Improving the Process Capability of SU8

Improving the Process Capability of SU-8 M. Shaw, D. Nawrocki, R. Hurditch and D. Johnson MicroChem Corp., Newton MA, 02464

ABSTRACT SU-8 negative resists designed to produce uniform thick films in a single spin-coating step are widely used in the fabrication of MEMS devices. A potential limitation in using resists of this type in large-scale production is the relatively long processing cycle time, which is determined by the choice of solvent and bake conditions. A well-known disadvantage of SU-8 is its inability to wet low surface energy substrates which can result in non-uniform coating. In this paper we present the results of the first part of a study aimed at improving SU-8 process capability in which the focus is on solvent replacement. Some of the important tradeoffs that should be considered in the selection of a practical solvent are discussed. The results of an investigation that led to the selection of cyclopentanone to replace gamma-butyrolactone are presented, the relative drying rates are compared and the drying mechanism is discussed. This study led to the commercialization of a new family of cyclopentanone based resists, SU-8 2000 (referred to as SU-8 CP in this paper) which shows significantly improved wetting, faster drying for film thicknesses up to about 50 microns and clean edge bead removal without the need for an intermediate bake step.

1. INTRODUCTION SU-8 photoresist compositions based on a multifunctional bisphenol A novolak epoxy resin and a photoacid generator as the curing agent were first disclosed by IBM (1). The resist combines the desirable properties of high transparency at wavelengths above 340 nm, relatively fast photospeed, high lithographic contrast and resistance to plating bath chemistries, making it uniquely suited for the fabrication of high aspect ratio MEMS devices. Another important component of a resist is the solvent, which plays a critical role in determining viscosity (film thickness), coating quality and cycle times for effective solvent removal. For standard SU-8, γbutyrolactone (GBL) is the chosen solvent, primarily because of its high solvency, low cost and low toxicity. GBL, with a boiling point of 204ºC has low volatility which is expected to result in longer drying times than might be the case with other more volatile resist solvents. Furthermore, spin-coated films tend to wet poorly leading to pull back or pin-hole formation on low surface energy substrates such as silicon (depending on surface condition), glass, titanium nitride and gold. In this study, the main objective was to identify an alternate solvent which would result in faster drying times than GBL, together with additional improvements in over-all process capability.

2. INVESTIGATION and SELECTION OF ALTERNATIVE SOLVENTS The main selection criteria used for initial screening of candidate solvents were as follows: 1. Ketone or similar solvency based on Hildebrand δ and Hansen δh solubility parameters 2. Low Toxicity 3. Flash point > 10ºC (ideally >25ºC) 4. Vapor pressure >3mm (10 x GBL) 5. Cost similar to or lower than GBL

A list of solvents that meet or are close to meeting these requirements is shown in Table 1. Of these, nitromethane (Cal Proposition 65 listed) was eliminated because of toxicity indications. Table 1 Solvent

BPt ºC

MIPK (3-methyl-2-butanone ) 2-pentanone Pinacolone ( t-butyl methylketone) Nitromethane MIBK (4-methyl-2-pentanone) PME Cyclopentanone Cyclohexanone methylamylketone (2-heptanone) PMGEA Gammabutyrolactone

Flash Vapor Molar Point pressure Volume ºC mm 25ºC cm3/mol

Solubility Parameters δh δ

95

6

45.0

107

17.2

105

7

30.0

108

4.7

17.8

106

23

23.0

125

4.3

17.0

101

35

22.4

54

5.1

25.1

117

16

17.3

125

118 130 157

32 30 44

10.9 10.0 3.8

128 88 103

4.3 5.1

18.0 19.3 19.6

150

39

3.8

139

4.1

17.5

146 204

46 98

3.3 0.3

136 81

4.8 7.4

18.3 25.2

17.1

Solvents that passed the screening criteria included; cyclopentanone (CP), methyl isobutyl ketone (MIBK), t-butyl methyl ketone (pinacolone), cyclohexanone, and conventional positive resist solvents such as methylamylketone, 1-methoxy-2-propanol and propylene glycol monomethyl ether. From these candidates, solvents having a vapor pressure > 5mm and molar volume < 120 were assigned the highest priority for subsequent experimental evaluation. The desirable properties to be defined by these experiments were as follows: 1. 2. 3. 4.

Ability to dissolve SU-8 resin under conditions acceptable for production Drying rate > 1.5 x SU-8 GBL Excellent coating quality Acceptable tolerance for manufacturing to a target film thickness based on slope of viscosity vs. solids curve.

