Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2001-05-02
2003-08-26
Ghyka, Alexander (Department: 2812)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S778000, C438S788000, C257S760000
Reexamination Certificate
active
06610609
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of manufacture of electronic devices. In particular, the present invention relates to photoresists and antireflective coatings used in the manufacture of electronic devices.
In the manufacture of electronic devices, such as printed wiring boards or semiconductors, a number of layers of material, such as photoresists or antireflective coatings, are applied to a substrate. Photoresists are photosensitive films used for transfer of an image to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask (reticle) to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. For most negative-acting photoresists, those coating layer portions that are exposed to activating radiation polymerize or cross-link in a reaction between a photoactive compound and polymerizable reagents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble. Photoresist compositions are known to the art and described by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, ch. 2, 1975 and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York, ch. 2 and 4, both incorporated herein by reference to the extent they teach photoresist compositions and methods of making and using them.
A major use of photoresists is in the manufacture of semiconductors where an object is to create features, such as vias, trenches or combinations thereof, in a dielectric layer. Proper photoresist processing is a key to attaining this object. While there is a strong interdependency among the various photoresist processing steps, exposure is believed to be one of the more important steps in attaining high resolution photoresist images.
In such processes, reflection of actinic radiation during exposure of the photoresist and “poisoning” of the photoresist by the dielectric layer are detrimental to fine feature formation. Reflection of actinic radiation, such as from the layer underlying the photoresist, often poses limits on resolution of the image patterned in the photoresist layer. Reflection of radiation from the substrate/photoresist interface can produce variations in the radiation intensity in the photoresist during exposure, resulting in non-uniform photoresist linewidth upon development. Radiation also can scatter from the substrate/photoresist interface into regions of the photoresist where exposure is not intended, again resulting in linewidth variations. The amount of scattering and reflection will typically vary from region to region, resulting in further linewidth non-uniformity.
Reflection of activating radiation also contributes to what is known in the art as the “standing wave effect”. To eliminate the effects of chromatic aberration in exposure equipment lenses, monochromatic or quasi-monochromatic radiation is commonly used in photoresist projection techniques. Due to radiation reflection at the photoresist/substrate interface, however, constructive and destructive interference is particularly significant when monochromatic or quasi-monochromatic radiation is used for photoresist exposure. In such cases the reflected light interferes with the incident light to form standing waves within the photoresist. In the case of highly reflective substrate regions, the problem is exacerbated since large amplitude standing waves create thin layers of underexposed photoresist at the wave minima. The underexposed layers can prevent complete photoresist development causing edge acuity problems in the photoresist profile. The time required to expose the photoresist is generally an increasing function of photoresist thickness because of the increased total amount of radiation required to expose an increased amount of photoresist. However, because of the standing wave effect, the time of exposure also includes a harmonic component which varies between successive maximum and minimum values with the photoresist thickness. If the photoresist thickness is non-uniform, the problem becomes more severe, resulting in variable linewidths.
With recent trends towards high-density semiconductor devices, there is a movement in the industry to shorten the wavelength of exposure sources to deep ultraviolet (DUV) light (300 nm or less in wavelength), KrF excimer laser light (248 nm), ArF excimer laser light (193 nm), electron beams and soft x-rays. The use of shortened wavelengths of light for imaging a photoresist coating has generally resulted in increased reflection from the upper resist surface as well as the surface of the underlying substrate. Thus, the use of the shorter wavelengths has exacerbated the problems of reflection from a substrate surface.
An approach used to reduce the problem of reflected radiation has been the use of a radiation absorbing layer either interposed between the substrate surface and the photoresist coating layer, called a bottom antireflective coating or BARC, or a radiation layer disposed on the surface of the photoresist layer, called a top antireflective coating or TARC. See, for example, PCT Application WO 90/03598, EPO Application No. 0 639 941 A1 and U.S. Pat. Nos. 4,910,122, 4,370,405 and 4,362,809, all incorporated herein by reference to the extent they teach antireflective (antihalation) compositions and the use of the same. Such BARC and TARC layers have also been generally referred to in the literature as antireflective layers or antireflective compositions. Typically, such antireflective compositions include a radiation absorbing component, or chromophore, a polymeric binder and one or more cross-linking agents.
Variations in substrate topography also give rise to resolution-limiting reflection problems. Any image on a substrate can cause impinging radiation to scatter or reflect in various uncontrolled directions, affecting the uniformity of photoresist development. As substrate topography becomes more complex with efforts to design more complex circuits, the effects of reflected radiation become more critical. For example, metal interconnects used on many microelectronic substrates are particularly problematic due to their topography and regions of high reflectivity.
One method of solving such problems resulting from variations in substrate topography is by placing a photoresist at the same height over a surface, as disclosed in U.S. Pat. No. 4,557,797 (Fuller et al.). This patent uses a multi-layer structure having a relatively thick bottom layer of poly(methyl methacrylate) (“PMMA”) to provide a planar surface, a thin middle layer of an antireflective coating and a thin top layer of a photoresist material. However, this system results in a thick polymer layer which must subsequently be removed. Such layers are typically removed by a variety of methods, such as chemical mechanical polishing (“CMP”), etching and wet chemical methods. Due to the added time and cost of such removal processes, it is desired that the polymer layers be as thin as possible to aid in their subsequent removal.
Another approach to solving the problems associated with variations in substrate topography is that disclosed in Adams et al.,
Planarizing AR for DUV Lithography, Microlithography
1999
: Advances in Resist Technology and Processing XVI
, Proceedings of SPIE, vol.
Linehan Leo L.
Rutter, Jr. Edward W.
Cairns S. Matthew
Ghyka Alexander
Shipley Company L.L.C.
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