524 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, NO. 4, APRIL 1982

resistance to a strongly soluble developer, and good thermal properties due to their crosslinking structure. These con- tribute to high electron sensitivity of 5 pC/cm2, ?(contrast) = 2.0 on the sensitivity curve, and finer resolution than 0.2-pm line patterns (65 pC/cm2). A good adhesion of @-MAC’S to the substrate is confirmed, which is important in microfabrication. The carboxylic acid groups in @-MAC’S are responsible for their adhesion. @-MAC’S have excellent dry-etching durabilities, e.g., 2 times stronger than PMMA in reactive sputter etching.

@-MAC’S are expected to be a suitable resist for submicrom- eter dry process with electron lithography.

ACKNOWLEDGMENT The authors are grateful to Dr. T. Tamamura, Dr. S. Suga-

wara, and J. Shimada for their helpful discussion and encour- agement.

REFERENCES [ l ] L. F. Thompson, Solid-State Technol., p. 41, Aug., 1974. [2] M. J. Bowden and L. F. Thomoson, J. Electrochem. SOC., vol.

121, p. 1620, 1974. 131 K. Harada, J. Appl. Polymer Sci., to be published. [4] A. W. Levine, M. Kaplan, and E. S . Poliniak, Polymer Eng. Sci.,

[SI K. Murase, M. Kakuchi, and S. Sugawara, in Proc. Int. Conf. on

[6] H. Saeki, in Proc. 39th Meet. Japan SOC. Appl. Phys., vol. 3a-E-7,

[7] T. Tada, J. Electrochem. SOC., vol. 126, p. 1635, 1979. [8] K. Sekikawa, Monthly Publ. Japan SOC. Appl. Phys., vol. 45,

p. 983, 1976. [9] H. Nakane e t al., in Proc. 12th Symp. on Semiconductors and

Integrated Circuits Technol., p. 74 (Tokyo, 1977). [ l o ] M. Kakuchi, S. Sugawara, K. Murase, and K. Matsuyama, J.

Electrochem. SOC., vol. 124, p. 1648, 1977. [ l l ] K. Harada, J. Appl. PolymerSci.,vol. 26, p. 1961, 1981. [ 121 -,J. Electrochem. Soc.,vol. 127, p. 491,1980. [13] S . Imamura,J. Electrochem. SOC., vol. 126, p. 1628, 1979. [14] M. Kakuchi, S . Sugawara, and K. Matsuyama, in ACS/CSJConf.,

Organic Coating and Plastics Chemistry, vol. 40, p. 192 (Hawaii, 1979).

1151 T. Kitakohji, Y . Yoneda, H. Okuyama, and K. Murakawa, J. Electrochem. Soc.,voI. 126, no. 11, p. 1881, 1979.

[16] C. A. Coulsen, Valence. Oxford, p. 301, 1952. [17] H. Mark and A. V. Tobolsky, Physical Chemistry of High Poly-

vol. 14,p. 518, 1974.

Microlirhography, pp. 261-269 (Paris, 1977).

p. 92, 1978.

meric Systems, 2nd ed., 1950 p. 143.

Characterization of AZ-2415 as a Negative Electron Resist

TERRELL D. BERKER AND D. DEAN CASEY

Abstract-The use of AZ-2415 as a negativeelectronresist is described. The effects of electron dose, optical dose, and development time on the resist tine profiles are investigated. A low optical dose leads to wider and thicker developed negative lines, but with a lower contrast than lines exposed with a higher optical dose. An increase in develop- ment time results in a higher contrast which is accompanied by a signifi- cant increase in the electron dose required to maintain a fixed tinewidth. A good overall process scheme would avoid both the low optical dose area and the shortest development times in favor of values of these parameters that offer greater linewidth control. Using a 0.5-pm initial f i i thickness, an electron dose of 80 to 120 pC/cm2, an optical dose of 333 mJ/cm2, and a 15-s development in 1:3.5 AZ-2401:H20 pro- duce submicrometer resist patterns that provide excellent resistance to plasma etching.

Manuscript received August 24, 1981; revised October 26, 1981. The authors are with Sperry Research Center, Sudbury, MA 01776.