SU-8 resin dissolution rate in the selected solvent was assessed using a lab scale version of standard production equipment. Drying times of 55% solids resist solutions at 95 and 115ºC were determined by means of an Omnimark solids analyzer. Those solvents that passed the first two tests above were used to formulate 60-67% solids content resists solutions, which were spin coated at 1000 and 3000 rpm onto RCA cleaned 4 inch silicon wafers. The coatings were inspected for uniformity and absence of striations.

MIBK produced poor quality coatings even at relatively high solids (67%) due to the formation of a surface skin, which partitioned from the film during spin coating. Pinacolone formulations exhibited bubbling during softbake. Cyclopentanone met all the requirements and was selected as the solvent of choice. Table 2 compares important properties with GBL. It was interesting to observe that SU-8 formulations based on CP, which has a vapor pressure approximately 30 times higher than GBL, dried only about 30% faster at 95ºC. This suggests that diffusion, which depends on the solvent molar volume and solvent polymer interaction, is the rate determining process for drying of high solids formulations, at temperatures significantly below the boiling point (130ºC). This topic will be discussed in more detail in section 3. Table 2 Vapor Surface Molar Solvent Pressure Tension Volume mm Hg 25ºC GBL CP

10 0.3

cm3/mol 40.4 29.5

81 88

Resist Viscosity Canon Fenske 25ºC 65% solids

Drying Time (to constant weight) ( Omnimark ) 70% min @ min @ solids 95 ºC 115 ºC

3634.8 12100 2004.5 7000

38 27

28 13

3. DRYING STUDIES AND FUNCTIONAL EVALUATION OF SU-8 CP Experimental Formulation SU-8 formulations were prepared by rolling the predetermined quantities of epoxy resin, PAG and solvent in a jar roller with heating to not greater than 65ºC for 4 hrs. The solutions were filtered through a 0.6 micron filter. The solids content of each formulation was chosen such that target film thicknesses of approximately 5, 10, 15-20, 25-30, 40-50 and 85-100 micron would be obtained at a spin speed of 3000rpm. Spin Coating A Brewer Science CEE CB100 coater was used for spin coating. The resist was statically dispensed onto a 4 inch RCA cleaned silicon wafer and spread at 500 rpm for 5 seconds. The speed was ramped to 3000 rpm and held for 30seconds to attain the target film thicknesses. The same spin coating conditions were used for all of the samples evaluated. Weight loss and film thickness change Following coating on the CEE CB100 under the conditions described above, each wafer was baked at 65°C for one minute on the hotplate. The wafer was cooled to room temperature. The film thickness was measured using an SCI FilmTek 1800 spectral interferometer and the weight was determined using an OHAUS GA110 balance. After recording the initial film thickness and weight, the wafer was heated on the hotplate at 95°C for predetermined intervals; initially 1 minute and gradually increasing to 5 minutes such that a total of at least 12 measurements were obtained up to the maximum heating time of 30 minutes. After each baking period the wafer was allowed to cool to room temperature and the film thickness and weight recorded.

Edge bead removal The edge bead was removed from the topside and/or backside using either acetone or EBR PG (MicroChem) and a spin speed of 500 rpm for 30seconds Lithographic processing After soft baking at 95ºC for the time determined to produce a “dry” film, the coated wafer was exposed in contact through a resolution mask using a broadband mercury-xenon source, followed by a post exposure bake at 95º C. Development was by immersion in SU8 Developer (propylene glycol monomethyl ether acetate) with slight agitation. The imaged wafer was rinsed with developer, spray washed with isopropyl alcohol and blown dry. Exposure times, PEB times and develop times were adjusted in a predetermined manner according to film thickness.

Results and Discussion Drying Times Figures 1 and 2 show typical thickness and weight change data obtained in this case during the 95ºC bake of SU-8 50 GBL and SU-8 50 CP. Changes in film thickness and weight diminish exponentially as the “dry” condition is approached. Drying times obtained from weight loss and film thickness data were similar. Differences were generally attributed to noise in the data. Both were measurable with similar precision, estimated to be in the 0.05 to 0.2 % range. The noise in the weight loss data was lower for thicker films and the noise in the film thickness data was lower for thinner films. Figure 2 50

11.64 SU-8 50GBL F

11.62

FILM THICKNESS (micron)

WEIGHT OF COATED WAFER (gms)

Figure 1

SU-8 50CP

11.6 11.58 11.56 11.54 11.52

SU-8 GBL SU-8 CP

45

40

35

0

5

10

15

20

BAKE TIME at 95C (min)

25

0

5

10 15 20 DRYING TIME at 95C (min)

