Optical surface roughness evaluation of phosphorus-doped polysilicon films

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Abstract

The surface roughness of phosphorus-doped polycrystalline silicon (polysilicon) film is widely believed to be related to its electrical property. In this work, the roughness of polysilicon films prepared in situ under varied processing conditions, is determined using an optical technique that is based on measuring the spectral absorbance of specularly reflected light. The roughness measurements attained are found to follow the logical trend of roughness anticipated from phosphorus-doped polysilicon prepared under controlled variations of temperature, pressure and phospine/silane flow ratio.

Introduction

Polycrystalline silicon (polysilicon) films are widely used in the fabrication of integrated circuits. Films deposited by the low-pressure chemical-vapor deposition (LPCVD) technique are popular due to their relative low deposition temperature, excellent uniformity, good step coverage, and superior cost effectiveness. During the deposition process, polysilicon films are doped for uses such as metal-oxide semiconductor (MOS) plates, load resistors, ohmic contacts and capacitor plates [1], [2]. A common dopant in use is phosphorus.

A recently reported literature revealed positive relationships between surface structure and electrical properties for phosphorus-doped polysilicon films [3], [4]. Since it is not possible to determine the electrical properties of polysilicon films during processing, this portends a feasible means of in situ quality monitoring via surface structure evaluation.

Much of the literature reported to date on surface structure evaluation of polysilicon films, however, appears to be based primarily on the use of either transmission electron microscopy (TEM) or scanning electron microscopy (SEM). Measurements made using TEM and SEM are limited to grain size determination alone; notwithstanding the high imaging resolutions, that both visualization techniques are capable of. In general, surface structure is better described using roughness as it provides a measure of the topographical contour of a surface.

Although there are various methods available to measure roughness, perhaps the most commonly known one is the mechanical profilometer. With this instrument, the surface is lightly and directly traced by a narrow diamond stylus. Despite the usefulness of the mechanical profilometer, it suffers two major drawbacks:

Optical techniques, alternatively, do not suffer from such limitations. Measurement of the spectral content of reflected light from a surface is one promising approach to characterize roughness. These measurements are based on theories of light-surface interaction developed initially by Davies [5] as well as Beckmann and Spizzichinno [6].

The spectral content of light is typically determined using an instrument known as a spectrophotometer. Surface roughness measurement using spectrophotometers was arguably first accounted for in the mid-1970s by Abdulkadir and Birkebak [7]. An adaptation of the technique was later reported in a technical note to evaluate the surface roughness of polysilicon films [8]. Despite this, no later work — to the best of the authors’ knowledge — has so far been reported to characterize polysilicon surface structures using the technique. The scarcity in adoption may be due to

Clearly, there is a need to investigate the suitability of the spectral technique in polysilicon surface roughness evaluation given the possibilities presented. In this work, the peculiarities of in situ phosphorus-doped polysilicon surface film structures are investigated in relation to their important processing conditions. These include the deposition temperature, pressure, and phosphine/saline flow ratio applied in the reactor chamber. In all cases, the spectral reflectance technique was used to evaluate surface roughness.

Section snippets

Spectral reflectance measurement of polysilicon surface roughness

The theoretical models used to determine surface roughness are based on the assumption that the normalized bidirectional reflectance consists of a component contributed by specular (coherent) reflectance and another contributed by diffuse (incoherent) reflectance. Mathematically, Davies’ theory expresses the ratio of spectral reflectance of a material to its polished complement byRR0=exp−4πcosφσλ2+32π3m2σλ4cos3ψΔω,where λ, φ and Δω indicate the wavelength, angle of illumination and solid angle

Experimental

A vertical low-pressure chemical-vapour deposition reactor was used to deposit the phosphorus-doped polysilicon. The reactor had five individual heater control zones along the tube. Wafers were seated in a silicon carbide boat which allowed the wafers to be uploaded into the reaction chamber. During deposition, both silane (SiH4) and phosphine (PH3) gases were released under controlled conditions. Phosphine adsorps onto the surface via the loss of hydrogen orPH3→P+3/2H2.At the same time, the

Results and discussion

Fig. 1, Fig. 2, Fig. 3 give the plots of absorbance against (1/wavelength)2 phosphorus-doped polysilicon prepared under variations of temperature, pressure and phosphine to silane gas ratio, respectively. Overall, it could be seen that the trends of the graphs are approximately linear. Nevertheless, it is interesting to note that the distributions related to variation in pressure (Fig. 2) possess identical slopes but different offset values. This behavior quite clearly contradicts the relation

Conclusion

The surface roughness of phosphorus-doped polycrystalline silicon films produced under variations of temperature, pressure and phosphine–silane gas ratio was evaluated using an optical technique based on measuring the spectral absorbance of specularly reflected light. Linear graphs of absorbance against (1/wavelength)2 was uncovered for samples prepared under variations of temperature, pressure and phosphine to silane gas ratio. The offset values found in these graphs were accounted for by the

Acknowledgements

The support rendered by Mr Tai Kai and the management of Tech Semiconductor Singapore in the course of this work is greatly appreciated.

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