Accuracy of structure transfer in deep X-ray lithography

MICROEL£CTRONIC ENGINEERING

ELSEVIER

Microeleetronic Engineering 35 (1997) 557-560

Accuracy of structure transfer in deep X-ray lithography G. Feiertag, W. Ehrfeld, H. Lehr, A. Schmidt, M. Schmidt Institute of Microtechnology Mainz GmbH, Carl-Zeiss-Stral3e 18-20, 55129 Mainz, Germany Deep X-ray lithography with synchrotron radiation (DXRL) constitutes the key microfabrication process step in the LIGA technology. Microcomponents with a height of some gm up to several mm can be manufactured with sub-gm precision. The pattern transfer accuracy is governed by technological constraints like thermal mask deformation as well as by various physical effects, e. g. Fresnel diffraction, emission of photo- and Auger electrons, fluorescence radiation, radiation scattering and divergence of the synchrotron radiation beam. A computer program has been developed to investigate the significance of these effects to the dose distribution in the resist material, which in turn determines the lateral structure resolution. The paper gives a brief introduction to the calculation procedure and outlines the weight of the different contributions with respect to transfer accuracy. It is shown that beam divergence and diffraction are much less important than the image blur caused by photoelectrons. Fluorescence radiation emitted from the mask membrane or the substrate contributes to the dose deposition in the resist if mask membrane or substrate consist of high atomic number material. Radiation scattering is negligible for resist layers which are less than some mm thick. A good agreement is found between calculated dose distributions and measured resist profiles. This allows a partial compensation of the above mentioned accuracy limiting effects in the mask design. I. I N T R O D U C T I O N

2. C A L C U L A T I O N P R O C E D U R E

Microstructures produced by means of the LIGA process are extremely precise and show smooth sidewalls when DXRL is applied. LIGA is therefore the appropriate technology to fabricate components for microoptical, microfluidic or micromechanical applications [ 1]. Best results are obtained in a simple shadow printing process, applying synchrotron radiation to project the high precision absorber patterns from a DXRL mask into a thick radiation sensitive polymer layer. Photoelectrons and Fresnel diffraction are the dominant effects which limit the structure transfer accuracy to about 0.2 gm for a 500 gm thick resist layer [1,2]. The thermoelastic deformation of a DXRL mask contributes to deviations from the ideal shadow print with less than 0.2 gm for a 500 gm thick resist layer if Beryllium or Diamond are used as mask membrane materials [3]. This paper presents a detailed analysis of the different contributions which limit the structure transfer accuracy. Model calculations are compared with experimental results which have been obtained by measuring the lateral distance of resist edges after DXRL and resist development using an SEM.

A computer code was developed to simulate the contribution of the • Fresnel diffraction at the absorber edges, • divergence of the synchrotron radiation, • photo- and Auger electrons, • fluorescence radiation from mask or substrate, • scattering of radiation. The significance of these effects with respect to structure transfer accuracy is weighted for the above mentioned processes performing model calculations to obtain the dose distribution in the resist. The radiation parameters taken into account correspond to the spectral distribution of the synchrotron radiation source DCI, Orsay, France, with a dose of 5 kJ/cm 3 at the bottom of the resist, a vacuum window and a mask membrane made from Beryllium with a total thickness of 1000 gm and a Gold absorber height of 15 gm. Calculated results will be discussed for the irradiation of a 500 gm thick PMMA resist layer.

0167-9317(97)/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII: S0167-9317(96)00158-X

2.1 Diffraction Diffraction at a semitransparent absorber edge was calculated in the Fresnel approximation [4]. The calculation of the dose distribution in the resist considers the intensity distribution at an absorber edge

G. Feiertag et al. / Microelectronic Engineering 35 (1997) 557-560

558

due to Fresnel diffraction at different wavelengths of the synchrotron radiation spectrum, the transmission of the filters (vacuum window, mask membrane, resist) and the energy dependent absorption coefficient of the PMMA resist. Integration with respect to the wavelength leads to the results displayed in figure 1, which exhibits calculated contour lines of equal dose. The thick black line indicates the absorber. The dissolution limit of PMMA corresponds to a deposited dose of about 1.8 kJ/cm 3, which represents the resist wall after development (bold line in fig. 1). A comparison with the geometrical shadow (broken line) results in a deviation from this ideal line of less than 0.1 ~tm. -100

0

...................

/

100 200o 300., ~400. 50010

-100, I

0 100.

~ /

.~ 200, E .9 300

~

400,

[ [[[1~/\

Dose [kJ/cm3] 10--

0.1

500 -l

.0 Distance to absorber edge [p.m]

I I

Fig. 2: Deposited dose, considering diffraction and the divergence of synchrotron radiation. I0

0i I

calculation the contribution of the total divergence to the diminishing of transfer accuracy is quite small.

