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Metallized photoresists: A new approach to surface imaging
Microelectronic Engineering 13 (1991) 93-96 Elsevier
93
METALLIZED PHOTORESISTS: A NEW APPROACH TO SURFACE IMAGING
L. N. Abali, S. M. Bobbio*, J. F. Bohland, G. S. Calabrese, M. Gulla, E. K. Pavelchek, and P. Sricharoenchaikit Shipley Company, Inc. 455 Forest St. Marlboro, MA 01752, USA
G-line photoresists are exposed at 248 nm and subjected to aqueous development and electroless plating to produce submicron, metallized photoresist patterns. The high plasma etch resistance of the photoresist/metal structure allows for efficient pattern transfer into silicon dioxide. A key feature is that optical exposure of only the near-surface of the photoresist is required to achieve selective metallization. This process represents a new approach to surface imaging, reducing complications from substrate reflectivity and topography. 0.4 #m line/space pairs have been transferred into 5000 A of SiO2 on Si.
1. INTRODUCTION The most widely known single-level resist systems capable of sub-0.5 #m optical lithography rely on surface imaging in combination with gas phase silylation and dry development [1-3]. The use of surface imaging [4,5] alleviates focus latitude problems which become more severe at shorter imaging wavelengths and allows use of heavily dyed resists to solve problems associated with substrate reflectivity. Unfortunately successful use of silylation requires selection of the proper photoresist chemistry and silylating agent as well as careful control of the silylation conditions. In addition, redeposition of Si or Si-containing species in unwanted areas during dry development is common, and although the incorporation of Si increases oxygen plasma etch resistance it does not substantially increase the resistance to other plasmas (eg. SF6). We wish to report here on a surface imaging scheme [6] which employs a 248 nm exposure of g-line positive-tone photoresist in combination with electroless plating and dry development to achieve sub-0.5 #m resolution. *MCNC, P.O. Box 12889, Research Triangle Park, NC 27709, USA 0167-9317/91/$3.50 © 1991 - Elsevier Science Publishers B.V.
L.N. Abali et al. / Metallized photoresists
94
2. EXPERIMENTAL Si wafers containing 5000/~, of thermally grown oxide were first primed for 30 s with HMDS and spin coated with Shipley S-1400-31 Microposit® photoresist to yield =1.5 #m thick films after drying at 60 s/100o C (vacuum hotplate). Printing was accomplished on a GCA ALS KrF eximer laser projection stepper (0.35 NA) equipped with a high resolution reticle. Exposure energies were 50 mJ/cm2. Selective metallization was accomplished as shown in figure 1. Wafers were treated with the plating solutions without agitation. Shipley Cataposit® 44 and Cataprep® 404 were used to catalyze the resist surfaces. An HCI-based accelerator was used. Photoresist development was accomplished using Shipley Microposit® developer concentrate diluted to 0.21 N. Plating was accomplished using Shipley Niposit® 468 electroless Ni diluted 1:20 with water and adjusted to pH 7. The etching apparatus used was a split cathode magnetron which has been developed at MCNC [7]. An O2/N2 mixture was used for the photoresist etch, and a CF4/O2 mixture was used for the SiO2 etch. Ion energies were <140 eV (=3 mA/cm2). 3. RESULTS 3.1. Resist exposure In order to obtain sub-0.5 #m imaging,-a g-line positive-tone photoresist was exposed with a 0.35 N.A. 248 nm excimer laser stepper. Since the optical density of the resist is >l/I.tm at 248 nm use of doses of =50 mJ/cm2 result in effective exposure of only the near-surface of the photoresist. An added feature of this top surface-only exposure is
photoresist exposure
I
I
~, colloidal catalyst
.
.
.
.
.
~ plate
i
~ dry etches
I
I
FIGURE 1 Photoresist metallization process for surface imaging.
FIGURE 2 Metallized photoresist at 0.9 p~m before (top) and after (bottom) 02 and CF4 RIE's. Substrate is 5000 ,&, SiO2 on Si.
L.N. Abali et a L / Metallized photoresists
95
the isolation of the underlying substrate from the solutions throughout the process. The uppermost electron micrograph shown in figure 2 represents a typical resist profile after exposure, catalyst deposition and acceleration, development, and metallization. It is apparent that <0•2 ~m of the resist has been developed away at the surface• Figure 3 shows results at 0.4 I~m feature size for the same process. 100 " 80O~
P
60"
> o rJ
40 2O 0
•
I
I
"
I
"
i
•
I
"
o.s Developer Concentration, N
0.0
FIGURE 3 0.4 I~m pattern transfer into 5000 ,~ of SiO2 on Si by RIE.