B INTRODUCTION

ECAUSE of the continuing miniaturization of devices, there is much interest in using electron-beam lithography

for direct writing of integrated circuits. An electron resist used for submicrometer technology requires high sensitivity, high resolution, and a high resistance to dry etching. Both positive and negative resist systems are needed because this allows maximum throughput with vector scan electron-beam machines and allows maximum process flexibility in subsequent device fabrication. Shaw and Hatzakis [ 13 have shown that the posi- tive diazo-type photoresists can be used as positive electron resists, with moderate sensitivity, high resolution, and high etch resistance. Using a method proposed by Pacansky and Lyerla [ 2 ] , AZ-1350 was exposed as a negative electron resist by Oldham and Hieke [ 3 ] . The sensitivity, resolution, and

0018-9383/82/0400-0524$00.75 0 1982 IEEE

BERKER AND CASEY: CHARACTERIZATION OF AZ-2415 5 2 5

etch resistance of negative AZ-1350 are similar to those of positive AZ-1350. Both tones of electron resist are thus possible by using a readily available photoresist with which the semiconductor industry has a good deal of experience.

Because throughput is a problem in direct-writing electron- beam lithography, several approaches are currently under evaluation for improving throughput. One of these approaches utilizes mixed (electron-beam/optical) lithography. Electron- beam lithography is reserved for those levels where the dimen- sions are very small and/or where registration to a previous level is critical. The noncritical levels are exposed using con- ventional photolithography. The use of a single resist system for the entire fabrication of the integrated circuit in mixed lithography is a very attractive option. The use of a diazo-type resist as both a positive and negative electron resist as well as a positive photoresist with either a light- or dark-field mask would allow a completely flexible process scheme while greatly simplifying substrate preparation, resist spinning, development, and baking processes.

The evaluations of Shaw and Hatzakis [ 11 showed that AZ- 2400 is an excellent positive electron resist. In this paper we will describe the use of AZ-2415 (a member of the family of AZ-2400 photoresists) as a negative electron resist. The effect of electron dose, optical dose, and development time on the linewidth and contrast of electron-beam-induced negative AZ-2415 will be examined.

EXPERIMENTAL Water plays a key role in the imaging process in AZ-type

photoresists [2]. Optical exposure in the presence of water converts the photoactive compound (PAC) to a carboxylic acid soluble in a basic developer solution. Positive resist be- havior results, In a water-free environment, such as in a vacuum, optical or electron-beam exposure produces an ester, which diminishes the development rate in a basic, aqueous developer. Negative resist structures can thus be formed by exposure of the pattern with an electron beam in vacuum followed by a flood optical exposure in air to change the back- ground into a highly soluble state. A short development time is used so that the negative nature of the electron-exposed areas is preserved while the background is developed away.

In this work, the starting substrates consisted of a 0.5-pm layer of doped polysilicon on oxidized single-crystal silicon wafers. A 0.5-pm film of AZ-2415 was spun and prebaked at 100°C for 30 min in a forced-air oven. An array of O S - , 1-, and 2-pm lines was defined by electron-beam exposure at 20 keV. The optical exposure was made using a contact printer without a mask. The exposure system utilized a 200-W Hg arc with no filters and glass optics; the integrated dose ranged from 150 to 850 mJ/cm2. To insure that the resist film regained the mois- ture lost during electron-beam exposure in a vacuum, the wafers were exposed to 35 percent RH air at room temperature for 15 min prior to optical exposure. The wafers were spray developed using a 1 : 3.5 mixture of AZ-2401: H20 . Develop- ment time varied from 8 to 25 s. Specimens were cleaved across the array of negative lines and viewed in a SEM for linewidth and resist height measurements.

2.5 I

"1 50 100 200 300 600

E L E C T R O N DOSE wC/crn?

Fig. 1. AZ-2415 developed linewidth versus electron dose for nominal 0.5- and 1-pm negative resist images. Also shown for comparison are curves representing nominal 0.5- and 1 - ~ m positive AZ-2415 resist images.

RESULTS

Electron Dose Effects

The effect of electron dose on the developed linewidth of nominal 0.5- and 1-pm isolated lines of negative AZ-2415 is shown in Fig. 1. The optical dose was 333 mJ/crnz and the development time was 15 s. As the electron dose increases, the line broadens. Due to intraline proximity effects, a lower electron dose is required to produce a 1-pm line (90 pC/cm2) than a 0.5-pm line (175 pC/cm2).

