[2.2]Paracyclophane의 열분해에 의한 poly-p-xylxylene 박막의 증착

김철진, 김의정

울산대학교 화학공학부

Deposition of poly-p-xylylene films by pyrolysis of [2.2]paracyclophane

Chul Jin Kim, Eui Jung Kim

School of Chemical Engineering, University of Ulsan

INTRODUCTION

As the feature scale reduces to the submicron range, line width control over high aspect ratios becomes increasingly difficult. Current materials and technologies are unable to overcome the barrier for the production of the future IC where high speed and packing density are demanded. In addition, as the devices continue to scale down, RC delay, cross talk, and power dissipation become serious performance issues, especially at very high frequencies [1,2]. Currently, silicon dioxide whose dielectric constant is about 3.9 is employed as an interlayer dielectric, but it should be replaced with a low dielectric material to increase the speed of the IC and to reduce the cross talk between wires.

Poly-p-xylylene (PPX) is a vapor-depositable organic polymer [3-6]. The dielectric constant of this material ranges from 2.35 to 3.15 depending on its type. This polymer has an extremely low water permeability and its breakdown voltage is very high. Unlike polyimide, PPX is capable of coating submicron crevice without developing pinholes. Since it is deposited at a low temperature, its thermal expansion coefficient is low and the stress is not high in the film. PPX is also very inert chemically and exhibits good solvent resistance. This is due to the simple aromatic ring structure that forms the bulk of the monomer unit. The conformality of the deposited film is very good even with high aspect-ratio trench structures.

In this work, PPX films were deposited using [2.2]paracyclophane as a precursor by the Gorham method. To better understand the PPX deposition mechanism, we investigated the effects of changing operating conditions on growth rate and film property.

EXPERIMENTAL

The substrate (2cm×2cm) was prepared by cutting 4-inch-diameter p-type (100) Si wafers with a resistivity of 1-30 □-cm with a diamond pencil. Prior to being loaded into the deposition chamber, the substrate was cleansed with H2SO4:H2O2 (vol% 3:1) solution, HF:H2O (vol% 1:7) solution, and deionized water consecutively and was purged with nitrogen (>99.999%) to remove moisture.

The PPX deposition apparatus is described in detail in Ref. 7. The precursor vessel that contained about 0.5 g of the source material was connected to a precursor feeding line. Then the line was evacuated and the feeding valve was closed. The deposition chamber was purged with nitrogen when loading the substrate to keep impurities such as dust, moisture, or oxygen from entering into the chamber. The system was evacuated to a base pressure of 10-3 torr. The diluent gas, argon, was then allowed to flow through a bypass line and the pressure stabilized to a desired value. With argon flowing the susceptor was cooled to a deposition temperature by turning on the circulating chiller. The upstream line was heated above 140℃. The precursor vessel was immersed into the oil bath and it was typically maintained at 110℃. After desired conditions were achieved, the precursor feeding valve was opened to start deposition. At the end of the run, it was shut off to finish deposition.

Atomic force microscopy (AFM) was employed to measure the film thickness. Since it is hard to etch PPX films by a usual etching solution, part of the substrate was masked with a tape before deposition to form a step required for thickness measurements. The dielectric properties of the deposited PPX film were measured on a metal-insulator-semiconductor (MIS) capacitor structure with C-V and I-V techniques.

RESULTS AND DISCUSSION

The base conditions for the PPX deposition experiments performed in this study are: diluent gas flow rate = 50 sccm, precursor vessel temperature = 110℃, precursor decomposition temperature = 700℃, deposition pressure = 1 torr, and substrate temperature = 1℃. We have explored variations of growth rate and film property with the above operating conditions. It was observed that as the deposition proceeded, the color of the substrate surface was changed. The deposition was typically performed for 3 h. The thickness of the deposited film was 3500-12000 Å. The surface of the film obtained from the base conditions was smooth.