0.1
0.2
0.3
0.4
0.5
FIGURE 4 Effect of developer concentration on electroless Ni coverage.
3.2. Catalyst Adsorption and Activation In order to obtain uniform, defect-free metal coatings on the order of 100-200/~, thick a modification of the conventional electroless plating process was required. The first part of the process involves adsorption of a PdCI2/SnCI2 colloidal catalyst [8] onto the surface of the exposed photoresist• We have employed a commercially available catalyst and used it without modification• Best results are obtained using no agitation and contact times of 5 or more minutes at room temperature• Once adsorbed, it is necessary to activate the catalyst particles (known as acceleration) by removing the Sn-rich outer layer, exposing catalytic Pd sites• This is usually done using acidic or basic solutions• We have found that HCI-containing solutions give the best results in the present application. We have investigated the effect of HCI concentration and found that the HCI level must be kept below =0.25 M to achieve good plating• Immersion times of 3-4 minutes give the best results. It should also be noted that the development step acts as an accelerator for the colloidal catalyst, but the HCI-based accelerator is required for best results• 3.3. Development of the Surface-Catalyzed Photoresist Following photoresist catalysis and acceleration, selective metallization is accomplished by first removing the catalyst and underlying photoresist from the irradiated areas• This is achieved by use of customary diazonaphthoquinone/novolak photoresist development in aqueous base. Care must be taken, however, to choose specific developer strength and contact times such that adsorbed and activated catalyst particles on the unexposed photoresist regions are not inactivated or removed• Removal could occur either by direct desorption from the resist surface or by removal of unexposed photoresist. The data in figure 4 show that developer
96
L.N. Abali et al. / Metallized photoresists
normalities in excess of 0.35 N have an adverse affect on the plating. In addition, we have found that immersion times longer than 2 minutes also lead to poor plating. Based on these observations, development times of 1-2 min. at 0.18-0.21 N are used. 3.4. Dry Etching of Metallized Photoresist Table 1 shows a comparison of dry etch rates for a variety of materials measured on the magnetron apparatus described in the experimental section. The data show that the very thin Ni films deposited on photoresist surfaces are extremely effective etch barriers, and allow for high-resolution pattern transfer using photoresist and oxide dry etches. TABLE 1. Comparison of Oxygen Plasma Etch Rates Material
Etch Rate, ,/Vsec.
Novolak polymers
140-280
Silylated novolaks a. non-silylated areas b. silylated areas
70-140 <25
Ni on novolak (70-170 A)
0.1-0.4
4. CONCLUSIONS Electroless plating is combined with conventional positive-tone photoresists to produce sub-0.5 p.m images. The high plasma etch resistance of the thin metal films deposited by electroless plating allows for efficient pattern transfer in both oxygen and fluorine chemistries. Finally, because exposure of only the top surface of the photoresist is required, the process represents a new approach to surface imaging. REFERENCES [1] Coopmans, F. and Roland, B.Proc. SPIE, 631,34 (1986). [2] Schellekens, J. P. W. and Visser, R. -J., Proc. SPIE, 1086, 220 (1989). [3] Thackeray, J. W., Bohland, J. F., Pavelchek, E. K., Orsula, G. W., McCullough, A. W., Jones, S. K., and Bobbio, S., Proc. SPIE, 1185, 2 (1990). [4] Taylor, G. N., Stillwagon, L. E., and Venkatesan, T., J. Electrochem. Soc., 131, 1654 (1984); Wolf, T. M., Taylor, G. N., Venkatesan. T., and Kraetsch, R. T., J. Electrochem. Soc., 131, 1658 (1984). [5] Taylor, G. N., Nalamasu, O., Stillwagon, L. E., Microelectron. Engr., 9, 513 (1989). [6] Gulla, M. and Sricharoenchaikit, P., U.S. patent application (unpublished). [7] Bobbio, S. and Ho, Y. -S., U.S. patent 4,738,761 (1988). [8] (a) Shipley, C. R., U.S. patent 3,011,920 (1961); (b) D'Ottavio, E. D., U.S. patent 3,532,518 (1970); (c) Gulla, M. and Conlan, W. A., U.S. patent 3,874,882 (1975); (d) Gulla, M. and Conlan, W. A., U.S. patent 3,904,792 (1975); (e) Matijevic, E., Poskanzer, A. M., and Zuman, P., Plating, 62, 958 (1975); (f) Feldstein, M., Schlesinger, N. E., Hedgecock, N. E., and Chow, S. L., J. Electrochem. Soc., 121, 738, (1974); (g) Cohen, R., and West, K., J. Electrochem. Soc., 120,502 (1973).
