Electron beam hardening of photoresist

w. R. Livesay, A. L. Rubiales, M. Ross, S. Woods, S. Campbell

Electron Vision Corporation, San Diego, California 92126

ABSTRACT

Electron beam hardening is investigated and compared with conventional thermal hardening on a diazoquinonenovolak (DQN) photoresist. The electron beam hardening is accomplished without significant heating of the resistthereby eliminating resist flow or melting. The electron beam cured polymer is fully cross-linked throughout itsentire thickness (full matrix cure). Thermal stability of the resist versus electron beam dose is examined. Theresults of varying amounts of electron beam dose show that the shrinkage of the photoresist can be reduced almost tozero by sufficient curing. The elimination of shrinkage of the resist also greatly reduces the amount of stress in thecured film. After this electron beam cure, no resist stress or shrinkage is experienced even when the resist issubjected to thermal bakes in excess of 200°C. In fact, thennal stability of better than 400°C has beendemonstrated. The resist shrinkage is eliminated due to the resist being fully cross-linked well below its glasstransition temperature. These fully cross-linked resists exhibit superior performance in plasma processing and yetremain strippable by conventional plasma ashing processes.

1. BACKGROUND

1.1 Thermal hardening

Hardening of photoresist via post baking after image development is performed to strengthen the resist film sothat it can withstand the rigors ofplasma etching and other harsh processes.' Conventional post-baking or hardeningof photoresist is typically a trade-off between resist stabilization and melting. Melting temperature is proportionalto the molecular weight or degree of cross-linking in the film. As the molecular weight is increased, the glasstransition temperature is raised. In DQN resists, melting occurs above 135°C. The postbake heating can cause theexterior of the patterned resist to form an oxidized crust Nitrogen gas evolved from the DQ decomposition istrapped by the crust inducing stress into the film which may cause edge lifting later in wet etching.2 DQN resistsbaked in air at temperatures above 170°C become insoluble due to oxidation and cross-linking. Below thistemperature the resist can be stripped in organic solvents. Although the chemical cross-linking of DQN competeswith physical melting, long slow bakes (120°C for 16-24 hours) in air can achieve a high degree of thermal stability(cross-linking) while minimizing resist flow (oxidation and ketene reactions harden the resist). However,throughput considerations dictate raising the resist temperature rapidly (to increase the rate of cross-linking) whilekeeping it below the glass transition temperature (Tg) to avoid melting the lithographic features.

1.2 Ultraviolet hardening

Radiation (UV) hardening has been a popular technique to harden the outer shell of the resist thereby allowinghigher posthake temperatures to be attained without destroying the pattern image. However, novolak resins absorbstrongly in the ultraviolet, preventing films thicker than 250-300 nanometers to be hardened. Combining UVradiation with hot plate (ramped) baking of the resist allows for a more time efficient process.3'4 A fast cure can beobtained by encapsulating the bulk of the resist (which is being cured by the hot plate) with the UV cured surfacefilm. The bulk resist temperature (ramped from a lower temperature) is kept below the glass transition temperatureof the surface cured film. The process requires careful balancing of the skin cure (it must be uniform) so that it willhold the bulk of the resist from flowing while the hot plate is ramped close to the Tg of the resist. The surface ofthe resist obtains a harder cure (tJV +bake) than the bulk (thermal cure only). The harder cured surface layer whileholding together lithography features, may also encapsulate some of the volatiles and solvents. When the resist islater subjected to temperatures which exceed the postbake temperature (i.e. plasma etching or ion implantation)solvents evolved can wrinkle or crack the brittle surface layer of the resist (reticulation).

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2.0 ELECTRON BEAM HARDENING

The advent of sub-micron and higher aspect ratios in lithography features has intensified the need to eliminatemelting or any flow of the resist in the hardening process as well as eliminating reticulation in subsequent processes.Sevemi years ago an electron beam exposure tool was proposed for curing and hardening photoresist.5'6 This newtype of hardening tool has shown promise in eliminating some of these process problems. In this paper we look atsome of the aspects of electron beam curing and the characteristics of an electron beam stabilized DQN photoresist

2.1 Electron beam interaction with polymers

An electron impinging on a polymer resist can have two kinds of encounters, nuclear and electronic. Nuclearcollisions (Rutherford scattering) are elastic and merely change the direction of the electron. Electronic collisions areinelastic and slow down the primary electron trsnsfemng its energy into excitation energy and kinetic energy ofsecondary electrons. As the electrons slow down from collisions with the polymer molecules they excite and impartenergy to the molecules and ionize them. Secondary electrons are emitted with relatively slow speeds and producemany more ions along the track of the primary electron beam. The impinging electrons have a very large numberof inelastic collisions with electrons of different energy states before they are slowed down enough to be absorbed.

