Forming a vertical interconnect structure using dry film processing for Fan Out Wafer Level Packaging

Transcription

1 2017 IEEE 67th Electronic Components and Technology Conference Forming a vertical interconnect structure using dry film processing for Fan Out Wafer Level Packaging Yew Wing Leong, Hsiang Yao Hsiao, Soon Wee Ho, Boon Long Lau and Huamao Lin Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), hsiaohy@ime.a-star.edu.sg Abstract In this study dry film photoresist material is employed to fabricate thicker Cu pillars and solder interconnects. The development of the dry film photoresist process for forming 200 μm thick copper pillar were introduced. Several experiments were conducted to find the optimized process parameters for the dry film photoresist. A Laminator was adopted to laminate the dry film. The lamination temperature was 110 C on the top roll and 65 C on the bottom roll are controlled. And the pressure was 20 psi. Lamination speed was about 2 FPM. For pattern with 200 m thickness and 150 m pitch Cu pillar, optimized exposure dose and developing time were 400 mj/cm 2 and 20 minutes respectively. Cu plating was performed on developed wafers and dry film photoresist was stripped using spray developer tool. Results show good performance for 200 μm copper pillar with flat surface and no dry film photoresist residues. Keywords-component; dry film; Cu pillar; laminate; developing; stripping I. INTRODUCTION The electrical interconnect technique of the advanced packaging process has been evolving. Generally, in order to produce increased functionality and performance in the same volume, the advanced packaging processes used multiple chips to integrate the systems into a single package. The rapid growth in mobile handheld devices such as smartphones has driven the development of these advanced packaging trends [1]. A lot of development and production are reached, inclusive of wafer level packaging, Cu pillar on through silicon via interposer, fan out wafer level packaged, and many more. The fine pitch copper pillar process is subjected to be replaced by the controlled collapse chip connection bump in the new system package designs. The wafer level packaging fabrication used the spun process of photo resist to plating the copper pillar [2]. However, when the thicknesses of photo resist had been increased to 100 m, there are many process and manufacturing challenges; the thickness of photo resist coating and the uniformity of photo resist, throughput and costs of materials. And emerging as an attractive alternative is the use of dry film resist materials. Dry film photoresist materials were used not only on fabricating the PCB but also on WLP fabrication process in recent years [3-5]. For WLP fabrication process, dry film resist materials were used to make photo-patterning, combined with printing the solder paste, electroplating solder joint and fabricating the RDL. After printing or plating process, dry film resist materials were able to strip out using wet bench process. Several years, many benefits of dry film photo resist material were bulletined and involved in wafer level packaged fabrication process engineering comparing with liquid type photo resist material. Moreover, the famous properties were the laminated multilayer on single substrate due to its planarization effect, some exclusive advantages such as low exposure energy, wide band UV light make the fabrication process easy and fast have been pointed out. Besides dry film photoresist materials were applied as temporary patterning masks, they are highly expected to be functionally better so as to be able to remain within packaging after wafer level process. Here the study of the lithography process for copper pillars, with focus on heights in excess of 200 m and diameters of 150 m, with the anticipation of the process requirements for the future fan out wafer level packaging application had been carried out. The features of the dry film photoresist are studied including the process development procedures. Some major concerns during process are pointed out. Sets of experiments are done to find solutions of those concerns mentioned above. Optimized process parameters are proposed. II. CHARACTERISTICS OF THE DRY FILM Dry film photo resist is one type of thick film. Although its resolution and aspect ratio are not very high compared with the other types of thick film, it is still attractive in the applications which do not require high resolution and high aspect ratio. Printed Circuit Board industry origins from Silk Screen Printing technology. This is the reason why it is called PCB (Printed Circuit Board). With the driving force of high density, high accuracy and low cost, printing method had reached its limitation. In 1968, DU-Pont Inc. developed dry film photo resist. Since then, dry film technology replaced most of the traditional image transfer technology. In early stage of the dry film development, photo resist is solvent type, it needs solvents for development and caused many inconvenience. As time goes on, it gradually became aqueous type and improved resolution. After that, packaging industry joined to use the dry film for research and process applications. Up to the present, dry film is still the best choice for PCB industry. New types of dry film are being fabricated toward higher aspect ratio for various applications. Dry film photo resist has numerous advantages. It is possible to laminate dry film simultaneously on several substrates with various contour shapes. It is also possible to laminate multilayer on single substrate due to its planarization effect. Low exposure energy and wide band UV light make the process easy and /17 $ IEEE DOI /ECTC

