Advances in thick photoresists for flip-chip bumping
08/01/2005
The push to smaller process geometries and higher-count I/O on ICs in portable and wireless applications has significantly increased the industry’s focus on chip-scale packaging and flip-chip assembly techniques, which deposit metal contact bumps across the entire surface of a die. Selection of thick photoresists for flip-chip processes also is becoming more important as manufacturers face new challenges in patterning smaller bumps and finer-pitch contacts with traditional metal masks. This article reviews the trends and tradeoffs for bump formation using three basic types of photoresists: naphthoquinone diazide positive-tone, acrylic negative-tone, and chemically amplified positive-tone resists.
Microbump formation processes generally include patterning resist by photolithography, electroplating to form metal bumps, and stripping the photoresist. During photolithography steps, the resolution required for bump formation is much lower than in frontend wafer fab processes, which continually push limitations of printing smaller feature sizes for devices on ICs. Even new gold bump processes for LCD driver chips requiring the highest resolution for contacts have ~30µm feature sizes, while frontend resists are dealing with minimum feature sizes <100nm. For thick resist applications in bumping processes, however, completely different characteristics are required. High viscosity and high solid content are needed to deposit films as thick as 10µm or more. Metal plating tolerance also is essential in flip-chip bumping. Specifically, high chemical resistance and mechanical strength are necessary to withstand plating solutions and to bear the plating stress that can occur in conventional polymer materials in resists.
Slight variations in photoresist composition can have major effects on plating performance. Photoresist is developed in aqueous developer and the plating solution is generally of a high pH and used at a high temperature. The resist must have a strong chemical tolerance to withstand the severe chemical environment of the plating solution. If adhesion is poor, plating agents will penetrate between the resist layer and substrate being bumped. If resist hardness is insufficient, the plating profile will change with pressure. On the other hand, if the hardness is too high, the film becomes weak and the resist will crack. Moreover, if the wettability of resist to plating solution is bad, the plating solution does not penetrate into hole patterns, leading to plating failure. Therefore, resist materials have to be well controlled to optimize wettability and plating tolerance.
There currently are two types of resists in practical use for plating: positive tone (based on a naphthoquinone diazide, or NQD, system), and negative tone (based on an acrylic system). In addition, chemically amplified (CA) positive tone resists are rapidly being developed for wafer-bumping processes.
In the final process step - resist stripping - photoresist films can be >10µm thick. For this reason, wet etching is required because ashing, the common method used to strip resist films in frontend wafer processes, cannot be applied. The resist must be stripped under mild conditions that will not corrode the metal. Because there often is a tradeoff between plating tolerance and the ability to strip thick films, the challenge facing photoresist manufacturers is to design the appropriate balance between these conflicting characteristics into new resists.
Assessing the tradeoffs
Thick-film photoresist performance for bump formation can be evaluated in five key areas: photospeed, resolution, process margin, plating resistance, and strippability (see table). Overall, NQD positive-tone resists provide the best combination of good strippability and proper process margins, but these photoresists suffer from slow photospeeds and poor resolution. Acrylic negative-tone resists score well in nearly every category, but strippability remains an issue in wafer-bumping processes. CA positive-tone resists continue to improve, but some challenges have existed in maintaining process margins when the photoresist is exposed to the environment (air) or applied to substrates that are basic, such as copper.
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NQD positive-tone photoresists consist of novolak and NQD photoactive compounds. In the unexposed area, solubility of the matrix in alkali is reduced by the interaction of the NQD compounds and novolak, which causes dissolution inhibition. In the exposed area, when the NQD decomposes, it changes to indencarboxylic acid, resulting in a high dissolution rate. Therefore, there is a dissolution contrast between the exposed and unexposed areas. To improve plating tolerance, polymeric plasticizers are generally added into the NQD system. Chemical resistance and adhesion to a substrate are not enough, however; resist sometimes dissolves into the plating bath, allowing plating agents to penetrate the resist film. Also, due to low transparency at a 365nm wavelength (Fig. 1), the resist tends to show a large thickness dependency; thus, exposure latitude narrows when the substrate has significant topography.
Bubbling in the exposed area is another potential issue when resist film is as thick as 60µm because N2 outgassing is inevitable. Other problems such as tapered profiles and low sensitivity (4000-5000mJ/cm2) can occur in positive-tone resists. For these reasons, while NQD positive-tone systems have been used in thin-film applications for generations, they are not suitable for thick-film applications.
Acrylic negative-tone photoresists are based on radical polymerization systems containing multifunctional acrylic monomers, photo radical initiators, and an acrylic polymer that is soluble in aqueous-base developer. In an exposed area, photo radical initiators decompose to generate radical species, and the multifunctional acrylic monomers are then polymerized by the radical. The 3D cross-linking results in poor solubility in developer, strong chemical resistance, and good adhesion to a substrate. Furthermore, because a highly transparent design is possible, thickness dependency is relatively small. Even with a large difference in thickness, there is sufficient exposure latitude and development margin. Rectangular profiles can be obtained at high film thicknesses >60µm - which are needed for solder bump (Fig. 2) and Cu post formation. Sensitivity of the resist is about 1000mJ/cm2, which is high compared to NQD positive-tone resists, and the contrast must also be high to achieve an aspect ratio of more than three, which has been required for wafer-level CSP manufacturing.
