Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1998-09-30
2001-01-30
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S322000
Reexamination Certificate
active
06180290
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to alternating type phase shifting photomasks and more particularly to multi-phase multi-layer phase shifting photomasks using phase shifting layers of less than 180° optical phase shift to form a mask having a total optical phase shift of 180°.
(2) Description of the Related Art
The use of phase shifting technology in masks used in projection systems to expose a layer of photoresist formed on a semiconductor substrate has become very important as the requirements of image definition and depth of focus have become more stringent. A number of workers have disclosed methods of forming and using phase shifting photomasks.
U.S. Pat. No. 5,358,808 to Nitayama et al. and U.S. Pat. No. 5,376,483 to Rolfson teach etching into a transparent mask substrate in addition to the formation of phase shifting material to provide photomasks having phase shifting regions with different optical lengths and different amounts of phase shift.
U.S. Pat. No. 5,403,682 to Lin teaches the use of phase shifting rims surrounding each pattern of a photomask.
U.S. Pat. No. 5,429,897 to Yoshioka et al. teaches the use of attenuating type phase shifting masks.
U.S. Pat. No. 5,393,623 to Kamon teaches the use of reflection type phase shifting photomasks.
U.S. Pat. No. 5,387,485 to Sukegawa et al. teaches the use of photomasks using annular arrangement of light shielding regions together with phase shifting material.
U.S. Pat. No. 5,246,800 to Muray teaches an alternating type phase shifting mask using a transition region having three different optical phases. The three phases are assigned to pixels, or subregions, in the transition region.
In the invention of this Patent Application uses an alternating type phase shifting photomask. The phase shifting region of the photomask is formed from a number of layers of phase shifting material. The total optical phase shift of the mask is 180° and the phase shift of each layer is less than 180°. The use of a multi-layer phase shifting photomask provides greater control of the phase shift during mask fabrication and a gradual transition between no phase shift and a phase shift of 180° which improves the quality of the image formed by the photomask.
The multi-layer mask is formed with an etching process having a wide tolerance with regard to etching times. The thickness control for the phase shifting layers is provided by control of the deposition of the layers.
SUMMARY OF THE INVENTION
The fabrication of integrated circuits places ever increasing requirements on photolithography to produce improved image definition and depth of focus. Conventional masks, such as shown in
FIG. 1A and 1B
, use an opaque pattern formed on a transparent substrate.
FIG. 1A
shows a cross section of such a photomask having an opaque pattern
12
formed of a material such as chrome formed on a transparent substrate
10
formed of a material such as quartz.
FIG. 1B
shows the top view of this type of mask.
Phase shifting photomasks, both rim type and alternating type, have been used to provide improved image definition and depth of focus however the fabrication of phase shifting photomasks is often difficult. Conventional alternating type phase shifting photomasks, such as shown in
FIGS. 2A and 2B
, provide a shift in optical phase of 180° at every other space.
FIG. 2A
shows a cross section of this type of mask showing an opaque pattern
12
and a pattern of phase shifting material
11
formed on a transparent substrate
10
.
FIG. 2B
shows a top view of this type of mask. At the edges of the pattern of the phase shifting material an abrupt phase shift of 180° occurs which can produce undesirable effects in the image. In fabrication of phase shifting photomasks of this type etching times must be carefully controlled to insure the correct amount of phase shift.
It is a principle objective of this invention to provide a method of forming an alternating type multi-phase phase shifting mask from a multi-layer thin film structure which provides a large latitude in etching times used to form the mask.
It is another principle objective of this invention to provide an alternating type multi-layer multi-phase phase shifting photomask which provides a 180° optical phase shift at pattern edges in steps of less than 180° such as 45°, 60°, or 90°.
These objectives are achieved by depositing a number of layers of phase shifting material on a transparent mask substrate having a patterned layer of opaque material formed thereon. A first layer of first phase shifting material is deposited on the transparent mask substrate over the patterned layer of opaque material. A second layer of second phase shifting material is deposited on the layer of first phase shifting material. A third layer of first phase shifting material is then deposited on the second layer of second phase shifting material. A fourth layer of second phase shifting material is then deposited on the third layer of first phase shifting material. This example is for four layers of phase shifting material however the process can be terminated earlier if fewer than four layers is desired or can be continued in this manner if more than four layers of phase shifting material is desired.
Continuing with the four layer example a pattern is then formed in the fourth layer of second phase shifting material using a photoresist mask and etching with an etchant which will not etch the third layer of first phase shifting material. Next a pattern is formed in the third layer of first phase shifting material using a photoresist mask and an etchant which will not etch the second phase shifting material. Next a pattern is formed in the second layer of second phase shifting material using a photoresist mask and an etchant which will not etch the first phase shifting material. Finally, a pattern is formed in the first layer of first phase shifting material using a photoresist mask and an etchant which will not etch the second phase shifting material or the transparent substrate material.
The thickness of the phase shifting layers is determined when the layers are deposited and is very easily controlled. The etching method used to form the patterns has no effect on the thickness of the phase shifting layers so that control of etching time is not critical. The total optical phase shift in all of the layers used is 180° so that for a mask using four layers, for example, the thickness of each layer will be chosen to provide a phase shift of 45°.
At alternating edges of the mask pattern the mask is arranged so that, in a four layer mask for example, the light at the edge of the mask pattern will pass through one, two, three, and four layers of phase shifting material. This provides a gradual phase shift of 45°, 90°, 135°, and 180°. This gradual phase shift will improve the quality of the image formed by the mask.
REFERENCES:
patent: 5358808 (1994-10-01), Nitayama et al.
patent: 5376483 (1994-12-01), Rolfson
patent: 5387485 (1995-02-01), Sukegawa et al.
patent: 5393623 (1995-02-01), Kamon
patent: 5403682 (1995-04-01), Lin
patent: 5429897 (1995-07-01), Yoshioka et al.
patent: 5487962 (1996-01-01), Rolfson
patent: 5759724 (1998-06-01), Rolfson
Hsu Jung-Hsien
Hsu Sung-Mu
Ackerman Stephen B.
Prescott Larry J.
Rosasco S.
Saile George O.
Taiwan Semiconductor Manufacturing Company
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