93
METALLIZED PHOTORESISTS: A NEW APPROACH TO SURFACE IMAGING
L. N. Abali, S. M. Bobbio*, J. F. Bohland, G. S. Calabrese, M. Gulla, E. K. Pavelchek, and P. Sricharoenchaikit Shipley Company, Inc. 455 Forest St. Marlboro, MA 01752, USA
G-line photoresists are exposed at 248 nm and subjected to aqueous development and electroless plating to produce submicron, metallized photoresist patterns. The high plasma etch resistance of the photoresist/metal structure allows for efficient pattern transfer into silicon dioxide. A key feature is that optical exposure of only the near-surface of the photoresist is required to achieve selective metallization. This process represents a new approach to surface imaging, reducing complications from substrate reflectivity and topography. 0.4 #m line/space pairs have been transferred into 5000 A of SiO2 on Si.
1. INTRODUCTION The most widely known single-level resist systems capable of sub-0.5 #m optical lithography rely on surface imaging in combination with gas phase silylation and dry development [1-3]. The use of surface imaging [4,5] alleviates focus latitude problems which become more severe at shorter imaging wavelengths and allows use of heavily dyed resists to solve problems associated with substrate reflectivity. Unfortunately successful use of silylation requires selection of the proper photoresist chemistry and silylating agent as well as careful control of the silylation conditions. In addition, redeposition of Si or Si-containing species in unwanted areas during dry development is common, and although the incorporation of Si increases oxygen plasma etch resistance it does not substantially increase the resistance to other plasmas (eg. SF6). We wish to report here on a surface imaging scheme [6] which employs a 248 nm exposure of g-line positive-tone photoresist in combination with electroless plating and dry development to achieve sub-0.5 #m resolution. *MCNC, P.O. Box 12889, Research Triangle Park, NC 27709, USA 0167-9317/91/$3.50 © 1991 - Elsevier Science Publishers B.V.
L.N. Abali et al. / Metallized photoresists
94
2. EXPERIMENTAL Si wafers containing 5000/~, of thermally grown oxide were first primed for 30 s with HMDS and spin coated with Shipley S-1400-31 Microposit® photoresist to yield =1.5 #m thick films after drying at 60 s/100o C (vacuum hotplate). Printing was accomplished on a GCA ALS KrF eximer laser projection stepper (0.35 NA) equipped with a high resolution reticle. Exposure energies were 50 mJ/cm2. Selective metallization was accomplished as shown in figure 1. Wafers were treated with the plating solutions without agitation. Shipley Cataposit® 44 and Cataprep® 404 were used to catalyze the resist surfaces. An HCI-based accelerator was used. Photoresist development was accomplished using Shipley Microposit® developer concentrate diluted to 0.21 N. Plating was accomplished using Shipley Niposit® 468 electroless Ni diluted 1:20 with water and adjusted to pH 7. The etching apparatus used was a split cathode magnetron which has been developed at MCNC [7]. An O2/N2 mixture was used for the photoresist etch, and a CF4/O2 mixture was used for the SiO2 etch. Ion energies were <140 eV (=3 mA/cm2). 3. RESULTS 3.1. Resist exposure In order to obtain sub-0.5 #m imaging,-a g-line positive-tone photoresist was exposed with a 0.35 N.A. 248 nm excimer laser stepper. Since the optical density of the resist is >l/I.tm at 248 nm use of doses of =50 mJ/cm2 result in effective exposure of only the near-surface of the photoresist. An added feature of this top surface-only exposure is
photoresist exposure
I
I
~, colloidal catalyst
.
.
.
.
.
~ plate
i
~ dry etches
I
I
FIGURE 1 Photoresist metallization process for surface imaging.
FIGURE 2 Metallized photoresist at 0.9 p~m before (top) and after (bottom) 02 and CF4 RIE's. Substrate is 5000 ,&, SiO2 on Si.
L.N. Abali et a L / Metallized photoresists
95
the isolation of the underlying substrate from the solutions throughout the process. The uppermost electron micrograph shown in figure 2 represents a typical resist profile after exposure, catalyst deposition and acceleration, development, and metallization. It is apparent that <0•2 ~m of the resist has been developed away at the surface• Figure 3 shows results at 0.4 I~m feature size for the same process. 100 " 80O~
P
60"
> o rJ
40 2O 0
•
I
I
"
I
"
i
•
I
"
o.s Developer Concentration, N
0.0
FIGURE 3 0.4 I~m pattern transfer into 5000 ,~ of SiO2 on Si by RIE.