Also shown for comparison in Fig. 1 are curves for nominal 0.5- and 1-pm positive resist images. Substrate, resist thickness, prebake temperature, and developer concentration were the same as for the negative process. For the positive process, no optical exposure is used. The development time was 30 s. For these particular development times, very similar positive and negative linewidths are produced by the same electron dose.' Fig. 2 shows SEM micrographs of a 0.6-pm negative line and a 0.6-pm positive line.

For the negative, two-step exposure process, the effect of electron dose on the developed resist thickness can be seen in Fig. 3. With a development time of 15 s and for three different optical doses, we observed a threshold sensitivity of 10 to 30 pC/cm2 and contrasts2 in the range of 0.7 to 1.3. The electron sensitivity, defined for a negative resist as the dose necessary to produce a film thickness corresponding to 50 percent of the original thickness [4] , ranged from 50 to 80 pC/cm2, depend- ing on the optical dose following electron-beam exposure.

'As can be seen in Fig. 1, a developed positive-image 1-Wm line re- quired an electron dose of 80 pC/cm2. This is greater than the 20-pC/ cmz nominal sensitivity (vertical resist walls in 1-pm-thick resist layer) reported by Shaw and Hatzakis in [ 11. We did employ the same developer concentration as in [ 11, but many of the other processing parameters were different, notably the initial resist thickness, softbake temperature, and development time.

2For a,negative resist, the contrast is d e f i e d as y = [log (D!$D$]-', where D k is the interface gel dose (the minimum dose required for gela- tion), and Dg is the dose required to produce 100-percent film reten- tion, which is determined by extrapolating the linear portion of the thickness versus dose plot to a value of 1.0 normalized film thickness.

526 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, NO. 4, APRIL 1982

Fig. 2. (a) SEM micrograph of a 0.6-pm negative AZ-2415 resist pro- file. Electron dose was 200 pC/cm'; optical dose was 333 mJ/cm2; development time was 15 s. (b) SEM micrograph of a 0.6-pm positive AZ-2415 resist profile. Electron dose was 150 pC/cm2; development time was 30 s.

1.0

$ 0.8 2

u + 0.6 O

N 2 0.4

w

z z

0.2

0

t

10 20 50 100 200 500

ELECTRON DOSE hCicm2)

rig. 3 . Normalized developed resist thickness versus log electron dose for a 0.5-pm AZ-2415 film processed as a negative resist. Curves for three optical doses are shown and the contrast y is noted for each.

Optical Dose Effects The effect of optical dose on linewidth, for three constant

electron doses, can be seen in Fig. 4. The nominal written linewidth was 1 pm and development time was 15 s. There is

3'01 2.5 I - - 2.0 - r 0

E,

5 5 1.5 - J

1.0 -

0.5 -

1 1 I I I I , , / , I I 150 200 300 500 1000

OPTICAL DOSE (mJlcrn2J

Fig. 4. Negative AZ-2415 developed linewidth versus optical dose for a nominal 1-pm line. The linewidth saturates at approximately 300 mJ/cm2, independent of electron dose.

L1 0.8 - M

f?

n N a z 0.4 0

Y

0.6 - w

-

0.2 -

01 I I I I I , 8 / 8 1

150 200 300 500 lo00

OPTICAL DOSE (mJicm2J

Fig. 5. Normalized developed resist thickness versus optical dose for a 0.5-pm AZ-2415 film processed as a negative resist. Curves for four electron doses are shown. Development time was 15 s.

a minimum optical dose that will cause complete development of the background areas for a fixed development time. As can be seen from Fig. 4, this minimum optical dose for a IS-s development time is approximately 170 mJ/cm'. As the optical dose is increased just above this minimum value, the linewidth first decreases rapidly, then saturates at a dose of approximately 300 mJ/cm2, independent of the initial electron dose.

Fig. 5 shows the effect of optical dose on the normalized resist thickness for several electron doses. Just above the mini- mum optical dose, the normalized resist thickness decreases as the optical dose is increased. A lower electron dose results in a greater decrease in resist thickness. As the optical dose increases further, the resist thickness decreases only slightly. At any optical dose, an increase in electron dose results in a thicker developed resist, but also in a wider line.