Dose [kJ/cm3]

'~ 0~51 - " ' " " -0'.5 . . . . Distance to absorber edge [~tm]

-1.0

Fig. 1: Dose distribution near absorber edge, only diffraction. The contour lines represent doses of 0.1, 0.18, 0.32, 0.56, 1, 1.8, 3.2, 5.6, 10 and 18 kJ/cm 3. The bold line at 1.8 kJ/cm 3 indicates the resist wall profile after development. 2.2 Divergence Synchrotron radiation is extremely parallel, but the small divergence of about 0.2 mrad is an additional source of image blur below the absorber edges. The divergence results from the angular deviation of the light emitting electrons in the storage ring, the natural divergence of the synchrotron radiation and the size of the electron beam. A Gaussian angular distribution of the electrons with a standard deviation of 0.2 mrad and the wavelength dependent natural divergence given by theory [5] was used to calculate the combined dose distribution due to diffraction and divergence. The result is displayed in figure 2. In comparison to figure 1 it can be seen that in spite of the rather large electron divergence used in the

2.3 Photoelectrons and Auger electrons The absorption of a photon in the resist gives rise to energy tranfer to a core electron of a resist atom. The kinetic energy of the electron corresponds to the difference of the photon energy and the binding energy of the electron, The excited atom relaxes either by the emission of an Auger electron or by the emission of another photon (fluorescence). The electrons undergo elastic scattering by the nuclei of the resist atoms and inelastic scattering by electrons. This leads to energy dissipation in the vicinity of the point of absorption. To calculate the image blur caused by the photoelectrons and the Auger electrons a Monte Carlo simulation has been performed [6]. In order to achieve reasonable computing times the Monte Carlo data have been parametrized by fitting the monochromatic results with a Gaussian distribution. The dose deposition due to diffraction, SR divergence and electron energy deposition was calculated by convolution. The result is shown in fig. 3. It should be noted, that, compared to figure 1 and 2, the scale of the abscissa has been changed. It is obvious that the contribution of the image blur caused by the electrons is much larger than the diffraction and divergence effects. This result holds for resist layers with a thickness of more than some 10 ~tm. The situation is completely different in X-ray lithography (XRL) which is used in microelectronics. Due to the longer wavelengths used to

G. Feiertag et al. / Microelectronic Engineering 35 (1997) 557-560 pattern thin resist layers the contribution of the electron range is negligible compared to diffraction [7]. -106 . . . . . . . . .

v .......... i

0 "~ 100

\ ~

.::k.

~

Dose [kJ/cm3]' -

1

0

membrane materials no relevant fluorescence radiation is emitted [9]. If Copper substrates are applied, the fluorescence dose is eight times smaller compared to the Titanium case, because less photons are available which have enough energy to excite the Copper-K shell. 250

~

.~~ 200 ~ 300" .~ 300

- , - , • , - I - , - , - , Dose [J/cm3]

i

/.(,

I

.~ 500 -2" " ' -II 0 1 Distance to absorber edge [lam]

I

"~ 35o. "~,

~400 2

Fig. 3. Deposited dose, considering diffraction, divergence, photo- and Auger electrons. 2.4 F l u o r e s c e n c e The emission of fluorescence radiation competes with the Auger effect and is strongly dependent on the atomic number (Z) of the atom involved. At low Z the K shell fluorescence yield is small, at Z=30 it is about 0.5 and for large Z it is nearly 1. For Titanium (Z=22) the K shell fluorescence yield amounts to about 0.2 [8]. The only relevant sources for fluorescence radiation in DXRL are mask membranes or substrates consisting of high Z materials. Fluorescence radiation from the Gold absorber may be neglected since the fluorescence photons are either reabsorbed or backscattered. The probability to hit the resist is therefore very small. The dose deposited by fluorescence was calculated by use of the Monte Carlo method. The result of a calculation for the irradiation of a 500 gm thick PMMA resist deposited on a Titanium substrate is shown in figure 4. Up to 500 J/cm 3 are deposited below the absorber (near the substrate). Fluorescence radiation may therefore significantly diminish resist adhesion in case of thin resist walls or columns, which are attacked by fluorescence from more than one side. If Titanium is used as mask membrane material a dose which is comparable to the dose shown in figure 4 is deposited at the top of the resist by fluorescence radiation from the mask. If materials with low atomic number like Carbon or Beryllium are used as substrate or mask

559

/2o0 100

~c~ 400"

~)~

IZ- ~ ~

,-~~.400

^ ~ --'~'~

o

450.

