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김의정

울산대학교 화학공학부

Low Dielectric Constant Materials for ILD Applications

Eui Jung Kim

School of Chemical Engineering, University of Ulsan

INTRODUCTION

As the feature dimensions of the devices continue to shrink, performance and reliability requirements for interconnect technology are more stringent. 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. Lowering the dielectric constant of the interlayer dielectrics can improve IC performance by reducing the RC delay, decreasing the cross talk, and lowering the power dissipation[1,2]. The power consumption of ICs is proportional to the operating frequency and total capacitance. The interlayer dielectrics must also meet the requirements of high surface planarity, narrow gap filling, low deposition temperature and low residual stress. 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 reduce delay time, noise and power consumption. One can consider a variety of low dielectric constant materials for future ILDs. These materials are either organic or inorganic and can be deposited by either CVD or spin-on techniques. Some of the promising low k materials that have widely been studied are discussed in this paper.

LOW k DIELECTRICS

1. CVD polymeric films

Organic polymeric materials have attracted great attention as ILDs since they have intrinsically low polarity molecular structure and thus have low dielectric constants. One of the major concerns for using organic polymers is the thermal stability of the polymer. Another concern is the mechanical strength of the polymers. The diffusion and adhesion of metals to polymer films, polymer film stress, and the thermal stability of the polymer film depend strongly on the decomposition temperature of the polymer[1]. A wide variety of low k polymer films can be deposited by CVD techniques.

Parylene(PA) is a vapor-depositable organic polymer[3-5]. 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, PA 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. PA is also very inert chemically and exhibits good solvent resistance. Because of its superior properties among low k materials, PA and its derivatives have been studied. Joesten[6] reported that the decomposition temperature of PA-F is 530℃, which is 100℃ higher than that of PA-N. In addition to the highest decomposition temperature, PA-F has the lowest dielectric constant(2.35 at 1 MHz) among the PA families. However, it is not easy to employ Gorham's method in order to fabricate PA-F because of the difficulties in making PA-F dimer. You et al.[1] developed a vapor deposition process of making PA-F films by pyrolysis of dibromotetrafluoro-p-xylene.

Recently, the Du Pont corporation has produced new teflon-like fluorinated copolymers which incorporate groups designed to inhibit crystallization. The bulk dielectric constant of this material is as low as 1.9 at room temperature. Nason et al.[7] developed a two step vapor deposition process to produce smooth, uniform and pinhole-free Teflon amorphous fluoropolymer(AF). Although Teflon AF possesses an outstanding combination of physical, electrical and chemical properties, the thermal stability of this polymer is still a major concern for ILD applications.

Lang et al.[8] investigated vapor deposition of polynaphthalene(PNT)-N and fluorinated PNT(PNT-F) from 1,2-diethynylbenzene or 1,2-diethynyltetrafluoro- benzene in a hot-wall reactor. PNT-F was observed to have higher dissociation temperature(about 590℃) and better thermal stability(>530℃)  in comparison with  PA-F. No PNT films crack until annealed at 600℃ in nitrogen. There still remains the challenge of synthesizing potential source materials for this deposition methodology.

The properties of CVD organic films are, in general, not well understood because they depend not only on the source materials and chemistry but also on the deposition equipment and operating conditions used in depositing the film. Table 1 summarizes the properties of CVD polymeric films.

Table 1. The properties of CVD polymeric films

Film	PA-N	PA-F	Teflon AF	PNT-N	PNT-F
Source Material	dimer (solid)	precursor (liquid)	polymer (solid)	precursor (liquid)	precursor (solid)
Dielectric Constant	2.6	2.3	1.9	2.4±0.1	2.3±0.1
Electric Breakdown	2×107V/M	5×107V/M	5×107V/M	3×107V/M	5×107V/M
Dissociation T(℃)	430℃ in N2	530℃ in N2	360℃ in air	570℃ in N2	590℃ in N2
Structure As Deposited	crystalline	crystalline/ amorphous	amorphous	micro- crystalline	micro- crystalline
Cracks	(yes) annealing at 300℃ in N2	(no) annealing at 510℃ in N2	(yes) -15℃ in Air 250℃ in N2	(no) annealing at 600℃ in N2	(no) annealing at 600℃ in N2

2. Spin-on polymeric films

Most of the work on organic dielectric is based on spin-on films. A wide variety of polyimide(PI) films are used as stress relieve layer over the chips and packaging applications. Because of its good local planarization capability, PI has also been used as ILD layer. However, the local planarization capability is no longer an important advantage because a variety of planarization techniques have been developed such as etchback, SOG and CMP. Lee et al.[9] observed in their experiments that the fluorinated polyimde films were thermally stable up to 450℃ and that no detectable moisture was absorbed. Auman[10] suggested fluorinated polyimides and copolyimides as promising candidates for ILDs. They reported that theses materials had low dielectric constant below 3.0, low moisture absorption, very high thermal stability, good mechanical properties, solvent resistance and low in-plane thermal expansion coefficient for low thermal stress.

PTFE(polytetrafluoroethylene) has constituted the lowest dielectric constant polymer(1.9-2.2) with good mechanical strength. It has a low water absorptivity less than 0.01%. However, problems with adhesion, plastic creep, and thermal stability have hindered its use in electronic applications. Since PTFE is insoluble in most organic solvents, a conventional spin coating is quite difficult. Kudo et al.[11] investigated plasma fluorination of hydrocarbon polymer films spin-coated onto Si wafers by exposing NF3 plasma in a microwave downstream plasma system. They found that fluorinated hydrocarbon polymers gave low dielectric constants of 2.0-2.4, which is close to that of PTFE. However, the thermal stability of the fluorinated polymers was inferior to that of the non-fluorinated polymers.

3. Inorganic films

Most low dielectric constant materials development emphasizes new organic polymers but temperature limitations of organic materials are also driving the development of more thermally stable inorganic materials.

One can reduce the dielectric constant of the conventional CVD SiO2 by doping it with fluorine. Homma et al.[12-14] obtained fluorinated SiO2 films by (i) CVD using fluorotrialkoxysilane and water vapor, (ii) liquid phase deposition, and (iii) fluorotrialkoxysilane vapor treatment for a spin-on glass. Liquid phase deposition was found to produce fluorinated SiO2 films with higher moisture resistance than other methods. They claimed that fluorination of SiO2 films is one of the realistic solutions in order to meet the interconnect requirements for the future ILDs. However, the decrease in dielectric constant is relatively small since the amount of F incorporated is limited. One advantage of F-doped SiO2 is its excellent gap filling capability. A main concern is that loosely bonded F will degrade interconnect and device reliability.

Porous silica offers advantages for ILD applications because of its intrinsically low dielectric constant below 2.0 as a result of the extremely high porosities, high temperature limit and compatibility with existing microelectronics processing. Further work is needed to determine if porous silica can be modified to meet the processing requirements for ICs.

SUMMARY

The need for developing low k dielectric materials to meet the interconnect requirements for the future ILDs has been recognized. In order to be useful in the semiconductor industry, new low k dielectric materials must be carefully characterized for their chemical, electrical, mechanical and thermal properties. Also, their impact on process integration, fabrication cost and device reliability must be taken into account.

REFERENCES

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