By plotting the normalized thickness versus the log of the electron dose, the contrast' of the resist can be obtained. Fig. 3 gives curves for three optical doses. As the optical dose

BERKER AND CASEY: CHARACTERIZATION OF AZ-2415 5 2 7

w

8 100 t

50 F 150 200 300 400 600 800

OPTICAL DOSE ImJlcrn2)

Fig. 6. The electron dose required to produce three different negative AZ-2415 linewidths as a function of optical dose. Development time was 15 s.

TABLE I DOSE AND RESIST THICKNESS FOR A 0.5-pm LINE

Opt ica i E-Bean, p s e Developed Resist (nc/cn 1 Thickness (&XI)

175 66 0.12

222 140 0.26

increases, so does the contrast (7). Thus for a fixed electron dose, a low optical dose results in both wider and thicker negative lines than obtained with a higher optical dose. The contrast, however, is lowered.

Since the negative lines widen as the optical dose decreases, the electron dose must be lowered to obtain a narrower line at a low optical dose. Fig. 6 shows the electron dose required to produce three different linewidths (0.5, 1, and 2 pm) as a function of optical dose. A low optical dose substantially reduces the electron dose required for each linewidth. How- ever, this low optical-dose area is a sensitive region: a slight increase in the optical dose greatly increases the electron dose necessary to maintain the required linewidth. In order t o increase throughput, it might be useful to tightly control the process parameters and operate in this low optical-dose/low electron-dose region. Ho,wever, a serious problem is apparent from Table I. This table shows the electron dose required to produce a 0.5-pm line at three optical doses and the resultant developed resist thickness. To obtain a 0.5-pm line with an optical dose of 175 mJ/cm2, the electron dose must be lowered to 66 yC/cm2. This results in a developed resist thickness of only 0.12 pm. Although a lower optical dose results in a wider and thicker line for a fixed electron dose than that produced with a higher optical dose, the widening effect is much greater than the thickening effect.

Fig. 7 shows SEM micrographs of three linewidths (0.5, 1 .O, and 1.8 pm) obtained with optical doses of 175 and 222 mJ/ cm2. The difference in the profiles is striking. It will be remembered from Fig. 3 that this low optical dose of 175 mJ/ cm2 results in a contrast 7 of 0.7. It is generally agreed that a good negative resist must have y 2 1. We have found that a minimum developed resist thickness of 0.2 pm is required to pattern 0.5 pm of polysilicon using plasma etching. A gr*eater

Fig. 7 . SEM micrographs of threelinewidths obtained with opticaldoses of 175 and 222 mJ/cm2. Development time was 15 s. (a) A line- width of 0.5 ,urn requires an electron dose of 6 0 pC/cm2 (150 pC/ cm2) for an optical dose of 175 mJ/cm2 (222 mJ/cm2). (b) A line- width of 1.0 pm requires an electron dose of 4 0 pC/cm2 (90 pC/cm2) for an optical dose of 175 mJ/cmZ (222 mJ/cmZ). (c) A linewidth of 1.8 pm requires an electron dose of 40 pC/cm2 (70 pC/cm2) for an optical dose of 175 mJ/cm2 (222 mJ/cm2).

1 ! E 1.5

DEVELOPMENT TIME (SECONDS)

Fig. 8. Negative AZ-2415 developed linewidth versus development time for a 1-pm nominal line. Curves for three electron doses are shown.

developed resist thickness may be necessary for other applica- tions. For plasma etching of 0.5 ym of polysilicon, an optical dose of 175 mJ/cm2 is unusable. An optical dose of 850 mJ/ cm2 offers a high contrast (y = 1.3), but requires a long expo- sure time. We have found that an optical dose of 333 mJ/cm2, which has a contrast of 1.1, is a good point at which to operate.

