EMULSITONE COMPANY
19 Leslie Court
Whippany, New Jersey 07981
TEL. (973)386-0053
FAX (973)503-0256

EMULSITONE SOLUTION 3739

EMS 3739 is a spin-on dopant system for producing arsenic diffused layers in silicon. Two types are available 3739N for shallow highly doped layers required in N-MOS for the source drains, and 3739B for less highly doped but thicker layers required for buried layers in bipolar ICs.

EMS 3739 is singularly free of such contaminants as sodium, iron, manganese, copper, etc. In addition, EMS 3739 is free of particulates and needs no filtering prior to use. While sensitivity to relative humidity is a problem with some spin-on arsenic solutions, this problem is greatly reduced with EMS 3739. Only extremely high R_h will affect the parameters obtained with EMS 3739, usually evidenced as a modest increase in sheet resistivity over normal. EMS 3739 has a shelf life in excess of six months when stored at room temperature. The solutions are stable at room temperatures for long periods of time with no need for refrigeration. In addition to yielding extremely uniform sheet resistivities and junction depths, diodes produced from these diffused layers will exhibit IV characteristics close to the theoretical and free of anomalies. EMS 3739 will not adversely influence the minority carrier lifetime in itself; however, although the arsenic concentration is high, the gettering capability of the diffused layer is not equal to that of phosphorous when present in concentrations near 5 x 10²⁰ or 1 x 10²¹.

EMULSITONE SOLUTION 3739N

Most processes for high speed N-MOS require shallow penetrations, usually 0.5 micron or less, which are heavily doped. Minimum diffusion under the gate is desire and is obtained with shallow penetrations. It is also desirable that there be no excessive accumulation or pile-up of the film along the gate edge, so as to avoid possible undercutting of the gate oxide when, after diffusion, the dopant film is removed by HF etching solutions. These characteristics are available with EMS 3739N when used as recommended.

Process Recommendation

Wafer Cleaning: No unusual wafer cleaning processes are required other than the usual industry practice.

Spinning: EMS 3739N may be applied with either static or dynamic spinning. In the former case the wafer is not rotating when the solution is applied to its surface. In the latter case the wafer is spinning slowly at 100-300 rpm prior to the application of the solution. After application of the solution, the slow spin is continued for a few seconds to allow the solution to spread over the surface whereupon the spin speed is rapidly accelerated to the final speed, usually 3000-6000 rpm.

Both methods will yield good results. Some spinning apparatus also provide for a negative pressure in the spinning chamber. In this circumstance, some solvent will be volatilized from the solution yielding a thicker film than is desirable. This difficulty can be resolved by reduction of the pressure difference or decrease in the slow speed spin time.

The freshly spun-on film will exhibit a somewhat wavy surface immediately after spinning; nevertheless, after the diffusion process, the film undulations will have disappeared. After heating the film at 450^oC for a few minutes the film thickness will be 3100 angstroms for spin speed of 3000 rpm. The film thickness will vary inversely as the spin speed in the range of spin speeds of 3000 rpm to 6000 rpm.

Baking: If the process schedule is such that the wafers will be diffused a short time after spinning, oven baking is not required. It will be sufficient to allow the wafers to equilibrate in the entrance of the diffusion furnace for 15 minutes prior to insertion into the hot zone. If diffusion is not carried out soon after spinning, the wafers should be baked at 150-200^oC. The wafers may be stored in the bake oven if such facility is provided until ready for diffusion since long bake times will not affect the results. Alternatively, after baking the wafers may be stored in a clean dry dust-free environment.

Diffusion: Diffusions are carried out in the temperature range of 1000-1100^oC. The nature of the arsenic diffusion in this temperature range is such that best results will be obtained either in 0₂ ambient for 15 minutes or in N₂:0₂ 1:1 for 30 minutes. Table I lists the sheet resistivities and junction depths which will be obtained with EMS 3739N. In the processing of 3" and larger diameter wafers, it is necessary to minimize the warpage of the wafer due to thermal stresses. It is common practice to insert the wafers into the furnace hot zone at some temperature less than 900^oC (e.g.800^oC) and to either remove the wafers slowly from the hot zone after diffusion or to reduce the hot zone temperature to 800^oC before removal. Some small from these listed values of resistivity will be realized depending upon the thermal response of the furnace. The film thickness observed on a flat monitor wafer usually included with the diffusion run, should be in the range of 1700-2200 angstroms, depending upon the spin speed of coating, the ambient atmosphere and the time in the furnace hot zone. The film will consist of and SiO₂ layer between the silicon and dopant film. This SiO₂ layer results from the oxidation of the silicon in the oxygen-rich ambient. The thickness of this layer will be in the range 300-500 angstroms, again depending on diffusion conditions. The remainder of the film will consist of the of the arsenic-rich dopant film. The index of Refraction of the arsenic-rich film is 1.45 to 1.47, close to the Index of Refraction of a thermal oxide.

Deglaze: After diffusion the arsenic doped layer is removed by etching in either dilute HF solution or in buffered HF. The etch rate of the arsenic doped film is 6.0 angstroms/sec. in 5% HF solution at 25^oC; the thin thermal oxide layer between the source and the silicon etches at a rate of approximately 2.9 angstroms/sec. in a 5% HF solution. Total removal of the doped silicon will be indicated when the etching solution does not wet the silicon. One will observe with EMS 3739N that the film will always etch away cleanly leaving no residue.

EMULSITONE SOLUTION 3739B

EMS 3739B has been developed to reduce, if not eliminate, common difficulties attendant with arsenic diffusion to produce buried layers. The problem with the formation of rosettes or other surface pitting (usually observed in the thermal oxide with spin-on sources, or in the silicon with solid sources) has been eliminated. The surface pitting problem has been obviated even for long-term one step diffusion processes where the source is not deglazed from the wafer after deposition. This has been achieved without reducing the film thickness to such an extent that normal processing variations introduce large variations in the sheet resistivity.

Process Recommendations

Wafer Cleaning: No unusual wafer treatment is required after photo etching other than the common practice in the industry.

Spinning: The same considerations as described above for the spinning of EMS 3739N apply in this instance. Either static or dynamic coating may be employed. Care should be taken that an excessively thick film is not deposited, especially in the case of dynamic spinning. When applied at 3000 rpm for 5-10 seconds of spinning and baked at 100^oC for 15 minutes in air, the film thickness should be in the range of 900 angstroms for best results.

Baking: Baking after spinning is optional; the same considerations apply as with EMS 3739N.

Diffusion: Diffusion is carried out in oxidizing ambients. Table II lists the sheet resistivities and junction depths which will be obtained for various temperatures, times and ambient atmospheres. Some small variation from these results will be observed depending upon the thermal cycle, i.e., wafer insertion into the hot zone at 800^oC. with ramping to the diffusion temperature, etc.

SAFETY CONSIDERATIONS

Both EMS 3739 and EMS 3739B contain arsenic and as such should be handled with care. Obviously one should avoid getting the solution in the mouth or in the eyes. Carefully testing has established that there are no volatile arsenic or arsenic compounds such a arsine or arsenic trichloride evolved from the solution during spinning or baking at 200^oC. Spills may be washed away with methanol or water methanol mixtures. Spinning bowls and other equipment may be cleaned by soaking in methanol. If any glassy remnants are to be removed, these may be dissolved in methanol with a few drops of HF.

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