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SILICAFILM

Silicafilm is an alcohol solution which is applied to a semiconductor surface to yield a pure Si02 film similar in characteristics to a pyrolitic oxide. Silicafilm yields a pyrolytic oxide essentially free of sodium ions or other undesirable elements deleterious to semiconductor devices. Films may be formed on a wide variety of flat surfaces by spinning, spraying or dipping. For precise control of flatness and greatest uniformity from wafer, spinning is the recommended procedure for applying Silicafilm.

After spinning, the film remaining on the surface of the wafer must be hardened or densified by heat treatment. The heat treatment is carried out at temperatures from 200oC to temperatures in excess of 800oC or 900oC in air, oxygen or nitrogen. The extent of the densification of the film is indicated by the etch rate in dilute HF solutions. A film densified by heat treatment at 200oC for 15 minutes will exhibit an etch rate 1000 times faster than a thermal oxide. A similar heat treatment at 900oC will yield a film whose etch rate is two to three times faster than a thermal oxide. In addition to the reduction in rate of etching by HF, heat treatment also affects the hardness of the film, as evidenced by its resistance to scratching with a steel styles. Heat treatment at 800oC will yield a film which will exhibit scratch resistance comparable to a thermal oxide. The thickness of a SiO2 film formed from Silicafilm is determined by the spin speed when the material is spun on. At 3000 rpm, a film thickness of 2000 angstroms is obtained after heat treatment at 200oC. The film thickness decreases as spin speed is increased. Films up to 8000 angstroms thick may be obtained by successive applications of material applied while the wafer is spinning and allowing 5 or 10 seconds between successive applications. Table I lists the film thicknesses obtained after heat treatment at 200oC at various spin speeds. Table I also shows the film thicknesses obtained by successive applications of Silicafilm.

The dielectric and optical characteristics of films formed from Silicafilm are quite similar to the characteristics of pure SiO2 formed by the oxidation of silicon at high temperature. In Table II the index of refraction, dielectric constant and conductivity of films of Silicafilm are listed.

APPLICATIONS OF SILICAFILMS

Diffusion Masking
Silicafilm may be used as a diffusion mask in semiconductor processing. After spinning, the films are densified at 250oC for 15 to 20 minutes. Windows may now be photo-etched in the film with 0.5% HF solution. A two thousand angstrom film will be completely dissolved in 10 to 15 seconds in a 0.5% HF solution.

Most photo-resists will mask against such short exposures to 0.5% HF solution even when post-baking of the resist is eliminated.

For most diffusion processes employed in IC processing, relatively thin films of Silicafilm will be adequate for masking, especially where doped oxides such as Borosilicafilm, Phosphorosilicafilm, Antimonysilicafilm and Arsenosilicafilm are used as diffusion sources. In Table III the diffusion coefficients of boron, phosphorous, arsenic and antimony in SiO2 formed from Silicafilm are listed. From this data it is apparent the use of Silicafilm in conjunction with doped oxide sources permits diffusion masking with much thinner oxide masks than is possible with vapor transport sources, where the volatilized doping compound dissolves through the oxide, requiring thick oxide masks with the attendant photo-resist undercutting problems. In addition, the use of thinner oxides is advantageous in subsequent metallization processes, in that shallower oxide steps are encountered. Large oxide steps resulting in metallization failures are a source of unreliability in ICs.

For deep diffusions, where the probability of mask penetration is high, Silicafilm may be used in conjunction with doped Silicafilm in the following way. The doped Silicafilm is spun over the bare wafer. The doped film is densified by heat treatment at 200oC for 20 to 30 minutes. Undoped Silicafilm is then spun over the doped oxide, and heat treated. A positive mask is now used to remove the oxides from regions which are not to be doped. A second layer of Silicafilm is then spun over the wafer as a blanket to protect the bare silicon areas from possibility of doping from volatilization of dopant from the oxide. By carrying out the diffusion in an oxidizing atmosphere, steps indicating the delineation between doped and undoped regions in the silicon may be produced if required for future registration. This process obviates the need for thick oxide masks and eliminates an oxidation process.

