- Email: [email protected]
Resist profile control of image reversal process in contact lithography
Microelectronic NorthHolland
Engineering
RESIST
F.
591
9 (1989) 591-594
PROFILE
POMMEREAU,
CONTROL
M.
OF IMAGE REVERSAL
IOST, F.
VOLLENBROEK*,
PROGESS
S.
IN CONTACT
LITHOGRAPHY
GDURRIER
LEP : Laboratoires d'Electronique et de Physique appliquee A member of the Philips Research Organization 3, avenue Descartes, 94450 LIMEIL-BREVANNES, France,
Image
reversal with AZ 5214 resist is characterized by contact lithography, a statistical experimental approach. Overhanging and quasi vertical L%?les which are of particular interest in GaAs technology (lift-off and anisotropic etching processing) are obtained simply by varying the UV3 exposure dose and submicron features are achieved with a low dispersion (3aL = 0.12 urn) using a thick resist.
1. INTRODUCTION
2.2. Second
promising reversal Cl] is a very Image technique because of its very large flexibility which can combine tne advantages of positive and negative resists and has been studied in detail with projection lithography [2]. To our little information exists on the knowledge, application of an image reversal system to contact lithography, although it is widely used in GaAs technology for sub-micron pattern definition.
A second order analysis was then used to build model for the process. A mathematical a "rotatable central composite design" (RCCD) [4 an optimum set of of order two, generates experimental conditions. Tne mathematical model is a polynomial least square fit for each quantitative response. The validity of the model was checked by a variance analysis [4].
3. This paper presents a study on resist profile control of the AZ 5214 image reversal resist 131. Df particular interest are the overhanging profiles for "lift-off" and the vertical profiles (with dimensional control) for pattern definition by anisotropic ion etching. This work has been carried out using a Statistical Experimental Approach (S.E.D.) coupled with a "response surface methodology" [4].
2.
EXPERIMENTAL
METHODOLOGY
2.1. First order analysis
(factorial
analysis)
select First, it necessary to the was processing variables which have the most important effect on the photolithographic process. This has been done using a two level factorial design [4] which consists of all the combinations obtained by selecting two values for each variables. Several responses are measured to characterize the result of the lithography and finally, a hierarchy for the effects of the variables on the responses can be established. * Philips Research Laboratories Nat. Lab. 5600 MD Eindhoven, The Netherlands
0167.9317/89/$3.50
0
1989, Elsevier Science Publishers
B.V. (North-Holland)
The
analysis
order
EXPERIMENTS following
constant
[53
parameters
been
have
held
:
- coating : AZ5214 E photoresist thickness Z = 1.1 pm on GaAs substrates - soft bake = 90°C for 30 mn in an oven, - imaging exposure is performed with a Karl Suss contact mask aligner in the UV3 band (310 nm) through a quartz mask containing grating patterns (equal lines and spaces). - reversal bake = 15 mn in an oven - flood exposure in the UV4 band (365 nm) - development with diluted MF 312 developer. For
the
following
first
order
variables
analysis,
we
used
the
:
imaging exposure dose E = 50 and 200 ml/cm2 Reversal bake temperature T B = 110' and 120°C fF(ood exposure dose FE = 150 and 450 ml/cm2 developer normality N = 0,24 and 0,27 N. The
development
times
tdev
were
adjusted
F. Pommereau
592
et al. 1 Image reversal process in contact lithography
as follows : for each condition, the minimum development time to (required to remove the soluble part of the resist) was determined. The development bv applying
time
tdev
is
then
deduced
from
t0
in the first order analysis a constant over development (10 %). A variable OVERDEV is defined by DVERDEV = (tdev - to)/to. OVERDEV is therefore 0.1 in the first order analysis and is varied from 0 to I in tne second order analysis. the dimensions and the After development, profile of the resist patterns were measured with a Philips 525 SEM after calibration. The corresponaing responses (see Figure 1) are TOPW, BOTW, and MEDW, defined as follows : TOPW = Topwidth/Pitch, and MEDW =
BOTW = Bottomwidth/pitch
Mediumwidth/Pitch
(i.e. mask dimensions i TOP
Figures 2 ana 3 show the response contour plots with TRB held respectively at the values 112°C and 118°C.
with Pitch = 2 urn
The effect of the exposure dose is to increase the responses ana the effect of TRB is to increase TDPW slightly. At low doses, the concentration of the photoproduct is higher at the surface than at the bottom of the resist, due to the relative high absorptivity of the resist in the lJV3 band. This results in a higher crosslink density after the reversal bake in the top of the latent image. This difference in crosslink density between top and bottom is more pronounced at high reversal bake temperatures [6]. Upon development, this leads to overhanging profiles. Thus low dose and high reversal bake temperature leads to negative slopea At high doses, resist walls. the concentration of photoproduct is more uniform from top to bottom, which leads to more or less vertical profiles upon aevelopment.
