Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Insulative material deposited upon semiconductive substrate
Reexamination Certificate
1999-12-29
2003-08-19
Fahmy, Wael (Department: 2814)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Insulative material deposited upon semiconductive substrate
C438S473000, C438S638000
Reexamination Certificate
active
06607991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the fabrication of semiconductor devices and, more particularly, to methods for curing spin-on dielectric materials used in semiconductor devices.
2. Description of the Prior Art
Interlayer dielectrics utilized in multilevel interconnection in manufacturing of ultra-large scale integrated circuits have requirements to provide gap filing into high aspect ratio gaps (between metal conductors) and a high flatness of the topology (planarization). To meet these requirements, numerous interlevel dielectric formation processes have been investigated. Tetraethylorthosilicate (TEOS) based chemical vapor deposition (CVD), biased high density plasma CVD combined with chemical mechanical polishing (CMP) have been developed. There are a number of problems with these technologies including: particle generation, process reliability, cost and gap filling capability. Spin-on-glass processes have been utilized and offer simplicity, better gap filling and planarization than these other techniques.
In integrated circuit process technology, the fabrication of reliable interconnect structures with high yields require the deposition of metallization layers of uniform thickness and their subsequent patterning while preserving critical dimensions and line widths. These process goals are difficult to realize unless the substrate is planarized prior to the metallization step. That is, the interlayer dielectric must fill the space between the closely packed vertical wall metal lines of the lower interconnect level so as to produce a smooth topography. Spin-on-glass materials are limited in terms of thickness by their tendency to crack when made in thick layers and cured. Spin-on-glass liquids typically consist of a silicon oxygen network of polymers, one of which is siloxane, dissolved in an organic solvent (typically a combination of a high boiling point solvent and a low boiling point solvent). The dissolved spin-on-glass material is coated onto the semiconductor wafer by spinning at high speed. The spin-on-glass material fills gaps and the uneven topography of the integrated circuit wafer, thereby planarizing it. After spinning onto a substrate, low boiling point solvents are expelled via a low temperature hot plate bake. The wafer is then heated in vacuum or nitrogen to 300°-400° C. This removes higher boiling point solvents and components which can cause cracking and corrosion at subsequent process steps. Very thin coatings are applied this way. If thick coatings are used, the spin-on-glass film cracks due to shrinkage in the baking steps. If a thicker coating is required, multiple coatings must be applied and vacuum baked. This is undesirable because of the time consuming process steps involved and the built up film can still crack in the final cure. The final step in the forming of the spin-on-glass layer is curing at very high temperature. While temperatures as high as 800° to 900° C. may be required to obtain preferable film properties, in integrated circuit fabrication, the maximum temperature at which spin-on-glass film can be cured is often limited to about 450° C. because of the possibility of melting aluminum interconnects or adversely impacting other parts of the integrated circuit.
After a cure at this lower temperature, some spin-on-glass materials contain significant amounts of residual silanols and carbon, and can readily absorb water. The dielectric properties (for example, dielectric constant) of a spin-on-glass film are influenced by the silanol and water content of the film. In the fabrication of integrated circuits it is important to have a low dielectric constant in the spin-on-glass since it becomes the insulating barrier between signal conductors and thus, will determine the upper operating frequency of a circuit. A major disadvantage of thermal methods of curing spin on glass at high temperature is cracking of the spin-on-glass film. Because the spin-on-glass is constrained in a horizontal plane (at the substrate interface), it can only shrink in the vertical direction. This creates great stresses in the spin-on-glass film when it has been baked at very high temperature. These stresses, and the subsequent cracking, have limited spin-on-glass applications despite their favorable attributes: planarization and good gap filling ability. Additionally, the etch rate of thermally cured spin-on-glass is poor compared to the etch rate of thermally grown oxide. It is, therefore, desirable to have some means of curing spin-on-glass at low temperatures to reduce the subsequent cracking of the spin-on-glass while improving its physical properties.
As the importance of low dielectric constant insulating materials has increased as characteristic dimensions of integrated circuits have decreased, organic polymer spin-on dielectric materials have been introduced. These materials also require a curing step at high temperature.
A number of different techniques have been proposed for curing dielectric materials. In U.S. Pat. Nos. 5,192,164 and 5,192,715, Sliwa proposed a technique where an etch back of the spin-on-glass creates unfilled voids between the metal interconnects allowing the spin-on-glass to expand and contract during hard curing without cracking. The drawbacks to this approach are extra process steps and potential of contaminants filing the unfilled voids. Subsequent high temperature baking can trap gases within the voids which can then subsequently cause corrosion of the metal conductors.
An alternative method of curing spin-on-glass is by ion implantation. In U.S. Pat. No. 5,192,697, Leong devised a method of curing spin-on-glass using ion implantation, which allows curing at lower temperatures while improving the oxide etch rate. The high energy ions impinge on the spin-on-glass layer causing, heating and crosslinking. Disadvantages of this technique are that only relatively thin layers can be cured (~1000-2000Å), it requires high vacuum environments (<2×10
−5
Torr) and expensive equipment. Also, high energy ions can cause damage to the lattice structure of the oxides and radiation damage to the underlying active circuits. Even higher and more damaging implant energies are required to penetrate thicker oxide layers. As shown by Moriya (N. Moriya et al., “Modification Effects in Ion-Implanted SiO
2
Spin-in-Glass,” J. Electrochem. Soc., Vol 140. No. 5, May 1993. pp. 1442-1450), damage induced by the high energy ions can drastically modify the spin-on-glass (SOG) film properties.
Another technique that has been proposed to cure spin-on-glass is utilizing ultra-violet radiation and a hotplate. In U.S. Pat. No. 4,983,546, Hyun et. al. claim to achieve spin-on-glass properties that are better than thermally cured spin-on-glass cured at 420° C. However. the disclosed process does not produce the superior qualities of the spin-on-glasses that have been cured at 800°-900° C. There are still carbon and silanols present that can cause subsequent cracking and delamination due to water absorption.
Young-Bum Koh el. al. (“Direct Patterning of Spin-on-Glass by Focused Ion Beam Irradiation,” Jpn. J. Appl. Phys., Vol 31, (1992) pp. 4479-4482) utilized focused ion beam irradiation to crosslink the spin-on-glass. They compare ion beam irradiation of the spin-on-glass with thermal treatments. Whereas carbon is eliminated in thermal cures of 850° C., high doses of ion beam irradiation show a reduction in the carbon but not elimination. They also report that electron beam irradiation requires 2-3 orders of magnitude higher dose than ion beam irradiation to crosslink the SOG material. This would indicate that electron beam processing of SOG would require long process times.
Crosslinking of siloxane type materials by electron beam irradiation have been reported by numerous workers for direct patterning and use in lithography. Electron beams have been considered for crosslinking of spin-on-glass films. A. Imai and H. Fukuda (“Novel Process for Direct Delineation of Spin on Glass (SOG),” Japanes
Livesay William R.
Marlowe Trey
Narcy Mark
Ross Matthew F.
Rubiales Anthony L.
Electron Vision Corporation
Fahmy Wael
Roberts & Mercanti LLP
Trinh (Vikki) Hoa B.
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