371
Microelectronic Engineering 11 (1990) 371-374 Elsevier Science Publishers B.V.
ONLINE
EXPOSURE DOSE CORRECTION DURING ELECTRON BEAM PATTERN DELINEATION BY BLANKING PULSE WIDTH MODULATION.
N.K.L.Raja, K.J.Rangra and M.Singh. SEM-VAX Group, Central Electronics Engineering Research Institute, Pilani, Rajasthan - 333 031, India.
The electron beam exposure over extended periods of time using guassion electron beam in a SEM like column is highly prone to variation in exposure dose due to changes in the incident beam current, charging on the resist surface etc. This leads to the loss of global linewidth control. This paper presents a method in which the exposure current is monitored in-situ and the required correction to the exposure time is provided automatically in order to maintain uniform exposure globally. 1.
Introduction
In direct write electron beam lithography, particularly in e-beam machines with SEM like columns with gaussion electron beam probe [l] the pattern delineation time is considerable extending to several hour6 per mask level.The electron optical condition6 electronics. change during this time due to drift in the control Thus leading to variations mainly in the electron probe current. In addition charging of the resist during exposure leads to reduced charge deposition. These cause exposure variations around the nominal preset values and lead to poor global linewidth control. Figure 1. Show6 the percentage deviation of obtained line of width as a function intended linewidth for different doses. As an example, for an involuntary change in dose from 75
PC/cm* to 150 ,uC/cm2 may enhance the loss of line-width control by nearly 40% for 3fimline.
110 loo-
A 75pC/cm’ 4 lOOpC/cm’ 0 125pC/cm’ 9 lSOpC/cm’
go $*O > 70 X
0 .
T6' 5 So y40
0
L 30JJ$
This paper presents a method to monitor the 'as deposited dose' by sampling the absorbed electron (AE) current through the resist
and substrate and a scheme to control the spot dwell time by controlling the beam blanking
2o 10
D 05
1
15
20
INTENED
25
30
LINE WIDTH
35
40
45
J 50
CUM)
pulse width in proportion to the FIG.l. PERCENTAGE DEVIATION OF AE current under computer control. OBTAINED LINE WIDTH VS INTENDED
The relation of the absorbed probe LINE WIDTH FOR current to the e-beam energy i6 VALUES. also studied. 0167-9317/90/$3.50
0 1990, Elsevier Science Publishers B.V.
DIFFERENT
DOSE
372
N.K.L. Raja et al. I Online exposure dose correction
2. to
The Method
:
The total charge a pattern pixel is -6X Qpik = I Ib dt
value
The specimen correlated
deposited given as
per
pixel
during
e-beam
-------
(1)
absorbed current is monitored in-situ to the primary beam current (Ib) given
Ib = K(t,E,Zr/rhol,Zs/rho2)
Ia
__-____
exposure
and as
its
(2)
K is the current and Where Ia is the absorbed electron correlation function dependent on the resist thickness t, primary beam energy E and the material properties (Average atomic number The exposure Z and the density rho) of the resist and substrate. duration per spot (excluding the proximity effect contributions) is given by [2] Tpix
= D(0.78
--------
d* /Ib)
Where D is the required dose in cou1/cm2, d is Eliminating and Ib the current of primary beam. (2) and (3) Tpix
-
D((0.78
dz)
/
(kIa))
--------
(3) the diameter Ib from eqns. (4)
The primary beam is turned on at every pixel location of the pattern for a time Tpix and the value of Tpix is varied according the beam to eqn.(l) which results in pulse width modulation of The absorbed current Ia is measured in-situ at blanking signal. reaular intervals and the value of K is obtained from calibration tables. 3.