: line/space = 1 pm/l urn) Finally, tne effect of OVERDEV is to decrease This effect is probably due to the BOTW. relatively low dose received by the bottom of the resist at the mask edges (diffraction effects).
WIDTH
We can notice that the process latitude is very good. For example,nTOPW=.04 for ~TRB=~“. N
i-.-
BOTTOM
:
WIDTH)---
c
PITCH
Figure
-1
1
4.3. Optimization
for "Lift-off"
processing
:
Overhanging profiles can be ontained with : low exposure doses : 50 ,(E,(70 ml/cm2 TRB = 118°C ana 10 %,(oVERDEV,(lOO % (see Figure 5).
4. RESULTS AND DISCUSSION
4.1. First order analysis
The main variables affecting the responses are the exoosure dose E and the reversal bake temperature TRB. Low exposure doses yield improved orofileswhich are overhanaina increasing. the (TOPW-BOTWincreases) by reversal bake temperature. On the other hana, for high doses quasi vertical profiles are observed.
Tne oest overhangs are ootained with a 50 mJ/cm2 exposure dose, but the corresponding pattern wiath is smaller than the nominal width (I urn). On the contrary, by increasing the dose up to 70 ml/cm2, the overhang is degraded but the pattern width is closer to the nominal wiath.
4.4. Optimization
for anisotropic
The optimum
to
100
The flood exposure dose and normality were held constant : FE = 150 mJ/cm2, N = 0.24.
%
the
conditions
are
:
120
were used : to 200 ml/cm2 to 120°C developer
:
Nigh exposure doses, 120,(E ,(140 mJ/cm2, and a moderate overdevelopment yield quasi vertical resist profiles.
4.2. Second order analysis
The following variables varied from 50 E varied from 110 TRB 0 OVERDEV varied from
etching
I< Pommereau
A
100
593
et al. / Image reversal process in contact lithography
!!
Response -->TOPW T RB=112 "C
Response -->TOPW T RB=ilB "C
100
s .4
.45
5
2 P 2 I_: : 0 50
E 100
I Dose
,n. E
(mJIcm2)
200
50
Dose
B
Response -->MEDW T RB=112 "C
E
(mJ/cm2)
200
Response --,MEDW T RB=118 "C
100
\ .45 S Y
0’
50
c 100
cDose ,’
.‘.
E
.‘.
(mJ/cm2)
200
Response -->BOTW T RB=112 "C .2.25.3 .35
0 Ifi 50 Dose
’
C
I _ s
I& z
(mJ/cm2)
200
-i5.'2. 5
/ .3
5 2 9 0
4
2
E
Response -->BOTW T RB=llB "C
100
s
E W
.4
45
5
55
0
50
Responses
Dose
E
(mJ/cm2)
0
200
50
contour plots with TRB = 112'C (Pitch = 2 Mm) Figure 2
SEM photograph
of a quasi vertical Figure 4
profile
Responses
Dose
E
(mJ/cm2)
200
contour plots with TRB = 118°C (Pitch = 2 urn) Figure 3
SEM photograph
of a typical overhanging Figure 5
profile
F. Pommereau
594
4.5.
Electrical
et al. / Image reversal process in contact lithography
Measurements
ACKNOWLEDGMENTS
The purpose of this work was to evaluate in terms of linewidth control, the AZ 5214 image reversal process for lift-off and anisotropic etching applications. The evaluation technique employed electrical linewidth measurements (metal thickness : platinium = 300 A) [7].
Tne authors wish to acknowledge B.G. Martin for helpful discussions, B. Bru for electrical measurements, ana J. Bellaiche and S. Andre for metal deposition.