Development Time Effects The effect of development time on the crosslinked linewidth

can be seen in Fig. 8. The nominal written linewidth was 1 pm and the optical dose was 333 mJ/cm2. The linewidth is sensi- tive to the development time, particularly for shorter develop- ment times. For an electron dose of 80 pC/cm2, a 10-s devel- opment results in a linewidth of 2.2 ,pm; this narrows to 0.9

528 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, NO. 4, APRIL 1982

Fig. 9. for a three

8 -

7 -

1 9 . 6 - 0 b-

.5 -

N a I .4 0

-

3 -

2 -

10 15 20

DEVELOPMENT TIME (SECONDS)

Normalized developed resist thickness versus development time 0.5-pm AZ-2415 film processed as a negative resist. Curves for electron doses are shown. The optical dose was 333 mJ/cm2.

_OO!

.3 200

B

10 12 14 16 18 20 22 24

DEVELOPMENT TIME (SECONDS)

Fig. 1 1 . The electron dose required to produce three different negative AZ-2415 linewidths as a function of development time. The optical dose was 333 mJ/cm2.

1.0

1 0.8 z 0

0.6

P - $ 0.4 I

n 0.2

0 10 20 50 100 200 500

ELECTRON DOSE IpClcm2)

Fig. 10. Normalized developed resist thickness versus log electron dose Fig. 12. SEM micrographs of two negative-image profiles of AZ-2415. for a 0.5-pm AZ-2415 film processed as a negative resist. Curves for Electron dose was 400 pC/cm2; optical dose was 333 mJ/cm2; initial three development times are shown. The optical dose was 333 mJ/cm2. resist thickness was 0.5 pm. (a) A 12-s development results in a line- The contrast y is observed to increase with increased development width (measured at the substrate-resist interface) of 7.0 pm. (b) A time, but the electron dose requirement also increases. 25-s development results in a linewidth of 2.2 pm,

pm for a 15-s development. Increasing the development time decreases the linewidth, but also decreases the developed resist thickness, as shown in Fig. 9. For an electron dose of 75 pC/ cm', the resist thickness decreases approximately 50 percent of its original thickness in less than 15 s of development time.

Fig. 10 compares the normalized resist thickness versus the log of the electron dose for three development times. The contrast y increases with increasing development time. How- ever, this increase in contrast is accompanied by alarge increase in the threshold electron dose. The electron dose required to produce three linewidths (0.5, 1, and 2 pm) as a function of development time is shown in Fig. 11. A 1-pm line requires an electron dose of 85 pC/cm2 with a 1 5 s development. When the development time is lengthened to 25 s, the required electron dose increases by almost a factor of five to 400 PC/ cm2 . Fig. 12 shows SEM micrographs of two negative-image profiles which have each received an electron dose of 400 pC/cm2, Fig. 12(a) was developed for 12 s while Fig. 12(b) was developed for 25 s. The perimeter areas, exposed primar- ily by backscattered electrons, have been developed away in Fig. 12(b).

For a 0.5-pm film of AZ-2415, we have found that a devel- opment time of 15 s is a good point of operation. Contrast is adequate (y = 1.1) and the electron and optical doses re- quired to define submicrometer features are reasonable.

A typical device pattern with a gate length of 0.6 pm is shown in Fig. 13. After electron-beam exposure at 100 pC/cm2, optical flood exposure at 333 mJ/cm2, and a 15-s development, the wafer was postbaked at 100°C for 30 min. Pattern trans- fer was achieved by anisotropic plasma etching of the poly- silicon. As can be seen from Fig. 13, negative AZ-2415 offers excellent plasma etching resistance.

DISCUSSION When AZ-24 15 is processed as a negative resist, two successive

exposure steps are required: one in vacuum and one in moist air. The first exposure, in vacuum, occurs under electron irra- diation. The second exposure occurs under optical irradiation in air. These two successive processes influence the chemistry of the resist in opposite directions. Pacansky and Lyerla [2] have shown that, in vacuum, optical irradiation of AZ-type resists promotes ester formation, via an intermediate ketene,

BERKER AND CASEY: CHARACTERIZATION OF AZ-2415 529

Fig. 13. SEM micrograph of a plasma-etched polysilicon pattern. The gate length is 0.6 pm and the resist has not been removed. The poly is 0.5 pm thick.