Within a pico—second (1012 )after ionization, molecular rearrangement takes place in the ions and excitedmolecules, accompanied by thermal deactivation or the disassociation of valence bonds.8 The radicals, created bythese broken bonds, form new bonds and reattach themselves to neighboring polymer chains which also have brokenbonds in their side groups. These additional bonds strengthen the molecules against backbone damage and tie smallermolecules together into larger clumps of higher effective molecular weight. Cross-linking continues for higherdoses and is accompanied by the formation of gel and ultimately by the insolubalization of the entire coating.

This extensive cross-linked network of molecules extending throughout the entire coating makes the polymerstable to heat such that it cannot be made to flow or melt. A fully cross-linked polymer is insoluble in all solvents.The resulting polymer coating can still be removed by oxygen plasma etching even though it cannot be removed bychemical stripping methods. Depending on the material, the electron beam curing can improve the plasma etchresistance of the coating but only to varying degrees. The plasma etch resistance of a polymer is mostly a functionof it's basic structure (backbone) and not the degree of cross-linking. However, electron beam curing cansignificantly improve the plasma etch stability of the polymer in eliminating reticulation and melting caused byheating by the plasma.

2.2 Efficiency of cure

Electron beam curing is very energy efficient compared to other processes. Rather than heating entire ovens orhot plates most of the beam energy goes into curing the resist. High efficiency is achieved by assuring that most ofthe beam energy is absorbed in the material to be cured and not in the underlying substrate.6 Thinner resists requireless dose than thicker resists to achieve the same hardening properties. By adjusting the energy of the beam suchthat the range of the incident electrons is nearly equal to the resist thickness provides for an efficient means ofmaximizing throughput while providing a nearly uniform cure throughout the full thickness of the resist.

2.3 Electron beam exposure system

The electron beam exposure tool used for these experiments is specifically designed for electron beam curing ofphotoresists and interlayer dielectrics.9 The electron optical column in this system produces a large collimatedelectron beam which totally covers the sample being processed up to 200 mm in diameter. This electron beam tool,further described elsewhere,'0 utilizes a cold cathode which is impervious to the gaseous products evolved from theresist and allows it to operate in a soft vacuum environment (1-5 Pa).

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2.4 Radiation and charging effects

In conventional electron beam exposure systems (SEMs, etc. ) surface charging by the eleciron beam is aproblem for insulating samples. However, by operating in a soft vacuum environment electhcal charging of thesample does not occur. The induced charge at the surface of the sample is neutralized by positive ions generated bythe electron beamS This allows exposure of insulating samples without the requirement of a conductive coating.Also the electrical charge injected into the interior of an insulating coating is diminished or drained by electron beaminduced conductivity (EBIC) created by the curing beam. This allows the charge to be drained by positive ions onthe surface of the sample. The effect on circuit devices (oxide gates, etc.) by the high dose of electron beam curing(typically lOx to 20x higher than E-beam direct write exposure doses) has not been determined. Researchers havereported both beneficial and deleterious effects of e-beam exposure on semiconductor devices."2'13 E-beam curingis being used today in areas where it is followed by an annealing step in a subsequent process or on substrates withno active devices (i.e. thin film heads, multichip modules, etc.), or where the active device is protected by anotherlayer. No exhaustive study has been done to determine the effect on unprotected oxides.

3.0 PROPERTIES OF ELECTRON BEAM CURED PHOTORESISTS - EXPERIMENTS

3.1 Thermal stability

A diazonapthoquinone novolak positive photoresist, Nova 2071 (40% solids)'4 was tested for thermal stabilityafter electron beam curing. Similar tests have been run on other DQN resists. The results presented here on Nova2017 are representative of the effect electron beam curing has on DQN resists in general. Nova 2071 (40%) is usedfor single spin-on application of 3-10 un thick resist layers. Thicker resists, in general, are more prone to in-planepattern distortion via melting-than thinner resists. Melting or pattern distortion can occur for temperatures above125°C with this resist using conventional thermal hardening. For these electron beam curing tests, wafers werecoated, prebaked, patterned, and developed (see Tables 1-5). They were than cured via electron beam at variousdosages. After electron beam curing the samples were baked at various temperatures, cleaved and their edge profiles