Lift off
REFERENCES
:
(1) (21 (31 Table 1 Linewidth measurements with OVERDEV = 10 % Anisotropic
etching
: TRB = 118°C ana
(41
: (51
H. Moritz, IEEE Trans. Electron Devices, ED-32, 3, 672, (19851 S.K. Jones and R.C. Chapman, ULSI Sci. and Technol., 11, 190, (1987) M. Bolsen, "AZ 5200 Resists for Positive and Negative Patterning", Hoechst technical note, (October 19861, Hoechst Japan Ltd, Electronic Materials Division, Tokio Box and N.R. Draper, "Empirical G.E.P. Model Building and Response Surfaces", Wiley Series in Probability ana Mathematical statistics, New-York, 1987 "AZ 5214 Positive Photoresist for Semiconductors and Microelectronics", Hoechst technical note (October 1985), American Corporation, Somerville, Hoechst New-
Jersey (61 (71 Table 2 Linewidth measurements with OVERDEV = 20 %
: TRB = 112°C and
the metal In the lift-off process, decreased with an increase in the dose ; is consistent with the increase in top (see Figure 31. In contrast, in the etching process the increased with the dose.
width this width width
show that submicron Electrical measurements patterns can be achieved with the image reversal process even with thick resists (more than dispersions are low ( 1 urn). The associated 0.12 pm at 34 for suomicron features) ana are comparable to those ootained with a tri-level or a chlorobenzene mid-lJV process with thinner resist layers (0.7 urn) [8]. This is clearly an advantage for image reversal, because the image is aefinea at the surface.
5. CONCLUSION
Image reversal is very suitable for GaAs processing using contact lithography due to its good latitude to define overhanging as well as quasi vertical profiles. In addition, submicron patterns can be achievea for a 1.1 urn resist thickness with low dispersions (3 O- of 0.12 urn).
(8)
08876,
(USA)
Vollenbroek and M.J.H.J. Geomini. F.A. Proc. SPIE, 920, 419, (1988) D.S. Perloff, T.F. Hasan and E.R. BLome, Solid State Technology, 23, 2, 81, (19801 M. Iost, S. Gourrier, B. Bru, P. Rabinzohn ana F. Pasqualini, Microelectronic Eng., 6, 69, (19871
Engineering
RESIST
F.
591
9 (1989) 591-594
PROFILE
POMMEREAU,
CONTROL
M.
OF IMAGE REVERSAL
IOST, F.
VOLLENBROEK*,
PROGESS
S.
IN CONTACT
LITHOGRAPHY
GDURRIER
LEP : Laboratoires d'Electronique et de Physique appliquee A member of the Philips Research Organization 3, avenue Descartes, 94450 LIMEIL-BREVANNES, France,
Image
reversal with AZ 5214 resist is characterized by contact lithography, a statistical experimental approach. Overhanging and quasi vertical L%?les which are of particular interest in GaAs technology (lift-off and anisotropic etching processing) are obtained simply by varying the UV3 exposure dose and submicron features are achieved with a low dispersion (3aL = 0.12 urn) using a thick resist.
1. INTRODUCTION
2.2. Second
promising reversal Cl] is a very Image technique because of its very large flexibility which can combine tne advantages of positive and negative resists and has been studied in detail with projection lithography [2]. To our little information exists on the knowledge, application of an image reversal system to contact lithography, although it is widely used in GaAs technology for sub-micron pattern definition.
A second order analysis was then used to build model for the process. A mathematical a "rotatable central composite design" (RCCD) [4 an optimum set of of order two, generates experimental conditions. Tne mathematical model is a polynomial least square fit for each quantitative response. The validity of the model was checked by a variance analysis [4].
3. This paper presents a study on resist profile control of the AZ 5214 image reversal resist 131. Df particular interest are the overhanging profiles for "lift-off" and the vertical profiles (with dimensional control) for pattern definition by anisotropic ion etching. This work has been carried out using a Statistical Experimental Approach (S.E.D.) coupled with a "response surface methodology" [4].
2.