 Figure 2. Solder straight bump formation is possible using negative-tone resist at a thickness of 70µm. |
The only issue with negative-tone resist is stripping difficulty, which results from the cross-linked structure. In recent years, however, precise optimization of resist and stripping materials has enabled complete stripping without any residue even at fine pitches that cannot be achieved by dry films [1].
CA positive-tone photoresists now are being considered for CSP applications to address the weak points of NQD positive-tone resists (resolution, photospeed, and bubbling) and acrylic negative-tone resist (stripping). CA photoresist systems have been widely used for frontend wafer-processing applications where good etch resistance is required.
The main ingredients of positive-tone systems are base polymer and photoacid generators (PAG). The polymer is not soluble in aqueous developer when its acidic functionality (such as phenol or carboxylic acid) is secured by protection groups. However, the polymer becomes alkaline-soluble by UV irradiation followed by post-exposure bake (PEB). During exposure, acid is generated by the PAG, and during bake, the protection groups are decomposed to reproduce acid functionality. Because the acid works as a catalyst, the photospeed is significantly higher than conventional photoresists. Acetal and t-butyl groups have been the leading choices for protection groups for KrF lithography applications, but acetal is chemically unstable and easily decomposes in the presence of acid. Because aldehyde will be generated when the acetal is decomposed, and plating solutions for Cu and solder are acidic, polluting the plating bath may be an issue. On the other hand, t-butyl generates isobutene gas when decomposed, resulting in bubbling during PEB. To solve these problems, a unique protection group having tertiary carbon has been incorporated into JSR Micro’s THB CA positive-tone series. The protection group has a high molecular weight so bubbling does not occur. The high molecular-weight polymer also works to increase plating tolerance [2].
One of the biggest issues with CA systems is environmental tolerance. Acid generated from the PAG can be quenched by amine compounds that are present in the environment, resulting in poor resolution. For this reason, it is necessary to shorten the time delay between exposure and PEB, which can narrow the process margin. Substrate dependency also is large. If the substrate is basic, such as Cu, the acid is quenched on contact, and footing at the bottom of the resist profiles can result.
 Figure 3. Cu post formation using CA positive-tone resist. |
Recently, improvements in environmental tolerance and substrate dependency have allowed for fine patterning, even on Cu. Figure 3 shows an example of Cu bumps that were made using a CA positive resist on a Cu-sputtered substrate where 80µm-pitch features (40µm hole) were created with almost straight profiles. Because there is no cross-linking with a positive-tone resist, stripping was also easily achieved using a standard, safe, organic solvent such as ethyl lactate or PGMEA at room temperature. NMP, DMSO, or MEK also can be used.
Solder paste and resists for fine-pitch bumps
While electrolysis plating has been widely used in wafer-level bumping, its productivity suffers because of the time required to complete the plating step. Additionally, because the solder is a multidimensional metal alloy, management of the plating liquid can become complicated, especially for lead-free solders, such as Sn-Ag-Cu and Sn-Ag-Bi. To address these issues, solder paste technology has been proposed in which a narrow bump pitch can be achieved by inserting solder paste into resist hole patterns. The paste is then reflowed to form the solder ball (Fig. 4).
 Figure 4. Solder paste technology using photoresist. |
Because the resist film acts as a mask, it is possible to prevent short-circuiting, which can occur between bumps when a metal mask is used. To accomplish this, the resist must have good mechanical strength that withstands the imprinting of paste, and heat resistance that does not allow the resist to melt and contaminate the solder. The biggest challenge in using a resist mask for solder paste technology has been stripping. Since high heat - 260°C or higher - is applied during the reflow process, conventional negative resists can cause a tightly cross-linked structure, and subsequent stripping becomes quite difficult.
 Figure 5. Solder ball height vs. resist thickness. |
Recent development of negative photoresist by JSR for solder-paste bump applications has resulted in improved stripping properties. Although there seemed to be a limitation in resolution of around 180µm pitch for conventional solder paste technology, a new JSR Micro THB negative resist series has achieved pitches of 100µm at 60µm thickness. Unlike the conventional printing method using a metal mask, the level of solder-paste fill can be well fixed by the resist. The result is very little bump height variation and good bump height control through changes in hole diameter and resist thickness (Fig. 5). This option allows additional flexibility in the choice of solder in contrast to conventional plating methods. Finally, process cost can be significantly reduced because no expensive steppers or special equipment are required.
Conclusion
Solder-bump pitch and feature requirements will continue to shrink as semiconductor manufacturers increase their emphasis on advanced flip-chip and CSP technologies for next-generation ICs in digital consumer electronics and wireless systems to serve the ubiquitous computing era. For these applications, thick photoresists must deliver an appropriate balance of tolerance to metal-bumping processes, ease of stripping, and total cost-effective solutions.
References
- M. Sotomura, Jisso Gijutsu Guidebook 2003, Denshi Zairyo Bessatsu, p. 60, 2003.
- S. Iwanaga, “Photo Reaction & Electronic Materials,” abstract, Kobunshi Gakkai, 2001-2002.
- S. Sakuyama, 7th Symposium on Microjoining and Assembly Technology in Electronics, 285, 2001.
Katsuji Doki received his ME in applied chemistry from Osaka U. and is a senior staff engineer in the business development department of JSR Micro Inc., 1280 N. Mathilda Ave., Sunnyvale, CA 94089; ph 408/543-8910, fax 408/543-8952, e-mail [email protected].