15th Annual Microelectronic Engineering Conference, 1997

The Design of an Inorganic BARC

Andrew VeterMicroelectronic Engineering

Rochester Institute of TechnologyRochester, NY 14623

Abstract — A methodology was arrived at for thedesign of an inorganic bottom antireflective coating(BARC). The design methodology consisted of fourparts. First, a material compatible with IC processingwas chosen. Second, simulation was performed todetermine the optimum optical properties of thematerial, where the materials extinction coefficient,and film thickness were varied to produce zerosubstrate reflectivity. Third, the stochiometry of thematerial was varied through experimentation toproduce a film with the index of refraction andextinction coefficient as close as possible to thesimulation results. Fourth, the results of step threewere used in simulation to re-determine the filmthickness required to produce zero reflectivity at thesubstrate. The methodology was implemented and aSixNy BARC was designed for RIT’s G-Line (436nm)process. The applicability of the design method to anyexposure wavelength is demonstrated by designing anI-Line BARC concurrently. The film conformality oforganic and inorganic BARCs were compared.

I. INTRODUCTION

As the semiconductor industry continues itsdrive towards higher and higher packing densities,manufacturers are continuously confronted with new andinteresting challenges. These challenges have presentedthemselves not as theoretical barriers or physicallimitations, but as high volume manufacturing issues. Forinstance, it has been demonstrated that devices on theorder of SOnm can be created under laboratory conditions,but the feasibility of manufacturing these devices costeffectively and in high volumes has yet to be determined.

One area that has drawn significant attention inrecent years is photolithography. Although exotictechnologies exist to define patterns well into the deepsub-micrometer regime, the cost associated with theseadvanced technologies, due to capital investment andthroughput issues, is staggering. It is no wonder then thata large amount of resource has been expended inextending the lifetime of conventional lithographictechnologies.

One common way of doing this is through theuse of a Bottom Anti-Reflective Coating (BARC).

Currently, the most commonly employedlithographic strategies involve the projection ofelectromagnetic radiation onto photosensitive materials,using step and repeat systems. These systems, calledsteppers, are tailored to expose photoresists at the specificwavelengths to which they are sensitive. A BARC is athin layer of some reflection suppressing materialdeposited between a photoresist and its underlying layer(substrate). It is designed to absorb light that penetratesthrough the resist and is usually tailored to the specificwavelength of the exposing radiation. It is important thatany penetrating light be absorbed because a certainpercentage of light incident on an interface or topographicfeature will be reflected. This is extremely undesirable,especially when imaging critical layers such aspolysilicon gates, due to its severe impact on patternresolution.

BARCs have been used to enhance patternresolution for many years in the IC industry.Traditionally, BARCs have been organic compositionsthat are generally easy to design, easy to apply, and caneliminate reflections entirely. Unfortunately, as devicedimensions are scaled into the deep sub-micrometerregime, these types of BARCs can no longer perform tothe increasingly stringent requirements placed on them.This is mostly due to the fact that organic BARCs areapplied via a spin-coat method that causes variation in thefilms’ thickness across a wafer. Currently, the best spin-coat processes boast variations of around ±50A.Unfortunately, a difference of 50 A can increasereflectivity by several percent and it is commonlyunderstood that reflectivity must remain below 1% forlinewidth control, especially when imaging critical layerssuch as at poiy gate definition.

It is because of these thickness variations thatother solutions are becoming the focus of considerableattention, the most promising of which are inorganicBARCs. Inorganic BARCs differ from organic BARCs inthat they can be sputtered or CVD deposited onto a wafer,both of which are conformal depositions. This eliminatesthe thickness variation effects of organic BARCs. Ingeneral, inorganic BARCs are also cleaner films and insome cases can be left behind after processing. Although

Veter, A: The Design of An Inorganic BARC 15th Annual Microelectronic Engineering Conference, 1997

they suffer from longer throughput times and requiremore expensive deposition equipment, they are more thanadequate as a replacement for conventional organicBARCS in the deep sub-micrometer regime.

This report details the design of an inorganicBARC. The design is shown to be a straightforwardexercise using the Prolith® lithography simulator, anellipsometer with WVASE32 interpretive software, aPerkinElmer spetrophotometer, and a PerkinElmer 2400Sputerer. Although the process is developed for G-line(436nm) technology, an i-line (365nm) equivalent isdeveloped to demonstrate the versatility of this designprocess. The conformality of films are qualitativelycompared between organic and inorganic compositions.