EXPERIMENTAL
METHODOLOGY
2.1. First order analysis
(factorial
analysis)
select First, it necessary to the was processing variables which have the most important effect on the photolithographic process. This has been done using a two level factorial design [4] which consists of all the combinations obtained by selecting two values for each variables. Several responses are measured to characterize the result of the lithography and finally, a hierarchy for the effects of the variables on the responses can be established. * Philips Research Laboratories Nat. Lab. 5600 MD Eindhoven, The Netherlands
0167.9317/89/$3.50
0
1989, Elsevier Science Publishers
B.V. (North-Holland)
The
analysis
order
EXPERIMENTS following
constant
[53
parameters
been
have
held
:
- coating : AZ5214 E photoresist thickness Z = 1.1 pm on GaAs substrates - soft bake = 90°C for 30 mn in an oven, - imaging exposure is performed with a Karl Suss contact mask aligner in the UV3 band (310 nm) through a quartz mask containing grating patterns (equal lines and spaces). - reversal bake = 15 mn in an oven - flood exposure in the UV4 band (365 nm) - development with diluted MF 312 developer. For
the
following
first
order
variables
analysis,
we
used
the
:
imaging exposure dose E = 50 and 200 ml/cm2 Reversal bake temperature T B = 110' and 120°C fF(ood exposure dose FE = 150 and 450 ml/cm2 developer normality N = 0,24 and 0,27 N. The
development
times
tdev
were
adjusted
F. Pommereau
592
et al. 1 Image reversal process in contact lithography
as follows : for each condition, the minimum development time to (required to remove the soluble part of the resist) was determined. The development bv applying
time
tdev
is
then
deduced
from
t0
in the first order analysis a constant over development (10 %). A variable OVERDEV is defined by DVERDEV = (tdev - to)/to. OVERDEV is therefore 0.1 in the first order analysis and is varied from 0 to I in tne second order analysis. the dimensions and the After development, profile of the resist patterns were measured with a Philips 525 SEM after calibration. The corresponaing responses (see Figure 1) are TOPW, BOTW, and MEDW, defined as follows : TOPW = Topwidth/Pitch, and MEDW =
BOTW = Bottomwidth/pitch
Mediumwidth/Pitch
(i.e. mask dimensions i TOP
Figures 2 ana 3 show the response contour plots with TRB held respectively at the values 112°C and 118°C.
with Pitch = 2 urn
The effect of the exposure dose is to increase the responses ana the effect of TRB is to increase TDPW slightly. At low doses, the concentration of the photoproduct is higher at the surface than at the bottom of the resist, due to the relative high absorptivity of the resist in the lJV3 band. This results in a higher crosslink density after the reversal bake in the top of the latent image. This difference in crosslink density between top and bottom is more pronounced at high reversal bake temperatures [6]. Upon development, this leads to overhanging profiles. Thus low dose and high reversal bake temperature leads to negative slopea At high doses, resist walls. the concentration of photoproduct is more uniform from top to bottom, which leads to more or less vertical profiles upon aevelopment.
: line/space = 1 pm/l urn) Finally, tne effect of OVERDEV is to decrease This effect is probably due to the BOTW. relatively low dose received by the bottom of the resist at the mask edges (diffraction effects).
WIDTH
We can notice that the process latitude is very good. For example,nTOPW=.04 for ~TRB=~“. N
i-.-
BOTTOM
:
WIDTH)---
c
PITCH
Figure
-1
1
4.3. Optimization
for "Lift-off"
processing
:
Overhanging profiles can be ontained with : low exposure doses : 50 ,(E,(70 ml/cm2 TRB = 118°C ana 10 %,(oVERDEV,(lOO % (see Figure 5).
4. RESULTS AND DISCUSSION
4.1. First order analysis
The main variables affecting the responses are the exoosure dose E and the reversal bake temperature TRB. Low exposure doses yield improved orofileswhich are overhanaina increasing. the (TOPW-BOTWincreases) by reversal bake temperature. On the other hana, for high doses quasi vertical profiles are observed.
Tne oest overhangs are ootained with a 50 mJ/cm2 exposure dose, but the corresponding pattern wiath is smaller than the nominal width (I urn). On the contrary, by increasing the dose up to 70 ml/cm2, the overhang is degraded but the pattern width is closer to the nominal wiath.
4.4. Optimization
for anisotropic
The optimum
to
100
The flood exposure dose and normality were held constant : FE = 150 mJ/cm2, N = 0.24.
%
the
conditions
are
:
120
were used : to 200 ml/cm2 to 120°C developer
:
Nigh exposure doses, 120,(E ,(140 mJ/cm2, and a moderate overdevelopment yield quasi vertical resist profiles.