The mass flow rate of the precursor entering into the tube reactor to produce PX monomers depends on the diluent gas flow rate, pressure, and precursor vessel temperature. The precursor feeding rate is plotted as a function of diluent gas flow rate in fig. 1a in which the pressure is 1 torr and the precursor-vessel temperature is 110℃. One can see that the precursor feeding rate increases from 0.848 mg/min to 1.12 mg/min as the diluent gas flow rate is raised from 20 sccm to 80 sccm. The reason for this is that the mass transfer is enhanced with increasing diluent gas flow rate, thereby augmenting the amount of the precursor sublimated. However, the concentration of the precursor in the gas phase diminishes due to an increase in diluent gas flow rate larger than precursor feeding rate. Fig. 1b shows the effect of pressure on precursor feeding rate. Here the precursor vessel temperature is fixed at 110℃ and the diluent gas flow rate is 50 sccm. It can be seen in fig. 1b that the precursor feeding rate decreases to about a half value from 1.13 mg/min to 0.593 mg/min as the pressure is increased 8 times from 0.5 torr to 4 torr. As a result, the precursor concentration in the gas phase is increased in spite of a decrease in precursor feeding rate. Shown in fig. 1c where the pressure is 1 torr and the diluent gas flow rate is 20 sccm is the effect of precursor vessel temperature on precursor feeding rate. In fig. 1c, the precursor feeding rate hugely increases from 0.100 mg/min to 0.789 mg/min as the precursor vessel temperature is lifted from 80℃ to 110℃. At a precursor vessel temperature above 110℃ some precursors were found to condense on the upstream process line and the precursor feeding valve causing a serious problem with reproducing the experimental results. Therefore, in our experiments the precursor vessel temperature was maintained below 110℃. From the above results the precursor feeding rate depends most strongly on the precursor vessel temperature. Accordingly, it is crucial to precisely control the precursor vessel temperature to obtain a constant precursor concentration during deposition. In this work, the precursor vessel was immersed into the oil bath to maintain a constant precursor vessel temperature with an accuracy of ±0.2℃.

MIS structures were fabricated by coating a thin aluminum layer on the sample after the PPX deposition by thermal evaporation to perform C-V and I-V measurements. The dielectric constant of the deposited PPX film was found to be 2.7±0.05 and the dielectric strength was in excess of 2[eqn: TIMES]105 V/cm.

The effect of substrate temperature on growth rate is shown in fig. 2. It is important to note that as the substrate temperature is raised from -4℃ to 20℃, the growth rate considerably decreases from 66 Å/min to 16 Å/min. In our experiments, the temperature of the reactor wall was maintained above 100℃, and no deposition was observed on the reactor wall, whereas the deposition took place only around the susceptor which was maintained below room temperature. The dimer is pyrolyzed into PX monomers which condense on the substrate surface and polymerize to produce PPX films. Polymerization proceeds by a free radical mechanism and consists of three steps: an initiation, propagation, and termination step. Polymerization is initiated by two or more PX monomers adsorbed on the substrate surface reacting to form a diradical. In the propagation step, film growth occurs at both sides of the diradicals. The termination step consists of the radicals being buried so far into the film that monomers cannot reach the polymer chain radicals, therefore living polymer still exists throughout the bulk of the film. From the results of fig. 2, the rate-determining step of the deposition of PPX films seems to be the adsorption of monomer onto the substrate surface.

REFERENCES

[1]	L. You, G.-R. Yang, C.-I. Lang, J. A. Moore, P. Wu, J. F. McDonald and	T.-M. Lu, J. Vac. Sci. Technol. A, 11, 3047 (1993).

[2]	P. K. Wu, G.-R. Yang, J. F. McDonald and T.-M. Lu, J. Electr. Mater., 24,	53 (1995).

[3]	W. F. Gorham, J. Polym. Sci. Part A-1, 4, 3027 (1966).

[4]	M. Szware, Polym. Eng. Sci., 16, 473 (1976).

[5]	B. L. Joesten, J. Appl. Polymer Sci., 18, 439 (1974).

[6]	N. Majid, S. Dabral and J. F. McDonald, J. Electr. Mater., 18, 301 (1989).

[7] S. Woong, C. J. Kim and E. J. Kim, Theories and Applications of Chem. Eng., 3(2), 3265 (1997).

Fig. 1. Dependence of precursor feeding rate on (a) carrier gas flow rate,

(b) pressure and (c) precursor vessel temperature.

Fig. 2. Effect of substrate temperature on growth rate.