II. THEORY

Reflections from the resist/substrate interface can havemultiple consequences, including linewidth variation andnotched or uneven profiles. Figure I demonstrates thevarious mechanisms by which reflections occur.

The main reflection phenomenon of interest is theproduction of standing waves in the photoresist layer.These standing waves occur as reflected light interfereseither constructively or destructively with incomingradiation, causing a periodic intensity distribution withinthe film. During exposure of the photoresist, the lateralintensity variations in a projected image must competewith the vertical intensity standing wave variation. Duringsubsequent development of the exposed photoresist film,the development action rapidly proceeds laterally alongconstructive interference nodes, while it is slowlyproceeding vertically through the destructive interferencenodes.’ Thus, standing waves will have an effect on edgeprofile and linewidths. Figure 2 shows a resist line thatwas produced by disabling the post-exposure bake inRIT’s standard lithography process. The standing waveintensity profile is clearly discernable along the lines’sidewalls.

The problems particularly associated withimaging on topographical structures include resistthickness variations due to the spin-coat process oversteps, standing wave interference, and scattered lightreflections, all of which cause exposure variations withinthe resist film. BARC under a resist film eliminates the

discordant effects of resist thickness variations andreflections.’

Figure 3 shows a resist line that was processedidentically to the one in Figure 2, except that it waspatterned on top of an organic BARC, Brewer® XLT.Notice the complete lack of a standing wave profile. Ingeneral, the addition of a BARC facilitates thesuppression of substrate reflections which leads toimproved resolution. Also, a focal depth improvementand an increase in aberration tolerance will occur.

Figure 2: StandingWave

The design of a BARC considers three basic parameters.These are the index of refraction, the extinctioncoefficient, and the film’s thickness.

The conventional ontical equation;

(1)

is a real expression. When considering a materialsproperty the complex quality described by its absorbancemust also be taken into account. The complex index ofrefraction is;

*ik (2)

where k is the materials extinction coefficient. Theextinction coefficient is determined from a materialsabsorbance (a) as;

a = —LN(Trans.) = ~ (3)

-~O5335 e.3r~ X5.9~ 5. I~

/ .~J•

Figure 3: Resist LineOver BARC

Figure 1: Film and Topographic Reflections

V

Veter, A: The Design of An Inorganic BARC 15th Annual Microelectronic Engineering Conference, 1997

Therefore, the common equation for reflectivity ismodified and can be expressed as;

R (fll—fl2)2 +k2(fll+fl2)2 +k2

Where ni is the index of refraction for a photoresist andn2 is the index of refraction for an underlying film.”

A BARC is designed so that any lightpenetrating through the resist is transmitted mto theBARC where it is completely absorbed. The conundrumis that for zero reflectance at the resist/BARC interface,the extinction coefficient must be zero. Obviously, abalance must be struck. Conventional spin-on BARCsutilize very low extinction coefficients on the order of .25and rely on the relative thickness of the film (.—2000A) tocompletely attenuate transmitted light. The design of aninorganic BARC, however, is complicated by the fact thattheir thicknesses are in the 200A to 900A range. Anothermethod of ensuring a minimum reflectance, other thanattenuation, must be achieved. For an inorganic, or as it isoften described, dielectric BARC, the other method istotal phase shift cancellation11. Any light that enters theBARC must be reflected with equal amplitude butopposite phase from the BARC substrate interface.Therefore, a properly designed inorganic BARC requiresthat a balance is achieved between the index of refraction(n), the extinction coefficient (k), and the film thickness(t).

Obviously, for phase cancellation the uniformityof the thin film thickness is paramount. Figure four andfive demonstrate the conformality of a spin-coat BARCand a sputtered BARC respectively.

III. DESIGN

The design of an inorganic BARC is a verystraightforward procedure once the theory is understood.The design developed in this report consists of four parts.