photographed (SEM). Figure 1 through Figure 5show the thermal stability imparted by the electron beam curingprocess. In Figure 1, a wafer (control) without an electron beam cure (dose=O) is subjected to a series of postcurebakes. The resist sidewall profile for a 5 pinfeature is shown before baking (Figure la). In Figure lb the samefeature is shown after a 130°C bake for 60 seconds (this temperature is 5°C higher than the recommended postbaketemperature for this resist). Note the slight bulging of the sidewall indicating the onset of melting of this resist. At180°C (Fig.lc) and higher temperatures (Fig. id and Fig. le) the resist bulges then flows. Figure 2 shows the effectof the same postcure bakes but on a sample that has received an electron beam dose of 500 pC/cm2. As shown byFigure 2b through Figure 2e this relatively low dose has stabilized the resist except for the two hour 225°Cconvection oven bake. With this longer bake a slight bulge is seen at the top of the feature with the sidewallshowing no deformation. With higher cures (Figure 3a-e: dose =1000 Uiii2 and Figure 4a-e: dose =2000j.unC/cm2) there is no detectable resist distortion or flow at all bake temperatures. Note how the standing wavepattern created by the optical lithography step is preserved in all of the electron beam cured cases. Further curingeven at a dose 20 times that required to thermally stabilize the resist, shows no ill effects in the resist profiles(Figure 5a-e) . Thus, even on very heavy doses where the resist is cured to a hard plastic state, no change in resistwall slopes or critical geometries is experienced.

Without deleterious effects caused by overcure, process windows can be widened, thereby enhancing processrepeatability and yield. Further, the uniformity of the exposure source (which is so critical in a UV/skin bakeprocess) is greatly relaxed. The degree of cross-linking reaches an asymptotic level such that any nonuniformity inthe curing beam (across the wafer) can be eliminated by simply increasing the dose.

3.2 Solubility vs. dose

When a DQN photoresist is irradiated with an electron beam at low dose, chain scissions predominate,increasing the solubility of the resist. As the dose is increased, cross-linking predominates making the resist

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insoluble. Exposures were made on Nova 2071 diazonapthoquinone novolak (DQNIn)resist (- 4 jim thick) coatedon bare silicon wafers. The solubility of the resist as a function of exposure dose was evaluated by immersing theexposed samples in undiluted developer (Morton DE-3 O.5N) and monitoring the development rate. Developmentrate was determined by measuring resist thickness remaining over a range of development times. Figure 6 shows thesolubility vs electron beam dose. In a separate set of tests, the resistance to organic solvents was evaluated. Formore aggressive strippers (i.e. acetone based) dose levels of 2000-4000 pC/cm2or greater are required to make theresist insoluable. At these and greater dose levels the resist is inert in wet solvents and hot strippers. It is surmised,at these higher dose levels the cross-linked matrix of polymer chains is so dense that openings in the film are toosniall for solvent molecules to penetrate. The amount of dose required for a given process is dependent on the resistcharacteristics desired and the thickness of the resist. For example a process may require that the resist hold up to anetching step or a plating bath and then be removed in a wet stripper (as opposed to plasma ashing). In Figure 7 thethermal stability and solubility are plotted vs dose. A hypothetical process window established by a minimum

solubility requirement (dose '8OO .tCIcm2)and a minimum resist thermal stability of 1800 C (dose 6OO j.Lcicm2)is shown by the vertical lines in Figure 7. If the entire resist layer is hardened uniformly than it is possible toselect a dose within this process window that either optimizes solubility or optimizes thermal stability, In aUV/skin bake process there are large differences in the degress of hardening throughout the resist layer. In denselithographic areas there is a greater ratio of resist surface to bulk. In a UV skin/bake, the surface (and areas close tothe surface) are cured to a higher level than the bulk of the resist. Therefore, the denser lithographic areas have aharder cure than those areas with fewer features. if the difference in resist hardness is larger than the process windowthen a portion of the resist may have the thermal stability but not be removable via a wet solvenL Another portionof the resist may be removable via wet stripper but not hold up to the thermal process. Electron Beam curing canobtain a uniform cure throughout the entire resist layer. Therefore with E-beam curing one may work at eitherextreme of the process window to optimize for either ease of removal or thermal stability. (i.e. if the resist isuniformly cured at a lower level of cross-linking it is much easier to strip in subsequent operations).

