Semiconductor device manufacturing: process – Cleaning of wafer as interim step
Reexamination Certificate
2001-01-08
2004-07-06
Zarabian, Amir (Department: 2822)
Semiconductor device manufacturing: process
Cleaning of wafer as interim step
C438S942000, C438S945000, C438S949000
Reexamination Certificate
active
06759351
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the manufacture of semiconductor integrated circuits (ICs) and more particularly to a method for eliminating development related defects referred as polymer blobs in photoresist masks at the end of the photolithography process.
BACKGROUND OF THE INVENTION
In the manufacture of semiconductor integrated circuits such as Dynamic Random Access Memory (DRAM) chips, polysilicon borderless contacts, referred to hereinbelow as the CB contacts, are extensively used to connect devices to the first level of metallurgy (M
0
), for instance, to interconnect the source regions and the gate conductors of the Insulated Gate Field Effect Transistors (IGFETs). In conventional DRAM chips, each elementary memory cell is comprised of an IGFET and its associated capacitor that is formed in a deep trench.
The essential steps of a conventional CB contact hole formation process will be briefly described by reference to
FIGS. 1A-1D
. After these steps have been completed, the CB contact holes are formed and then filled with a conductive material to create the so-called CB contacts.
FIG. 1A
schematically illustrates a state-of-the-art semiconductor structure
10
which is part of a wafer at the initial stage of the CB contact hole formation process. Structure
10
comprises a silicon substrate
11
with diffused regions formed therein and a plurality of gate conductor stacks
12
formed thereon. A gate conductor stack consists of a composite SiO2/doped polysilicon/tungsten silicide structure.
Referring to
FIG. 1B
, structure
10
is coated with a boro-phospho-silicate-glass (BPSG) layer
13
and a tetra-ethyl-ortho-silicate (TEOS) oxide layer
14
above it. These layers are conformally deposited onto structure
10
by LPCVD as standard. As apparent in
FIG. 1B
, structure
10
has a substantially planar surface.
Now, CB contact holes are formed, using a common deep UV (DUV) photolithography process. To that end, the wafer is placed in an equipment comprised of a clean track system and a DUV exposure tool allowing a fully clusterized operation. For instance, the clean track system is an ACT8 tool manufactured by TEL (Tokyo Electron Limited), Tokyo, Japan and the DUV exposure tool is a Micrascan 3 scanner manufactured by SVG (Silicon Valley Group), Wilton, Conn., USA.
Turning to
FIG. 1C
, structure
10
is coated first with a 90 nm thick organic Bottom Anti-Reflective Coating (BARC) layer
15
then with a 625 nm thick layer
16
of a DUV photoresist material. After deposition, the photoresist layer
16
is baked, exposed, baked again, then developed as standard to leave a patterned layer referred to hereinbelow as the CB mask still referenced
16
for the sake of simplicity. The purpose of this CB mask
16
is to define the locations of the CB contacts at the first level of metallurgy (M0).
The BARC material supplied by SHIPLEY USA, Malborough, Mass., USA under reference AR3 900 and DUV photoresists such as KrF M20G supplied by JSR Electronics Co, Yokkaichi, Japan or UV80 supplied by SHIPLEY USA are adequate in all respects. The essential process parameters of the different steps to which the wafers are submitted during the photoresist development process are given below. All these steps are conducted in the ACT8 tool.
1. BARC layer: after coating, bake at 225° C. during 60 sec, then cool down to 22° C. for 60 sec.
2. Resist layer: after coating, post apply bake (PAB) at 140° C. during 90 sec, then cool down to 22° C. for 60 sec.
3. Post exposure bake (PEB): bake at 140° C. during 90 sec followed by cooling at 22° C. for 60 sec.
4. Development: conducted in four sub-steps using surfacted TMAH 0,263N that is dispensed with the H nozzle at 22° C.:
a) developer puddle formation with a 50 sec wait;
b) developer refresh (PDD: post development dispense);
c) rinse with 22° C. deionized water (DIW); and,
d) dry by spin rotation.
After the CB mask
16
has been defined, the process continues with the etching of layers
13
and
14
at locations not protected by said CB mask
16
to create CB contact holes
17
. At this final stage of the CB contact hole formation process, the resulting structure is shown in FIG.
1
D. Now the CB contacts are fabricated. A doped polysilicon layer is conformally deposited onto structure
10
to fill CB contact hole
17
in excess. Next, the doped polysilicon is etched in a plasma until the TEOS layer
14
surface is reached, and the etching is continued to produce a recess (CB recess) in the polysilicon fill that will be subsequently filled with metal to produce the desired M
0
metal lands for the word lines.
To control the defects or contamination added by the photolithography process itself, it is common to inspect patterned monitor wafers using a defect inspection equipment such as the TENCOR AIT, a tool manufactured by KLA-TENCOR, San Jose, Calif., USA, right after the end of the photolithography process. Total or partial wafer surface can be inspected resulting in a defect density measured by the number of defects/cm
2
. A map of the defects is generated. The defects can then be viewed using an optical microscope with laser imaging to analyze the size and shape of the defects in an attempt to determine the root cause. Bare silicon monitor wafers patterned with the CB mask
16
are used to control the defect level of the above described CB contact hole formation process.
The step of creating the CB mask
16
in DRAM chips is essential to the whole chip fabrication process, CB contact holes not etched can lead to the rejection of the chip. This step is normally a clean process which leads to a defect free photoresist CB mask
16
. More generally, less than 15 defects/wafer in the array area has been an acceptable level in the photolithography process for current technologies so far. Unfortunately, the total defect density at the CB mask level has been increasing with the introduction of a new generation of DUV photoresists in the manufacturing lines for unknown reasons.
Recent advances in high resolution DUV photoresists incorporating ESCAP (Environmental Safe Chemically Amplified Photoresist) chemistries have allowed to extend the life of a number of technologies in DUV photolithography beyond 0.20 &mgr;m. A side effect of this improved resolution for certain photoresists is the appearance of defects of a new kind that can be widely found in several high resolution DUW photoresists that are commercialized by different vendors on the market to date. These defects, known under the name of “polymer blob defects” because they are “blob” shaped, are seen right after development and can also be qualified as post development residues. Most of the time, they are seen in large unexposed parts of the photoresist layer in the “support/kerf” area but they are also present in the “array” area. If we still consider the CB contact hole formation process described above, the blobs can be redeposited over openings of the CB mask
16
preventing the contact hole formation during the etch step. Blobs are very critical defects as they have a real impact on test yields. The big concern for photoresist users and manufacturers is that as DUV photoresist systems evolve to even higher resolution, polymer blobs will soon become a major yield detractor.
The polymer blobs can vary in size from approximately 1 &mgr;m (referred to as small blobs) up to 20 &mgr;m or even more (referred to as large blobs). Typical small and large blobs are shown in
FIGS. 2A
,
2
B and
2
C respectively. As apparent in
FIG. 2A
, the small blob located at the center of the photograph covers two CB contact holes and there are some polymer residues over the surrounding CB contact holes.
FIGS. 2B and 2C
show a typical large blob in the “array” and “support/kerf” areas respectively. Large blobs often have a donut-like shape with an inside circle. A large polymer blob is able to cover a great number of CB contact holes and in that regards, can be considered as a manufacturing yield killer. SEM analysis shows a 10 nm thick circular structure surrounded by little spots.
International Business Machines - Corporation
Novacek Christy
Zarabian Amir
LandOfFree
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