First, a material compatible with IC processing ischosen. The material of choice for this experiment was anSixNy composition. Si3N4 is the common film used inLOCOS isolation and deposited films of these types arefairly well known. Further, these films are easily formedby simply varying the nitrogen flow of a silicon sputterer,such as the PE2400 of this report. Second, simulation isperformed to determine the optimum optical properties ofthe material, where the materials extinction coefficientand film thickness are varied to produce zero substratereflectivity. Varying all three factors would undulycomplicate the simulation and n is initially assumed to beinvariant at 2.1. This is allowable, as a wide variety of n-kcombinations will achieve the desired result of 0reflectivity, the only affect being a change in theappropriate film thickness range. Therefore, the initialfocus is on obtaining an acceptable extinction coefficient.The simulator of choice at RIT is Prolith® by ChrisMack. Figure 6 and 7 were generated from the Prolith®simulator.

Third, the stochiometry of the material is variedthrough experimentation to produce a film with anextinction coefficient within the acceptable rangedetermined through simulation. The only factor varied forthis design was the nitrogen flow during the siliconsputter deposition. Evaluation of the films opticalcharacteristics was performed using the WVASE®software, which is an optical data analysis tool used inconjunction with any optical data-producing tool. In thiscase, an ellipsometer was used as the data producer.When using an ellipsometer, the experimental dataobtained must be fit to a model, with the fit parametersproviding the desired information”. WVASE® is aparticularly powerful tool in extracting optical properties.

Fourth, the results of step three are used insimulation to re-determine the film thickness required toproduce zero reflectivity at the substrate.

IV. DATA AND RESULTS

The initial simulation showed that there was afairly large range of acceptable extinction coefficients.The range for both G and I line is taken to be —.3 to .8.Although extinction coefficients below .3 are useable, thefilm thickness necessary quickly becomes very large. The.3 value is taken relatively arbitrarily. For a specificprocess, acceptable film thickness ranges should be wellknown and explicit.

Experimentation was performed by sputteringsilicon and varying the nitrogen gas flow. Data was

(4)

:.~ ~

Figure 4: BrewerXLT Over Si02

Figure 5: SputteredSiNx Over S1O2

Veter, A: The Design of An Inorganic BARC 15th Annual Microelectronic Engineering Conference, 1997

extracted using the WVASE32 and the measurementsshowing the least error are shown in Table 1. These filmsalso show extinction coefficients within the acceptable .3to .8 range.

EXPERIMENTAL DATA ExtRAcTioNSample VET2 VET5

Wavelength 365nm 436nmArgon 28sccm 28sccm

Nitrogen 3.Osccm 3.5sccmPower 500W 500W

n 2.619 2.751k 0.647 0.344

t(Ano~ 1514.6 1296.8

Table 1: Experimental OpticalConstants

It is important to note that the sputter conditionsbetween the two samples cannot be compared because thesputterer underwent extensive repair between runs.

Once the optical parameters were extracted, thevalues were used in simulation to re-determine the filmthickness required to produce zero reflectivity at thesubstrate. Figure 6 and 7 are the results of thatsimulation.

The simulation demonstrates essentially zerosubstrate reflectivity at discrete film thicknesses. Table 2contains these values and the range in which the substratereflectivities remain below 1%.

IV. CONCLUSION

The design of inorganic BARC was shown to bea straightforward exercise. Four steps were proposed forthe design, including the selection of an appropriatematerial, initial simulation to determine the range ofusable extinction coefficients, experimentation, andfinally re-simulation using optical parameters extractedfrom experimentation. Both a G-Line and I-line BARC ofSixNy composition were designed using these steps.

V. ACKNOWLEDGEMENT

The author wishes to acknowledge Dr. BruceSmith for his technical guidance and Dr. Santosh Kurinecfor her support.

‘RIT Document Title ARC.MEM, VAXD, AccountKHHEMC, Directory EMCR575

11Smith, Bruce W., Principles of Microlithography111Bencher Ngai, Roman, Lian, Vuong, “Dielectric

antireflective coatings for DUV lithography” Solid StateTechnology, p.109, March 1997

IV J~ A. Woollam Co., “A Short Course in Ellipsometry”,

Guide To Using WVASE32TM

Andrew Veter received hisB.S. degree from theRochester Institute ofTechnology in 1997. He iscurrently employed by DigitalEquipment Corporation inHudson, MA, where he is asustainment engineer in thephotolithography department.

G-Line I-Line(Angstroms) (Angstroms)

Table 2: BARC ThicknessRange

:

Figure 6: C-Line Figure 7: I-Line

BARC THICKNESS RANGE(For Zero Substrate Reflecthiity)

510-370 I 450-290