3.3 Resist shrinkage and stress

Shrinkage occurs during resist post baking due to the evolution of diazoquinone and solvents not evolved in theprebake . Since the resist layer is constrained by the substrate in two dimensions, shrinkage of the film only occursin the vertical dimension. A 15% - 18% resist thickness loss can occur during baking due to the evolution ofsolvents, diazoquinone, water, nitrogen, hydrogen etc. Baking of the resist also induces stresses in the films due tothe mismatch of thermal expansion between the resist and the substrate. Stress induced in thermal curing has beenfound to be independent of the coating thickness and solution concentration.'5 Narrow molecular weight polymers(usually required for high contrast development) without added plasticizers can become brittle and crack High bakesmay induce internal stresses in DQN resists to cause edge lifting in aqueous etchants. The edge lifting, due tointernal stresses, can cause severe undercutting of small resist islands.

Resist shrinkage is greatly reduced in electron beam curing. Film thickness measurements made before and afterE-beam curing indicate little or no shrinkage of the resist. Measurements on Nova 2071 electron beam cured resistsshowed zero to less than 2% resist thickness loss (within experimental error) as compared to 15% - 18% resistthickness loss using thermal curing. Induced stress in electron beam cured films have shown similar improvements.Measurements made after electron beam curing and during subsequent baking have also shown little or no stress inthe cured films. To evaluate resist shrinkage after E-beam cure, DQN resist coated samples were exposed at varyingdose and subsequently baked at high temperature. Figure 8 shows the amount of shrinkage of a DQN based resist(2.3 pin thick original thickness) as a function of dose when postcured baked at 225°C. Note that dose levels of2000-4000 jiC/cm2are required to minimize shrinkage. It is believed that the shrinkage which occurs belowexposure doses of 4000 iC/cm2 is due to the expulsion of solvents or other volatiles not yet evolved (or not brokendown and cross-linked into an unreactive component of the film). For doses below 4000 j.tCIcm2 components inthe solvents may react with some etchant gases (in certain processes) to create surface films which would necessitatean additional descum process. Therefore, for resist films that will see very harsh process environments or are requiredto become permanent structures and remain inert, an electron beam curing dose of at least 4000 ixC/cm2 appearsnecessary. With little or no resist shrinkage using electron beam curing, the possibility exists for eliminating somesteps required to planarize resist that has shrunk unevenly over circuit features of differing elevations.

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4.0 CONCLUSIONS

Electron Beam cunng/har&iñng ofDQN photoresists was investigated and compared with conventional thennalcuring. Electron Beam hardening of photoresist can attain distortionless stabilization of DQN resists by utilizingfull matrix cross-linking of the resist. The thermal glass transition temperature can be raised to over 400°C withoutresorting to long slow bakes. Thermal stability can be improved which preserves lithographic features. Solubiityof the resist is dose dependent which means etch resistance can be determined by needs of downstream processes.Little or no shrinkage is experienced in the cured resist layer providing a means of improving planarization on waferswith large topographical differences. The elimination of shrinkage also reduces the stress in the film even forsubsequent thermal processes. In many applications E-Beam curing of resist provides improved performance overtraditional curing methods.

5.0 ACKNOWLEDGEMENTS

The authors gratefully acknowledge the contributions of the following individuals whose support made thispaper possible: Tom Eichenberg and Ken Bell of Morton International for helpful discussions and the thennalstability micrographs and processing, Tyler Curtis of AMCC for thin film measurements.

6.0 REFERENCES

1. W. Moreau, Semiconductor Lithography, 545,Plenum (1988)2. H. Vanazawa, N. Hasegawa and K. Donta, J Appl. Polym. Sci. 30,547(1985)3. J. C. Matthews, J. Wilmott, SPifi Proc. 470, 194 (1984)4. U.S. Patent 4,548,688 (1985) Fusion Semiconductor Systems5. P. Burggraaf, Semiconductor International, P.89 April 19876. W. Livesay, Radtech '90, Pg 195. 19907. A. Chapiro, Radiation Chemistry of Polymeric Systems, 42, Wiley (1962)8. F. Bilhneyer, Texthook of Polymer Science, P.372, Wiley (1971)9. ElectronCure 3Oxfl&200A manufactured by Electron Vision Corporation.

10. To be published - Symposium on Electron Ion and Laser beams -June 9311. J. Dunn, B. J. Gross, C. G. Sodmi - IEEE Trans. on Electron Devices, Vol. 39 No. 3 P.677, March 199212. J. M. Aitken, C. Y. Ting, IEDM, December 198113. M. Peckerar, R. Fulton, P. Blaise, D. Brown, R. Whitlock, J.Vac. Sci. Technol. 16(6) Nov.IDec. 197914. K. Bell, N. Acuna, S. Dixit, R. Lazuarus, G. Talor, SPIE Vol. 1672, 597 (1992)15. S. Croll, J. Appi. Polym. Sci. 23, 847 (1979).

