Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
1997-12-23
2001-01-23
Duda, Kathleen (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S950000, C216S056000, C438S960000, C438S964000
Reexamination Certificate
active
06177235
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the photolithographic techniques used in the fabrication of integrated circuits and, more particularly, to high-resolution optical lithography techniques. It relates more particularly to techniques for reducing the undesirable effects of the reflection of optical waves from the layer subjacent to a photosensitive resin.
BACKGROUND OF THE INVENTION
In the steps for fabricating an integrated circuit, layers of semiconductor materials or metallic layers have to be lithographically patterned. These layers can have variable topographies, either plane or in relief. Conventionally, the lithography is carried out by depositing a photosensitive reason on this layer to be lithographically patterned, only certain desired areas of which are sensitized, these being subsequently developed in a developer appropriate to the chemical nature of the resin, thus revealing specific areas of the layer to be lithographically patterned.
This technique is used, for example, to define the level of the gate of an MOS transistor in a layer of polycrystalline silicon. The dimensions of the gates of a transistor are so small that the slightest spurious exposure is deleterious and unacceptable in transistors. This spurious exposure is, in particular, generated by reflection of incident light from the layer to be lithographically patterned subjacent to the resin, increasing the interference effects in the resin and the scattering of the light on the topological variations. Such effects are becoming less and less tolerable because of the dimensions characteristic of submicron technologies.
One approach for reducing the influence of reflected light in photolithography techniques consists in depositing an antireflection layer called an ARC (antireflective coating), which may either be inorganic or organic, directly on the reflective surface. The photosensitive resin in then deposited directly on the ARC layer.
However, the use of these ARCs requires a separate deposition and annealing steps appropriate to these materials. Apart from the expense of the additional equipment necessary for implementing these deposition techniques, it is necessary to add the precautions to be taken for controlling all the parameters of the process of depositing these layers (annealing times, defect minimization, etc.).
In addition to these problems of implementation of the process, ARCs do not have the properties anticipated.
Although they have good antireflective properties, organic ARCs have problems of variation in the thickness of the layers, due in particular to the topography of the surface on which they are deposited. These problems are in fact directly associated with the effect of deposition planarization. Since the thickness of the ARC is not the same over the entire surface, there is a risk of side overetching in the areas where the ARC is thinner during etching of the ARC by a specific plasma.
With regard to inorganic ARCs, these require adjustments and steps additional to the photolithography process, in particular adjustment of their refractive index n and their extinction coefficient k depending on the thicknesses deposited, a specific etching step after exposure and development of the photosensitive resin, and their removal after photoetching is an additional step. On the other hand, they have the advantage over organic ARCs of giving conformal coatings.
Another approach for reducing the reflection of incident light on a reflective layer in a photolithographic etching step is described in U.S. Pat. No. 5,139,974. This approach consists in increasing the roughness of a reflective metal surface so as to increase the absorption of incident light. However, the roughness of the surface is increased by a process which does not guarantee uniformity of the physical state of the surface, a phenomenon which is aggravated by the fact that the layer to be treated includes features and therefore has a surface in relief. This technique modifies the optical properties of the material only in a random manner which is difficult to reproduce.
SUMMARY OF THE INVENTION
One object of the invention is to improve the photolithography step in the fabrication of integrated circuits.
Another object of the invention consists in reducing, or even eliminating, the undesirable effects due to reflection of the incident light from a reflective layer during exposure of a photosensitive resin, while overcoming the drawbacks associated with the approaches of the prior art.
The inventors have demonstrated that the formation of a porous layer within an actual material to be lithographically patterned and on the surface of the latter, so that this novel porous layer lies between the layer of material to be lithographically patterned and the photosensitive resin, allows considerable reduction, of more than 95%, of the reflectivity of the layer to be lithographically patterned, this being achieved in a stable and reproducible manner. Furthermore, the optical properties of the porous absorbent layer can be controlled so as to obtain the desired reflectivity.
Thus, the invention provides an improved photolithography process in the fabrication of integrated circuits, the main characteristic of which is that, prior to deposition of the photosensitive resin, a porous layer is formed within the material to be lithographically patterned and on the surface of the latter, this porous layer being of this same material and having a given thickness.
The material of the layer to be lithographically patterned, subjacent to the photosensitive resin and within which the porous layer is formed, is composed of a conductive material or of a semiconductor material. According to one aspect of the process of the invention, the material is a silicon-based material capable of forming porous silicon. This silicon-based material may have various crystallographic structures, for example it may be amorphous silicon, polycrystalline silicon or single-crystal silicon.
The porosity and the thickness of the layer of porous material formed within the layer to be lithographically patterned are determined depending on the intended optical properties (refractive index and extinction coefficient of the layer of the porous material) or else on the desired reflectivity for a given wavelength. The thickness of the layer of porous material is typically between 200 and 800Å.
The layer of porous material may be formed, within the layer to be lithographically patterned, by a chemical, electrochemical or plasma method, or any other method capable of producing porous material from the material of the layer to be lithographically patterned. Using a chemical method, the preferred method of the invention, the porosity and the thickness of the porous layer are adjusted by varying at least the treatment time or at least the properties of the chemical bath.
According to a preferred way of implementing the invention, a porous silicon layer is formed on the surface of a layer of a silicon-based material using a chemical method.
According to the process of the invention, the photosensitive resin is then deposited and exposed at a given wavelength. As the source, a UV lamp is used in the standard way for the exposure. The exposure of the photosensitive resin may be carried out at a wavelength varying between 193 and 500 nm. The process of the invention is particularly suitable for high-resolution optical lithography techniques using a short UV wavelength, called deep UV, of between 200 and 300 nm, as well as the g, h and i lines of the spectrum of mercury. However, these ranges do not exclude shorter wavelengths capable of sensitizing a resin and of being absorbed by the porous layer.
According to the process of the invention, the resin is then developed and the material of the layer subjacent to the resin to be lithographically patterned is etched. This plasma etching step, which is conventional for etching the material of the layer to be lithographically patterned, has the advantage according to the process of the invention of not requiring an add
Francou Jean Marc
Halimaoui Aomar
Schiltz Andr{acute over (e)}
Duda Kathleen
France Telecom
Fulbright & Jaworski L.L.P.
LandOfFree
Antireflection treatment of reflective surfaces does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Antireflection treatment of reflective surfaces, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Antireflection treatment of reflective surfaces will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2552298