600 --

500 . . . . . . . , . . . . . , -200 - 100 0 100 Distance to absorber edge [[am]

200

Fig: 4. Dose deposited by fluorescence radiation from a Titanium substrate. The lower 250 ~tm of a 500 ~tm thick resist layer are displayed. 2.5 Scattering Photon absorption is the dominant phenomenon for photon energies below 10 keV, since the absorption coefficient is much larger than the scattering coefficient. But at some 10 keV the probability for photon scattering becomes comparable to absorption. Scattering can therefore not be neglected when very thick resist layers are patterned. Monte Carlo simulations have been performed to calculate the dose which is deposited by scattered photons. Scattering cross sections given in [10] as well as elastic (Thomson)and inelastic (Compton) scattering events have been taken into account. The calculations show that in a 500 ~tm thick resist layer less than 100 J/cm 3 are deposited below the absorber by radiation which is scattered in the mask, in the resist or in the substrate. Dose deposition due to photon scattering in the mask membrane and the substrate is also negligible for thick resist layers. But a high dose is deposited due to radiation scattering from the resist material into the absorber shadow region. This is illustrated in figure 5. A dose of up to 1.5 kJ/cm 3 extends into the absorber shadow in case of a 10 mm thick layer and a trench width of 5 mm. The SR parameters cor-

G. Feiertaget al. I MicroelectronicEngineering35 (1997)557-560

560

respond to the radiation from a dipole beamline at the ESRF, Grenoble. The resist damage by scattered radiation and fluorescence can be drastically reduced if masks with a high absorber coverage are used.

o

SW7

2-

0.~llllll

[kJ/cmS]

1111113_ I -

"~ ' illllll ~X~...~2 .E "~6. I Iltll ~N.~ j /]]]i [ i'4 i ~ 4-

~

/lllili60!4

!

1.8

8-

1 0 - n

-5

-4

-3

-2

-1

0 1 x [mm]

2

3

4

5

Fig. 5: Dose deposited by radiation scattered in the resist for an irradiation of a 5 mm wide trench at a dipole beamline at ESRF, Grenoble, France. 3.

M E A S U R E M E N T S

In order to compare the calculations with experimental data, resist edge profiles have been determined utilizing an SEM. The results of the measurements are in good agreement with the calculated 1.8 kJ/cm 3 contour line (see fig. 6). -100,-

---

,

. . . .

' --

-I

. . . .

' . . . .

' . . . .

'

I I

'-~ 100

1

=I.

ctl

200

.~ 300 400 500 -1,5

i

-1,0 -0,5 0,0 0,5 1,0 Distance to absorber edge [lam]

1,5

Fig. 6: Calculated dose distribution due to diffraction, beam divergence, photo- and Auger electrons, fluorescence. Closed circles: Measured resist edge profile.

The good agreement allows the calculation of absorber offsets to compensate for the shift of the resist edges in advance, which leads to a precision of better than 0.05 p.m per 100 ~tm resist height. The small curvature in the vicinity of the resist surface may be attributed to an extended developer attack. The contribution of the dose resulting from fluorescence radiation in the substrate region is underestimated in the present calculation. 4. CONCLUSION We presented a detailed analysis of the different contributions which limit the structure transfer accuracy in DXRL. Model calculations revealed the importance of photoelectron emission in the resist compared to Fresnel diffraction and beam divergence for typical DXRL conditions. Fluorescence radiation turned out to contribute to the dose distribution if Titanium is used as mask membrane or substrate material. Radiation scattering effects are significant when thick resist layers are patterned utilizing short wavelength radiation. Applying DXRL it has been shown that the accuracy of the resist structures exceeds 0.05 ~m per 100 ~tm resist height. R E F E R E N C E S

1. W. Ehrfeld, H. Lehr, Radiat. Phys. Chem. 45, 3 (1995) 349. 2. W. Ehrfeld, D. Mtinchmeyer, Nucl. Instr. and Meth. in Phys. Research A 303 (1991) 523. 3. G. Feiertag, M. Schmidt, A. Schmidt, Microelectronic Engineering 27 (1995) 513. 4. K. Heinrich, H. Betz, A. Heuberger, S. Pongratz, J. Vac. Sci. Technol. 19(4) (1981) 1254. 5. E.-E- Koch (Ed.), Handbook on Synchrotron Radiation, North-Holland, Amsterdam, (1983) 6. A. Schmidt et al., Microelectronic Engineering 30 (1996) 215. 7. H. Betz et al., J. Vac. Sci. Technol. B 4 (1) (1986), 248 8. X-ray Data Booklet, Ed. by D. Vaughan, Lawrence Berkley Laboratory, Berkeley (1986) 9. W. Ehrfeld et al., German Patent 4310976 C1 10. W. M. J. Veigele, Atomic Data Tab. 5 (1973) 51