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Table 1

Resist:

Film Thickness:

CD:

Softbake:

Exposure:

Develop:

Resist Hardening:

Morton-NOVA 2071 (40%)

3.7 microns

5 microns

85°C/i 10 seconds120°C/hO seconds

360Ultratech 1000 (NA=0.315)

Immersion (1 Minute)DE-4, 23°C

None - Control Wafer

Figure id. 230°C160 Seconds Figure le. 225°C/2 Hours Convection

Figure 1 Resist thermal stability zero dose

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Figure lb. 130°C/60 Seconds Figure 1 c. 180° C/60 Seconds

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Morton-NOVA 2071 (40%)

3.7 microns

5 microns

85°C/i 10 seconds120°C/i 10 seconds

360 mi/cm2Ultratech 1000 (NA=0.315)

Immersion (1 Minute)DE-4,23°C

500 pC/cm2 @ 15 keVElectronCure 30X-200A

Table 2

Resist:

Film Thickness:

CD:

Softbake:

Exposure:

Develop:

Resist Hardening:

Figure 2a. No bake

Figure 2b. i30°C/60 Seconds Figure 2c. 180°C/60 Seconds

Figure 2d. 230°C/60 Seconds Figure 2e 225°C/2 Hours Convection

Figure 2 Resist thermal stability after 500 pC/cm2 dose

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Morton-NOVA 2071 (40%)

3.7 microns

5 microns

85°C/i 10 seconds120°C/i 10 seconds

360 mJ/cm2Ultratech 1000 (NA=0.315)

Immersion (1 Minute)DE-4,23°C

1,000 .tC/cm2 @ 15 keYElectronCure 30X-200A

Table 3

Resist:

Film Thickness:

CD:

Softbake:

Exposure:

Develop:

Resist Hardening:

Figure 3a. No bake

Figure 3b. 130°C/60 Seconds Figure 3c. 180°C/60 Seconds

Figure 3d. 230°C/60 Seconds Figure 3e. 225°C/2 Hours Convection

Figure 3 Resist thermal stability after 1,000 RC/cm2 dose

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Resist:

Film Thickness:

CD:

Softbake:

Exposure:

Develop:

Resist Hardening:

Morton-NOVA 2071 (40%)

3.7 microns

5 microns

85°C/i 10 seconds120°C/i 10 seconds

360 ipJ/cm2Ultratech 1000 (NA=0.315)

Immersion (1 Minute)DE-4,23°C

2,000 .tC/cm2 @ 15 keVElectronCure 30X&200A

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Table 4

Figure 4a. No bake

iIpI7 5KV X15888 h'i 1D19

Figure 4b. 130°C/60 Seconds Figure 4c. 180°C/60 Seconds

Figure 4d. 230°C/60 Seconds Figure 4e. 225°C/2 Hours Convection

Figure 4 Resist thermal stability after 2,000 iC/cm2 dose

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Table 5

Resist:

Film Thickness:

CD:

Softbake:

Exposure:

Develop:

Resist Hardening:

Morton-NOVA 2071 (40%)

3.7 microns

5 microns

85°C/lb seconds

120°C/lb seconds

360 mi/cm2Ultratech 1000 (NA=0.315)

Immersion (1 Minute)DE-4,23°C

20,000 p.C/cm2 @ 15 keVElectronCure 30X&200A Figure 5a. No bake

Figure 5b. 130°C/60 Seconds Figure 5c. 180°C/60 Seconds

Figure Sd. 230°C/60 Seconds Figure Se. 225°C/2 Hours ConvectionFigure 5 Resist thermal stability after 20,000 p.C/cm2 dose

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C)

EC

.00(I)

0

Figure 6. Solubility vs. dose

i3O0o 250. 200

1500

C,)

50

Resist: Nova 2071

Develop: DE-3 0.5N

Exposure: ElectronCure 30 System

Figure 7. Solubility & Tg vs. dose

1

160ci)0)12•E 10C,)

C,)0)4.2

500 100015002000250030003500 4000Dose (p.C/cm2)

Figure 8. Resist shrinkage vs. dose for postcure 225°C bake

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100 200 300 400 500 600 700Dose (RC/cm2)

0 500 1000150020002500300035004000Dose (pC/cm2)

0

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