Protective Coatings
Silicafilm may also be used in various "glassivation" processes for the protection of uncased chips against scratches and for passivation. For the former application "Silicafilm with Boron" is available. The addition of a small percentage of boron in the glass lowers the softening point and yields a harder coating than pure Silicafilm. Heat treatment at 400oC in air for 10 to 15 minutes is sufficient to provide a scratch resistant coating. "Silicafilm with Boron" is spun over the wafer after metallization. The wafer is heat treated at 200oC for 15 minutes. Photo-resist is applied and windows are etched with 0.5% HF solution over the bonding pads. Most metallization systems are able to withstand exposure to this dilute solution. After stripping the glass is hardened by heating at 400oC for 10 to 15 minutes to impart the desired hardness.

It is well known that SiO2 films do not mask against the diffusion of sodium ions. The drift of sodium ions in the fringing fields when the p-n junctions are biased, leads to instability of the device. Studies have shown that the incorporation of phosphorous in the SiO2 layer can prevent the sodium ion drift which causes the instability.

However, there is additional evidence which indicates that the concentration of phosphorous in the SiO2 induce additional polarization phenomena which can lead to device instability. "Phosphorosilicafilm for Passivation" has been formulated to provide what has been reported to be an optimum range of phosphorous concentrations to provide adequate phosphorous gettering and yet low enough to indicate an absence of other polarizations. Phosphorosilicafilm for Passivation is spun over the wafer after metallization. The film is heated at 200oC for 15 to 20 minutes. Windows are photo-etched over the bonding areas, using 0.5% HF solution. After stripping, the glass film is further hardened by heat treatment at 400oC 15 to 20 minutes in air.

Additional Uses For Silicafilm
Silicafilm may be applied in a variety of applications in addition to those mentioned above. For example, the material may be spun or sprayed over Ge, GaAs, and GaP semiconductors and used as a diffusion mask in a manner analogous to its use with silicon. It may be used in place of silicon monoxide to protect thin film resistors such as nichrome. One particularly useful application is its use over the heavily phosphorous doped SiO2 prior to etching contact windows. Used in a thin layer of approximately 1000 angstroms, it eliminates adhesion problems which frequently occur between phosphorous doped SiO2 and some photo-resists.

TABLE I



Film Thickness Variations

Film Thickness* (angstroms) Spin Speed (rpm)
2000 3000
1500 6000
1100 10,000



Film Thickness* (angstroms) No. of drops Silicafilm
(Spin speed - 3000 rpm)
2000 1
3000 2
4000 3
5200 4
6300 5
*After Heat Treatment at 200oC for 15 minutes in air.



Film Thickness**(angstroms) Heat Treatment(15 min. in N2)
2100 200oC
1900 500oC
1700 800oC
1700 1200oC
**Spin Speed 3000 rpm




TABLE II



Physical Characteristics of SiO2 Films Formed by Spinning SILICAFILM on a Silicon Wafer

Index of Refraction - 1.44
Dielectric Constant - 4
Thermal Expansion Coefficient - 5x10-7/oC
Thermal Conductivity - 4x10-3gr. cal./(sec)(sq. cm.)(oC/cm) @250C
Sheet Resistance (300oC) - 1x1015 ohms/sq. (2000 angstrom film)




TABLE III



Diffusion Coefficient of Dopants in Silicafilm at 1200oC

Boron - 1x10-15cm2/sec

Phosphorous - 1.8x10-15cm2/sec

Arsenic - 2.7x10-14cm2/sec

Antimony - 1x10-15cm2/sec

Diffusion Sources: Borosilicafilm
Phosphorosilicafilm
Arsenosilicafilm
Antimonysilicafilm



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