4.2. Second order analysis
The following variables varied from 50 E varied from 110 TRB 0 OVERDEV varied from
etching
I< Pommereau
A
100
593
et al. / Image reversal process in contact lithography
!!
Response -->TOPW T RB=112 "C
Response -->TOPW T RB=ilB "C
100
s .4
.45
5
2 P 2 I_: : 0 50
E 100
I Dose
,n. E
(mJIcm2)
200
50
Dose
B
Response -->MEDW T RB=112 "C
E
(mJ/cm2)
200
Response --,MEDW T RB=118 "C
100
\ .45 S Y
0’
50
c 100
cDose ,’
.‘.
E
.‘.
(mJ/cm2)
200
Response -->BOTW T RB=112 "C .2.25.3 .35
0 Ifi 50 Dose
’
C
I _ s
I& z
(mJ/cm2)
200
-i5.'2. 5
/ .3
5 2 9 0
4
2
E
Response -->BOTW T RB=llB "C
100
s
E W
.4
45
5
55
0
50
Responses
Dose
E
(mJ/cm2)
0
200
50
contour plots with TRB = 112'C (Pitch = 2 Mm) Figure 2
SEM photograph
of a quasi vertical Figure 4
profile
Responses
Dose
E
(mJ/cm2)
200
contour plots with TRB = 118°C (Pitch = 2 urn) Figure 3
SEM photograph
of a typical overhanging Figure 5
profile
F. Pommereau
594
4.5.
Electrical
et al. / Image reversal process in contact lithography
Measurements
ACKNOWLEDGMENTS
The purpose of this work was to evaluate in terms of linewidth control, the AZ 5214 image reversal process for lift-off and anisotropic etching applications. The evaluation technique employed electrical linewidth measurements (metal thickness : platinium = 300 A) [7].
Tne authors wish to acknowledge B.G. Martin for helpful discussions, B. Bru for electrical measurements, ana J. Bellaiche and S. Andre for metal deposition.
Lift off
REFERENCES
:
(1) (21 (31 Table 1 Linewidth measurements with OVERDEV = 10 % Anisotropic
etching
: TRB = 118°C ana
(41
: (51
H. Moritz, IEEE Trans. Electron Devices, ED-32, 3, 672, (19851 S.K. Jones and R.C. Chapman, ULSI Sci. and Technol., 11, 190, (1987) M. Bolsen, "AZ 5200 Resists for Positive and Negative Patterning", Hoechst technical note, (October 19861, Hoechst Japan Ltd, Electronic Materials Division, Tokio Box and N.R. Draper, "Empirical G.E.P. Model Building and Response Surfaces", Wiley Series in Probability ana Mathematical statistics, New-York, 1987 "AZ 5214 Positive Photoresist for Semiconductors and Microelectronics", Hoechst technical note (October 1985), American Corporation, Somerville, Hoechst New-
Jersey (61 (71 Table 2 Linewidth measurements with OVERDEV = 20 %
: TRB = 112°C and
the metal In the lift-off process, decreased with an increase in the dose ; is consistent with the increase in top (see Figure 31. In contrast, in the etching process the increased with the dose.
width this width width
show that submicron Electrical measurements patterns can be achieved with the image reversal process even with thick resists (more than dispersions are low ( 1 urn). The associated 0.12 pm at 34 for suomicron features) ana are comparable to those ootained with a tri-level or a chlorobenzene mid-lJV process with thinner resist layers (0.7 urn) [8]. This is clearly an advantage for image reversal, because the image is aefinea at the surface.
5. CONCLUSION
Image reversal is very suitable for GaAs processing using contact lithography due to its good latitude to define overhanging as well as quasi vertical profiles. In addition, submicron patterns can be achievea for a 1.1 urn resist thickness with low dispersions (3 O- of 0.12 urn).
(8)
08876,
(USA)
Vollenbroek and M.J.H.J. Geomini. F.A. Proc. SPIE, 920, 419, (1988) D.S. Perloff, T.F. Hasan and E.R. BLome, Solid State Technology, 23, 2, 81, (19801 M. Iost, S. Gourrier, B. Bru, P. Rabinzohn ana F. Pasqualini, Microelectronic Eng., 6, 69, (19871