between the photolysed PAC and OH on the phenolic resin. If more than one diazo group exists on the PAC, crosslinking of the resin can occur. That is, negative resist behavior is pro- moted. On the other hand, if irradiation is carried out in air, the ketene intermediate reacts with water to form a base- soluble carboxylic acid, yielding positive resist behavior upon development in a basic aqueous solution. The chemistry of these processes is dominated by the availability of water and not by the radiant source. Hiraoka and Gutierrez [5] have studied similar resist systems under electron irradiation and observed ester formation similar to that suggested by Pacansky and Lyerla in vacuum. Of course, direct crosslinking of resists under electron irradiation is well known. We want to empha- size that it is not this direct, electron-induced crosslinking that generates negative behavior in AZ-2415 in the electron-dose range studied here, nor is the negative image formation due to additive properties of electron and optical irradiation. Cross- linking is not promoted by optical exposure in the range of 3000 to 4500 A [ 6 ] . The final negative image results from the change of the background area into a highly soluble state com- pared to the less soluble area irradiated with electrons under vacuum conditions.

As shown in Figs. 4 and 5, for a fixed electron dose, a low optical dose results in both wider and thicker negative lines than obtained with a higher optical dose. To help understand these results, we have found it useful to employ a phenomeno- logical model of the process, which is shown in Fig. 14. This model is a very simplified view which considers only the for- mation of ester linkages in vacuum under electron-beam expo- sure, and the formation of acids in moist air under optical exposure. A narrow line is exposed in the resist with a 20-keV electron beam. Due to forward and backscattering of the elec- trons, the absorbed energy distribution in the resist film is nonuniform, with an approximate distribution as shown in Fig. 14(a) [4] , [7]. For this example, we assume an exposed linewidth of 2 pm and a resist thickness of 0.5 pm. The high- est energy density is at y = 0 and - 1 pm <x < 1 pm. The

ESTER FORMATIONS NUMBER OF

t ESTER FORMATIONS

NUMBER OF

t

ACID FORMATIONS NUMBER OF

Fig. 14. A phenomenological model of the two-exposure, negative diazo-type resist process. (a) A - nominal 2-pm line is exposed in vacuum in a 0.5-pm-thick resist film with an electron beam. The absorbed energy distribution in the resist film is nonuniform. (b) The number of ester formations per unit volume at y = 0 for a 2-pm electron-beamexposed line. (c) The number of ester formations per unit volume at x = 0 for a 0.5-pm resist thickness. (d) The number of acid formations per unit volume versus x (at y = 0) after flood optical exposure at two doses. A low (high) optical dose will cause Nlow (Neigh) acid formations in the background areas. Any area of the reslst that contains less than Ndev acids per unit volume will remain after development. Llow @high) is the developed linewidth resulting from a low (high) optical dose. (e) The number of acid formations per unit volume versus y (at x = 0) after flood optical exposure at two doses. A developed resist thickness of flow (thigh) results from a low (high) optical dose.

larger the absorbed energy density is in the film, the larger the probability of an ester being formed. Thus we would expect the number of ester formations per unit volume (y = 0) to appear as shown in Fig. 14(b). Likewise, we would expect the population of ester formations per unit volume through the resist (x = 0) to appear as shown in Fig. 14(c).

If the resist is now flood exposed in air, PAC's not involved in ester links with the resin will be converted to carboxylic acids. Since there are a fixed number of PAC molecules per unit volume, there will be fewer acid formations in the elec- tron-irradiated areas because some of these PAC's have formed esters. Fig. 14(d) and (e) illustrates how the number of acid formations per unit volume versus x and y , respectively, might occur. Curves for two optical doses are shown. A low (high) optical dose will result in Nlow (Nhigh) acid formations per unit volume in the background areas. For a set development time, at least Ndev acid formations per unit volume are assumed to be required for complete development of the resist. Any area of the resist that contains less than Ndev acids per unit volume will remain after development. Thus in Fig. 14(d),

530 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, NO. 4 , APRIL 1982

Fig. 15. SEM micrograph of a nominal 2-pm line. Two distinct angles are apparent. The steep angled center portion is mainly exposed by the primary electrons while the more gradually sloping outside por- tions are exposed by the backscattered electrons.

Lhigh @low) gives the negative linewidth for the high (low) optical dose, while in Fig, 14(e), thigh(tlow) gives the developed resist thickness. Based on this model, a low optical dose should result in both a wider and thicker negative line than that ob- tained with a high optical dose. Furthermore, the widening effect is greater than the thickening effect due to the contribu- tion of backscattered electrons from the substrate.