0.1
0.2
0.3
0.4
0.5
FIGURE 4 Effect of developer concentration on electroless Ni coverage.
3.2. Catalyst Adsorption and Activation In order to obtain uniform, defect-free metal coatings on the order of 100-200/~, thick a modification of the conventional electroless plating process was required. The first part of the process involves adsorption of a PdCI2/SnCI2 colloidal catalyst [8] onto the surface of the exposed photoresist• We have employed a commercially available catalyst and used it without modification• Best results are obtained using no agitation and contact times of 5 or more minutes at room temperature• Once adsorbed, it is necessary to activate the catalyst particles (known as acceleration) by removing the Sn-rich outer layer, exposing catalytic Pd sites• This is usually done using acidic or basic solutions• We have found that HCI-containing solutions give the best results in the present application. We have investigated the effect of HCI concentration and found that the HCI level must be kept below =0.25 M to achieve good plating• Immersion times of 3-4 minutes give the best results. It should also be noted that the development step acts as an accelerator for the colloidal catalyst, but the HCI-based accelerator is required for best results• 3.3. Development of the Surface-Catalyzed Photoresist Following photoresist catalysis and acceleration, selective metallization is accomplished by first removing the catalyst and underlying photoresist from the irradiated areas• This is achieved by use of customary diazonaphthoquinone/novolak photoresist development in aqueous base. Care must be taken, however, to choose specific developer strength and contact times such that adsorbed and activated catalyst particles on the unexposed photoresist regions are not inactivated or removed• Removal could occur either by direct desorption from the resist surface or by removal of unexposed photoresist. The data in figure 4 show that developer
96
L.N. Abali et al. / Metallized photoresists
normalities in excess of 0.35 N have an adverse affect on the plating. In addition, we have found that immersion times longer than 2 minutes also lead to poor plating. Based on these observations, development times of 1-2 min. at 0.18-0.21 N are used. 3.4. Dry Etching of Metallized Photoresist Table 1 shows a comparison of dry etch rates for a variety of materials measured on the magnetron apparatus described in the experimental section. The data show that the very thin Ni films deposited on photoresist surfaces are extremely effective etch barriers, and allow for high-resolution pattern transfer using photoresist and oxide dry etches. TABLE 1. Comparison of Oxygen Plasma Etch Rates Material
Etch Rate, ,/Vsec.
Novolak polymers
140-280
Silylated novolaks a. non-silylated areas b. silylated areas
70-140 <25
Ni on novolak (70-170 A)
0.1-0.4
4. CONCLUSIONS Electroless plating is combined with conventional positive-tone photoresists to produce sub-0.5 p.m images. The high plasma etch resistance of the thin metal films deposited by electroless plating allows for efficient pattern transfer in both oxygen and fluorine chemistries. Finally, because exposure of only the top surface of the photoresist is required, the process represents a new approach to surface imaging. REFERENCES [1] Coopmans, F. and Roland, B.Proc. SPIE, 631,34 (1986). [2] Schellekens, J. P. W. and Visser, R. -J., Proc. SPIE, 1086, 220 (1989). [3] Thackeray, J. W., Bohland, J. F., Pavelchek, E. K., Orsula, G. W., McCullough, A. W., Jones, S. K., and Bobbio, S., Proc. SPIE, 1185, 2 (1990). [4] Taylor, G. N., Stillwagon, L. E., and Venkatesan, T., J. Electrochem. Soc., 131, 1654 (1984); Wolf, T. M., Taylor, G. N., Venkatesan. T., and Kraetsch, R. T., J. Electrochem. Soc., 131, 1658 (1984). [5] Taylor, G. N., Nalamasu, O., Stillwagon, L. E., Microelectron. Engr., 9, 513 (1989). [6] Gulla, M. and Sricharoenchaikit, P., U.S. patent application (unpublished). [7] Bobbio, S. and Ho, Y. -S., U.S. patent 4,738,761 (1988). [8] (a) Shipley, C. R., U.S. patent 3,011,920 (1961); (b) D'Ottavio, E. D., U.S. patent 3,532,518 (1970); (c) Gulla, M. and Conlan, W. A., U.S. patent 3,874,882 (1975); (d) Gulla, M. and Conlan, W. A., U.S. patent 3,904,792 (1975); (e) Matijevic, E., Poskanzer, A. M., and Zuman, P., Plating, 62, 958 (1975); (f) Feldstein, M., Schlesinger, N. E., Hedgecock, N. E., and Chow, S. L., J. Electrochem. Soc., 121, 738, (1974); (g) Cohen, R., and West, K., J. Electrochem. Soc., 120,502 (1973).