25

The “dry” condition was defined as that corresponding to change of less than 0.1% which was close to the noise level in the experimental data for both weight and film thickness measurements. In Figure 3, the dry weights thus obtained are plotted as a function of film thickness for all of the samples investigated. Variations in the expected linearity are small and are caused by film thickness non-uniformity primarily due to the edge bead. Figure 4 plots the drying times obtained the by averaging the results from weight and film thickness changes, as a function of film thickness, for SU-8 GBL and SU-8 CP. It is evident that the CP formulations dry

somewhat faster than GBL formulations, however, differences in drying times for the different solvent formulations are relatively small especially for thicker films. Figure 3

Figure 4 35 30

0.8

AVERAGE DRYING TIME at 95C (min)

RESIST FILM WEIGHT (gm) AFTER 95C BAKE

1.0

0.6

0.4 SU-8 GBL 0.2 SU-8 CP

25 20 15 10 SU-8 GBL 5

SU-8 CP

0

0.0 0

20

40

60

80

0

100

20

40

60

80

100

FILM THICKNESS (microns)

FILM THICKNESS (micron)

The difference in soft-bake drying times between the two solvent systems at the lower end of the film thickness range is of the same magnitude the result obtained from the drying experiments done with the Omnimark solids analyzer shown in Table 2. At first sight these results appear to be unexpected because of the significant differences in the vapor pressures and hence evaporation rates of the two solvents. The effect of the difference in evaporation rates during spin coating is clearly shown in Figure 5, which plots the solvent lost during soft bake from the as deposited film (expressed as % of film weight) as a function of film thickness for all of the formulations investigated the two resist systems. Figure 5

Figure 6 1.6

20

RATIO SU-8 CP:SU-8 GBL

WEIGHT % SOLVENT LOST

25

15

10

SU-8 GBL

5

SU-8 CP 0 0

20

40

60

FILM THICKNESS (micron)

80

100

1.2

0.8 solvent retained

0.4

drying tim e

0 0

20

40

60

80

FILM THICKNESS (micron)

100

The weight % solvent loss shown in Figure 5 is equal to the weight % of solvent retained in the resist after spin coating minus the small (typically 3-5%) of solvent remaining in the film after drying to constant weight. Thus, a 5 micron film of SU-8 GBL retains about 8 % GBL whereas a 5 micron film of SU-8 CP retains only 4%. The amount of solvent retained by SU-8 GBL increases rapidly with film thickness up to about 20-30 microns, then gradually levels off to about 22.5% at about 85 microns. Whether the apparent maximum of 24% at about 25 microns is real is unclear since the data was not statistically validated. The SU-8 CP system behaves similarly; except the increase is more gradual and the amount retained at 100 microns is estimated to be 21%. The total solvent of the resist as manufactured, is about 27% (GBL or CP) plus 2.5% of propylene carbonate. Therefore, despite the much higher volatility of CP compared with GBL, solvent loss during spin coating is minimal for both resists at high solids (film thickness). As expected, the amount of solvent retained is much smaller for thin films (low solids). Figure 6 plots the ratios of the retained solvents and drying times for the two systems. The curves shown were obtained by fitting the raw data of Figures 4 and 5 to “best fit” curves, then using these to calculate the ratios as shown. It is clearly evident and consistent with expectations that the relative drying times are inversely correlated with the relative concentrations of retained solvent after spin coating. The above results are in accordance with studies on the drying kinetics of polymer-solvent systems (3), in which diffusion is the dominant rate determining step for a film baked in a weakly convective environment at temperatures both above and below the system Tg. The rate of solvent loss is determined by the solvent diffusion coefficient, which increases exponentially with temperature and free volume in the drying polymersolvent system (4). The free volume decreases with decrease in the concentration of retained solvent. Thus, the higher levels of GBL retained after spin coating due to its low volatility diffuse much more rapidly in the “wet” film (high free volume). As shown in Figure 7, the weight loss in the first minute of baking is significantly greater for the SU-8 GBL than for SU-8 CP formulations. Furthermore, as shown in Figure 8 which plots the weight change from the dry condition divided by the film thickness for SU-8 GBL formulations, most of the solvent loss occurs in the first minute or two of baking, when the amount of solvent remaining is greater than about 1% per micron corresponding to a relatively high free volume and therefore relatively rapid diffusion Figure 7

Figure 8 NORMALIZED WEIGHT CHANGE FROM "DRY" WEIGHT (mg/micron

3.0

INITIAL WEIGHT LOSS (mg) 95C BAKE 1 MIN :