I t should be stressed that the discussion above is based upon a phenomenological model of the process. We have made no attempt to arrive at a quantitative model and so the shapes of the curves in Fig. 14 are approximate. Even so, we are able to arrive at a good working model of the process that is occur- ring when an AZ-type photoresist is exposed as a negative electron resist. According to the model, the developed profile of a negative line should follow an absorbed energy contour. Fig. 15 shows a SEM micrograph of a nominal 2-pm line which received an electron dose of 100 pC/cmZ followed by an op- tical dose of 850 mJ/cm2. In this case, the initial resist thick- ness was 1.2 pm and development was for 15 s. Two distinct angles are apparent. The steep angled center portion is mainly exposed by the primary electrons, while the more gradually sloping outside portions are exposed by the backscattered electrons. To our knowledge, this is the first time a resist pro- file of this shape has been reported. The swelling inherent in most negative electron resists, such as COP, most likely distorts the profile to the point where this two-angle form is lost.

We recognize the importance of linewidth control in any lithographic technique. Linewidth repeatability is currently under investigation and will be treated in a future report.

SUMMARY We have shown that AZ-2415 can be processed as a negative

electron resist with sensitivity and resolution similar to that observed when this resist is developed positively. The effects of electron dose, optical dose, and development time on the line profiles were investigated. A low optical dose leads to wider and thicker lines, but with a lower contrast than lines exposed with a higher optical dose. The developed patterns are only weakly crosslinked. An increase in development time results in a higher contrast which is accompanied by a signifi- cant increase in the electron dose required to maintain a fixed linewidth.

A good overall process scheme would avoid both the low optical-dose area and the shortest development times in favor of values of these parameters that offer greater linewidth con- trol. When a 0.5-pm film of AZ-2415 with a prebake at 100°C is used, an electron dose of SO to 120 pC/cm2, an optical dose of 333 mJ/cmZ, and a development time of 15 s yield submi- crometer resist patterns that provide excellent resistance to plasma etching. The use of a readily available diazo-type photoresist such as AZ-2415 thus provides a good negative electron resist that is fully compatible with existing semicon- ductor fabrication equipment and procedures.

ACKNOWLEDGMENT The authors would like to thank J. Donahue for his helpful

discussions. The expertise of R. Shuman, who produced the SEM micrographs, is acknowledged. S. Bernacki performed the plasma etching.

REFERENCES [ I ] J. M. Shaw and M. Hatzakis, “Performance characteristics of diazo-

type photoresists under e-beam and optical exposure,”IEEE Tvans. Electron Devices, vol. ED-25, pp. 425-430, 1978; also M. Hatzakis and J . M. Shaw, “Diazo-type photoresist systems under electron- beam exposure,” in Proc. VIII Int. Con5 Electron and Ion-Beam Technol., R. Bakish, Ed. (Seattle, WA), pp. 285-302, 1978.

[2] J. Pacansky and S. R. Lyerla, “Photochemical decomposition mechanisms for AZ-type photoresists,” ZBM J. Res. Develop., vol.

[3] W. G. Oldham and E. Hieke, “A high resolution negative electron resist by image reversal,” IEEE Electron Device Lett., vol. EDL-

[4] L. F. Thompson and R. E. Kerwin, “Polymer resist systems for photo- and electron lithography,” Annu. Rev. Mater. Sci., vol. 6 ,

[S] H. Hiraoka and A. R. Gutierrez, “Electron-beam-induced reactions of orthonaphthoquinone-diazide-sulfonyl derivatives in phenolic- type resins,”J. Electrochem. SOC., vol. 126, pp. 860-865, 1979.

[6] B. Broyde, “Exposure of photoresists, 11: Electron and light expo- sure of a positive photoresist,” inProc. IVInt . Con5 Electvon and Ion Beam Sci. Technol., R. Bakish, Ed. (Los Angeles, CA), pp.

[7] J . S. Greeneich and T. Van Duzer, “Model for exposure of electron- sensitive resists,” J. Vac. Sci. Technol., vol. 10, pp. 1056-1059, 1973.

23, pp. 42-55, 1979.

1, pp. 217-219, 1980.

pp. 267-301, 1976.

575-580,1970,