Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Antihalation or filter layer containing
Reexamination Certificate
1998-12-09
2001-05-22
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Antihalation or filter layer containing
C257S437000
Reexamination Certificate
active
06235456
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the manufacture of semiconductor devices as components of integrated circuits, specifically to processes for photolithography in which anti-reflective coatings are used to increase the accuracy of the photolithographic processing steps.
BACKGROUND OF THE INVENTION
I. Photolithography and Photoresists in Semiconductor Manufacturing
The size of integrated circuits is at least partially limited by the ability of the manufacturing methods to be carried out on a small scale. Many steps in the manufacture of integrated circuits can involve photolithography, in which patterns of features are produced in photoresist materials. Photoresist materials are sensitive to electromagnetic radiation, and upon development of the photoresist layer, portions of the photoresist layer are removed, by a process termed photolysis, revealing the underlying semiconductor material. Subsequent exposure of the underlying semiconductor material to etchants can result in the removal of the semiconductor material only where the photoresist layer had been photolyzed. Subsequently, the remaining regions of photoresist material are removed, leaving the un-etched areas of the semiconductor exposed for further processing.
II. Limitations of Feature Size Photolithography
The minimum size of a feature that can be manufactured on a semiconductor wafer is called a critical dimension, which can be limited by photolithography processes. For certain photolithography methods, it is desirable for the incident beam of electromagnetic radiation to penetrate into the photoresist layer in a direction perpendicular to the photoresist layer and the semiconductor wafer. Vertical orientation can provide desirable high resolution of photolithography, thereby minimizing critical dimensions. However, resolution of a photolithography step can be limited, for example, by diffraction of electromagnetic radiation by the edges of the mask and reflection of electromagnetic radiation by underlying layers. Collectively, there effects widen the area of photoresist exposed to electromagnetic radiation, a process termed herein “beam spreading.” Nitride and oxynitride layers can amplify the problems inherent in photolithography, and thereby can limit the size of device features.
III. Diffraction, Reflection and Interference Effects in Photolithography
A. Diffraction
Diffraction of electromagnetic radiation by the edge of a mask (“edge effect diffraction”) can displace the direction of incident electromagnetic radiation toward more lateral areas of photoresist which underlie the mask. Lateral displacement of the beam can expose undesired areas of photoresist, including areas of photoresist under the mask itself. Angular displacement of electromagnetic radiation is dependent on the wavelength of the radiation, with longer wavelengths being deflected by a larger angle than shorter wavelengths. This has led to the use of higher-energy, shorter wavelengths in photolithography.
Additionally, after angular displacement of a beam of electromagnetic radiation, the total lateral distance away from its intended path that a beam can travel is dependent on the thickness of the layers through which it passes. A thicker layer permits a greater lateral displacement of the beam. Therefore, another approach to decreasing the effect of diffraction is to decrease the thickness of the photoresist film. By decreasing the thickness of the film, there can be less opportunity for diffracted electromagnetic radiation to undercut the photoresist. However, as the photoresist film thickness is reduced, there can be increased variation in thickness of the photoresist layer, leading to errors in transfer of an image to the photoresist. Moreover, as the layer of photoresist becomes thinner, the transparency of the photoresist layer increases, thereby increasing reflection of electromagnetic radiation by underlying surfaces.
B. Interface Reflection
Interfaces between layers of materials can reflect incident electromagnetic radiation. Interfaces relevant to semiconductor manufacturing include, by way of example only, interfaces between silicon oxides and silicon. When a source-drain stack is manufactured using layers of oxide, stoichiometric nitride or oxynitride, and photoresist, the electromagnetic radiation can pass through the photoresist layer, the nitride or oxynitride, and the oxide, and can be reflected back upwards through the stack. Lateral reflection can cause absorption of electromagnetic radiation by photoresist underneath the mask edge, undercutting the mask edge and resulting in additional inaccuracies in the transfer of the mask image to the photoresist.
Where barrier or polish-stop layers underlie photoresists, they can add to the critical dimension problem. Polish-stop and barrier layers serve several purposes in manufacturing semiconductor devices. Polish-stop layers can be used when it is desired to provide a surface below which an etching or chemical mechanical polishing step will not remove substantial amounts of material. Barrier layers are commonly used in another type of isolation, termed the local oxidation of silicon (the “LOCOS”) method. Barrier layers typically retard diffusion of contaminants into semiconductor structures.
Silicon nitride and silicon oxynitride are examples of materials commonly used to form barrier or polish-stop layers in photolithography processes. As used herein, the term “barrier layer” can refer to films that act either as diffusion barriers or as polish-stop layers. The chemical formula of silicon nitride is: Si
3
N
4
, and the formula for silicon oxynitride is: Si
3
N
4
O
x
, where x can vary from less than about 1 to about 3. Silicon nitride films can be made using chemical vapor deposition (CVD), wherein precursors, by way of example only, SiH
4
and NH
4
are introduced into a deposition apparatus. A source of energy dissociates the precursors into reactive intermediates, which then can combine on the wafer surface to form the layer of nitride. Oxynitride films can be made by introducing N
2
O or NO into the reaction chamber. A desirable property of these materials for use as polish-stop layers include high mechanical strength, and a desirable property of these materials for use as barrier layers include high resistance to diffusion of contaminant molecules. These desirable properties of nitride and oxynitride are the greatest for stoichiometric films, that is, films in which the ratio of silicon to nitrogen is 3:4. However, conventional, stoichiometric nitride and oxynitride layers can provide problems in photolithography, including reflection and standing wave effects which make the manufacture of small, reproducible semiconductor device features difficult.
As manufacturing processes become more miniaturized, barrier and polish-stop layers become thinner. However, it is desirable to maintain desired mechanical and chemical properties of barrier and polish-stop layers. Low Pressure Chemical Vapor Deposited (LPCVD) silicon nitride layers can be made with these desirable qualities because nitride layers can be made which are thin and stoichiometric, thus comprising Si
3
N
4
. However, stoichiometric nitride layers can be transparent. In patterning using monochromatic electromagnetic radiation, transparency poses a limitation as the critical dimensions become smaller. Additionally, silicon oxide layers underlying the nitride layers also can be transparent. In contrast, interfaces between oxide and silicon substrate layers can reflect electromagnetic radiation, permitting the incident electromagnetic radiation to be reflected upwards back into the photoresist layer. Therefore, the incident path length of electromagnetic radiation from the top surface of the photoresist to the reflective layer can be larger for transparent barrier layers than for opaque layers. An increased incident path length permits greater lateral displacement of the beam. For each incremental increase in lateral displacement of the incident beam, there is a corresponding incremental increase in lateral d
Advanced Micros Devices Inc.
Fliesler Dubb Meyer & Lovejoy
Young Christopher G.
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