings
20
and along the periphery
22
(
FIG. 4
) defined by silicon substrate
12
. The growth of oxide layer
24
can form small bird's beak regions
26
underlying sidewall edges
17
of nitride layer
16
.
Referring to
FIG. 6
, a high density plasma oxide
28
is formed to fill openings
20
(
FIG. 5
) and overlie nitride layer
16
. High density plasma oxide
28
merges with oxide layer
24
(
FIG. 5
) to form oxide plugs
30
within openings
20
(FIG.
5
). Oxide plugs
30
have laterally outermost peripheries
33
within openings
20
.
Referring to
FIG. 7
, wafer fragment
10
is subjected to planarization (such as, for example, chemical-mechanical polishing) to planarize an upper surface of oxide plugs
30
. The planarization stops at an upper surface of nitride layer
16
.
Referring to
FIG. 8
, nitride layer
16
is removed to expose pad oxide layer
14
between oxide plugs
30
.
Referring to
FIG. 9
, pad oxide layer (
FIG. 8
) is removed. The removal of the pad oxide layer leaves dips
32
at edges of oxide plugs
30
.
Referring to
FIG. 10
, a sacrificial oxide layer
34
is grown over substrate
12
and between oxide plugs
30
.
Referring to
FIG. 11
, sacrificial oxide layer
34
(
FIG. 10
) is removed. Formation and removal of sacrificial oxide layer
34
can be utilized to clean a surface of substrate
12
between oxide plugs
30
. As such surface of substrate
12
can be ultimately utilized to form an active area of a transistor device, it is desired that the surface be substantially free of defects. The removal of sacrificial oxide layer
34
can also undesirably exacerbate dips
32
.
Referring to
FIG. 12
, a silicon dioxide layer
36
is regrown between oxide plugs
30
, and a polysilicon layer
38
is formed over oxide plugs
30
and oxide layer
36
. Polysilicon layer
38
can ultimately be formed into a word line comprising transistor gate regions. Such transistor gate regions can lie between oxide plugs
30
. Plugs
30
can then function as trenched isolation regions between transistor devices. Dips
32
can undesirably result in formation of parasitic devices adjacent the transistor devices and ultimately have an effect of lowering a threshold voltage for the transistor devices. Accordingly, it would be desirable to alleviate dips
32
. Dips
32
can also interfere with subsequent fabrication processes and, for this reason as well, it would be desirable to alleviate dips
32
.
SUMMARY OF THE INVENTION
In one aspect, the invention encompasses an isolation region forming method. An oxide layer is formed over a substrate. A nitride layer is formed over the oxide layer. The nitride layer and oxide layer have a pattern of openings extending therethrough to expose portions of the underlying substrate. The exposed portions of the underlying substrate are etched to form openings extending into the substrate. After etching the exposed portions of the substrate, portions of the nitride layer are removed while leaving some of the nitride layer remaining over the substrate. After removing portions of the nitride layer, oxide is formed within the openings in the substrate. The oxide within the openings forms at least portions of isolation regions.
In another aspect, the invention encompasses another embodiment isolation region forming method. A silicon nitride layer is formed over a substrate. A masking layer is formed over the silicon nitride layer. A pattern of openings is formed to extend through the masking layer and to the silicon nitride layer. The openings are extended through the silicon nitride layer to the underlying substrate. The silicon nitride layer has edge regions proximate the openings and has a central region between the edge regions. The openings are extended into the underlying substrate. After e

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