80

60

40

SU-8 GBL

20

SU-8 CP

SU-8 5 SU-810

2.5

SU-8 25

2.0

SU-8 35 SU-8 50

1.5

SU-8 100

1.0 0.5 0.0

0 0

20

40

60

80

FILM THICKNESS (micron)

100

0

10 20 BAKE TIME 95C (min)

30

The diffusion coefficient and hence drying rate reduces exponentially with decrease in free volume. As the solvent content of the CP and GBL films approach the same value (below about 0.3% per micron), the diffusion constants become approximately equal and drying proceeds at an equally slow rate. The similarity of diffusion coefficients for the two solvents might be expected because of similar molar volumes and solubility parameters as shown in Table 1. Edge Bead Removal It is well known that clean removal of the edge bead of spin coated SU-8 GBL films cannot be accomplished without first performing an intermediate bake step. Figure 9 shows the resulting jagged edge produced as a result of attempting to remove the edge bead of a freshly deposited (unbaked) 50um SU-8 GBL film using acetone as the removing solvent. Figure 10 shows the result of the identical process done with SU-8 CP. Clean removal will be obtained if a) the rate of penetration of the of the solvent into the resist film is large enough so that during the contact time the distance penetrated is of the order of the film thickness of the resist, but not significantly greater, and b) the solvent wets the resist uniformly. Since the SU-8 CP films will have a reduced amount of retained solvent (Figure 5) , hence reduced EBR penetration and a lower surface tension (Table 2) both of these conditions are favorable compared with SU-8 GBL under the same conditions.

Figure 9

SU-8 GBL on silicon after edge wipe

Figure 10

SU-8 CP on silicon after edge wipe

The ability to perform the edge bead removal step without an intermediate bake step leads to the potential advantages of reduced process cycle time and reduced hot-plate utilization.

Surface Wetting It is well-known that standard SU-8 does not spread uniformly and may pull-back after spin coating on silicon wafers having a thin native oxide layer, titanium nitride, certain glass compositions, gold and other low surface energy substrates. This phenomenon is normally attributed to poor wetting by the resist solution, and in the case of SU-8 GBL is attributed to the relatively large surface tension as shown in Table 2. Figure 11 shows the effect of poor wetting after spin coating a 50 micron film of standard SU-8 on a silicon wafer which had been RCA cleaned then stored in the clean room for several weeks. Figure 12 shows the result of the same process using SU-8 CP, which shows complete spreading to the edge of the wafer. Additional tests confirmed

the superior spreading characteristics of SU-8 CP on gold, borosilicate, silica and titanium nitride. The significant improvement is attributed to the lower surface tension of the solvent as shown in Table 2.

Figure 11

Figure 12

SU-8 GBL on silicon

SU-8 CP on silicon

Lithographic Processing Standard Lithographic processing of SU-8 CP using the softbake times determined from the experiments described in the forgoing showed no discernable differences compared with standard SU-8 processed under the same conditions. The SEM photograph of Figure 9 shows dense arrays of 5, 10 and 20 microns in a 50 micron thick film. Figure 13

Conclusions 1) After spin-coat, more solvent remains in films of SU-8 GBL compared with SU8-CP, the difference decreases with increase in film thickness. This is due to the higher evaporation rate of CP compared with GBL. 2) Although SU-8 CP films dry faster than SU-8 GBL at 95ºC, the differences in drying times for a given film thickness are relatively small and diminish as film thickness increases This can be explained as follows: Drying of a spin coated film that is depleted of a large fraction of solvent is essentially determined by the rate of diffusion of solvent through the drying film. Initially, this is faster in the SU-8 GBL films due to the higher free volume caused by the greater amount of retained solvent. SU-8 CP films contain less retained solvent after spin coating which speeds up the overall drying process despite the lower initial diffusion rate. As the drying proceeds to the "near-dry state", the diffusion rates diminish, becoming similar for both solvent systems due to a similar degree of polymer-solvent interaction (similar molecular volume and polarity). 3) The decreased solvent content of SU-8 CP after spin coating, results in clean removal of the edge bead prior to soft bake. This cannot be done with SU-8 GBL, which requires an intermediate bake. 4) The lower surface tension of CP compared with GBL results in significantly improved wetting and therefore, more uniform spread on low surface energy substrates.

References 1) N. LaBianca, J.D. Gelorme, SPIE Vol. 2438, 846-849 (1995). 2) G. Wypych, Ch. 7 in ‘Handbook of Solvents”, Ed G. Wypych, Chem Tec Publishing, (2001). 3) J. S. Vrentas, C.M. Vrentas, J. Polmer Sci., Part B: Polym Phys., 30(9), 1005-11 (1992).