Experimental
Setup
:
25C) was The SEM (JELL Ic modified [3] for e-beam exposure (figure 2). The position of beam and its blanking is controlled by The specimen IBM-PC based system. current (Ia) and primary beam current (Ib) are collected and digitized through A/D cup a;FC) * interface. Faraday collecting Ib used for a:: measurements of beam diameter.The dependence of correlation function K on the above parameters has been in a look up studied and stored show the table. Figures( 3a,3b) variation of K as a function of beam energy for substrate combinations incident probe currents. configuration with the substrate/resist along beam energy value is input to the which computer program the loads automatically disk appropriate K-tables from files. A computer program
SCHEMATIC DIAGRAM OF F1G*2* THE SEM MODIFIED FOR E-BEAM WRITING.
373
N.KL. Raja et al. I Online exposure dose correction
0.8.
0.8.
0.6-
< =" 0.6-
-in
-. 5
b
+ {
O,&_
Ac. position 1130 oc. position 1200 IC.position 1230
0.2.
oc. position 1300
G
0.2
IC bosition 1230 oc position 1300 PMMA
i
f 0.53uml
1
on SiO,
10 Beam
Energy (in KeV)
Beam
K-FACTOR (Ib/Ia) VS FIG,3(a). BEAM ENERGY FOR PMMA(O.26 ,uM) ON SILICON
evaluates the value of Tpix for a preset value of dose and values of ‘d’ and Ia measured in-situ. The total charge deposited during pattern writing is maintained constant by blanking modulatting the width such that it pulse the varies inversely with monitored value of specimen absorbed Ia. current shows the Figure(4) width blanking pulse The top modulation scheme. the curves show representative variation of probe (Ib) and absorbed current (Ia) in time. The nominal exposure pulse width twn increases to twl when the current probe falls to low values (Ia also falls correspondingly). The exposure time reduces to
twh when the probe current and thus absorbed the current increases to a value higher than the nominal value.
;1 8 $ 3 j
Energy (in KeV)
FIG.3(b). K-FACTOR (Ib/Ia) BEAM ENERGY FOR PMMA(0.53 ON Si02
vs NM)
HG~.________________________________ Ib LOW ---- _----- --_“,OH___----_--_-___-------_-_----I,,. LOW .---___-____2% r?“: -lfWl I‘-
BLANKlNG n,,,,.r r”’
DOSE CONTROL SCHEME. FIG.4. Ib AND Ia ARE THE PROBE CURRENT ABSORBED CURRENT AND RESPECTIVELY.
N.K.L. Raja et al. I Online exposure dose correction
374
FIG.5.
WITH
CORRECTION
4.
DELINEATED IN PMMA (ON Si) WITH (FIG. 5A) AND LINE WIDTH DEVIATION WITH (FIG. 5B) DOSE CORRECTION. Is 10% AND WITH 0uT CORRECTION IT Is 50% (b/a -1.44).
PATTERNS
OUT
Results and Discussions
Figures(5a,5b) line-widths of patterns compare the delineated in PMMA (on silicon) with and without the in-situ correction to the exposiires dose. Both of these patterns correspond to exposures made after 45 minutes of setting the nominal SEM parameter values to delineate 4pm periodic line patterns. The patterns were stepped and repeated over the wafer after a specified delay. To demonstrate the effect the electron gun was operated in the unsaturated region. The pattern with correction shows a line width deviation of 10% where as the linewidth deviation with out the correction is above 50% (b/a = 1.44). In our experiments the effect of specimen topography on the The effect on lack of value of K has not been considered. planerisation during resist coating on the value of K (which is site dependent) is under study. Acknowledgements G.N. The authors wish to thank Dr. Acharya,Dr.W.S. Deshmukh for stimulating discussions. Khokle, Mr. P.R. Techinal assistance provided by Mr. A.K. Mistry, Mr. N.L. Saini and Mr. R. Sharma is also acknowledged. References ,
1.
Yoshikawa R et. al Jan/Feb 1987 ,pp65
J.
2.
Raja NKL ERL Memo , UCB/ERL-M Berkeley, USA Sept. 1984
3.
Raja NKL ,Karkare VG and David SK ,IEEE Meas. August 1989, To be published.
Vat.
Sci.
84/79
Univ.
and
Tech. of
Trans.
5(l),
California, Instrum.