2 fast. In addition, simple development, vertical sidewalls are all the advantages for the dry film. Sodium carbonate solution and sodium hydroxide solution are the only solutions needed in the development step and in the striping step, respectively. That is, there is no need for organic solvents which are harmful to human health. No significant hazards have been reported about dry film [6]. Most important of all, its cost is very low. However, the resolution is the main disadvantage. Resolution is in proportion to the film thickness. The aspect ratio is limited by its low resolution, hence the dry film can only achieve about 1 to 1.5. This is not as good as other types of thick film photoresist such as SU8 which has been reported to be as high as 20. Dry film photoresist looks quite different than the common liquid photo resist. From the appearance, it looks like a sandwich. The top layer is a separation sheet, composed of Polyethylene film (PET), the thickness is about 25 m. The bottom layer is a support or protective film, composed of polyester (PE), the thickness is also about 25 m. The middle layer is a photosensitive layer. Its thickness depends on the application and ranges from few micron to one hundred fifty micron meters. Generally speaking, the components of photosensitive layer are monomers, photo initiators, polymer binder, and some additional additives such as adhesion promoters and dyes. Monomers are the main components of a dry film. A monomer is initially solvable in developer and becomes unsolvable after UV light exposure and heat treatment. Photo initiators generate free radical under the UV light exposure. Polymer binders are used as the backbone of the dry film. As implied by its name, polymer binders bind all of the components together to form a dry film. A polymer binder does not participate in the chemical reaction during polymerization process but it does enhance the surface adhesion and plating-resist ability. Usually, dyes are being added to increase the process convenience for the sake of easy inspection. III. PROCESS TECHNIQUES FOR DRY FILM PHOTORESIST This research selected a dry film photo-resist material apply to the thicker Cu pillar fabrication process. The standard procedures for this application are listed as follow. Preprocess laminating exposure developing plating stripping The photoresist dry film materials are produced in rolls or sheets where the film is coated in between two protective layers. The dry film material is rolled directly onto the Si wafer by lamination method using the laminator. The result after lamination is a uniform thick coat which requires no lengthy wafer bakes or ancillary solvent processing. Depending on the dry film material, the temperature, pressure and rolling speeds will control the throughput and coating quality. This discussion is related to a dry film suited to the electroplating application being developed. Once the lithography process is completed, the process flow that is being discussed is identical through electroplate, dry film photo-resist strip and field metal etching. Figure 1. Double laminated dry film photoresist Process IV. EXPERIMENT RESULTS & DISCUSSION A. Laminated dry film Photoresist Materials Laminating is the most important step in dry film process. It has great influence on the quality of the patterns and the further processes. A laminating machine is adopted to paste a dry film on a Si substrate. Temperature, pressure and speed of a laminating machine are set according to the type of the dry film and the environment condition. Thereby, pressure is 20 psig, roll speed is 2 FPM and temperature on the top roll is 110 C and 65 C on the bottom roll are controlled. The Negative-tone dry film photoresist is used in this study. The thickness of dry film is 112 μm. Double laminated the dry film photoresist on 300 mm wafer with Ti and Cu seed layer to increase the thickness of film to 220 μm for achieving thick film thickness. Figure 2 shows the profile result of two layer dry film photoresist with EBR. A uniform thickness profile at the wafer edge can be observed. Figure 2. Double laminated dry film photoresist Process B. Exposure and Developing Process Exposure and developing is a system. Both variable influence each other and will affect the final results for the shape and quality of the plated Cu pillar. When the dry film photoresist is exposed under UV light at broad band 879

3 wavelength, photo initiators absorb UV energy and generate free radical. The monomers, initially spread uniformly in the dry film, start to polymerize under the stimulation of these free radical. After that, polymers cross link to each other and become unsolvable in developer solution. When an exposed dry film is ready for development, it will be immersed or spread under developer, which is usually composed of 1-3% sodium carbonate solution. The sodium carbonate then reacts with hydroxyl ion on the binder polymer and produces solvable polymer. These solvable polymers will be taken away by liquid solutions and reveal a new surface for developer. These two reactions happen over and over until reach the bottom of dry film photoresist. The reaction equation is shown below. The cross sectional technique is used to check the sidewall profile after dry film developing. Due to the optical microscopy image on plan view is not enough to inspect the features. Figure 4 shows the cross sectional SEM images of 350 and 400 mj/cm 2 dose. The results is dry film photoresist profiles were non-optimized developing parameters. It observed the under developing feature that is on the bottom vias has dry film photoresist residues. The other nonoptimized developing is over-developing or under crosslinking which will result in notching and footing defects on the bottom vias. However, vertical photoresist sidewall is achieved. Thick scum residue appeared at the bottom of the vias despite using long immersion bath developing time (1.5 hr). The exposure dose and focus offsets of the stepper were optimized for 150 μm diameters in the dry film. Board band UV exposure was used for the dry film photoresist. Exposure dose is ranging from mj/cm 2 with 50 mj/cm 2 increments. For dry film photoresist developing, it is very difficult step for thicker Cu pillar fabrication used dry film. The developing chemical is not easy go into thicker vias. Otherwise, how to remove the dry film residues in thicker via after developing reaction is also a big issue. We used some methods to do developing the thicker dry film. Try to solve above problem. We start with immersion bath for dry film photoresist developing with 1%wt. Na 2CO 3 developer. After immersion bath developing, DI water are used to rinse and N 2 to dry the wafer. The developing time is critical and depends on the pattern size. The first step used the unexposed dry film immersed 1%wt. Na 2CO 3 developer to check the break point. The break point is about 45 mins. Therefore, the developing time is about two times of the break point. Figure 3 shows the optical microscopy pictures with various exposure doses after immersion bath developing. It is found that lower 250 mj/cm 2 dose causes insufficient cross-linking of the dry-film leading to distorted feature. It implied that the exposure energy is not enough for fully polymerization. When increase the doses to 300 mj/cm 2, it causes thicker scum and under developing at the bottom of the vias. Figure 4. The cross sectional SEM images with various exposure doses after developing (a) 350 and (b) 400 mj/cm 2. From the above developing results, the immersion bath developer is not suitable for thicker dry film developing, due to need to take a long time for developing one wafer. The spray type developer used can help to improve the developing process. Therefore, the spray developer had been employed to do the thicker dry film developing. The diagram of the spry developer shows in Figure 5. Non pattern wafer had been used to find out the breaking time used for spray developer. It is found that the breaking point is 4 min for 220 μm thickness dry film. And the break point for spray developer is only one tenth of the time required for immersion bath. Figure 3. The top view optical microscopy pictures with various exposure doses after developing. Figure 5. The diagram of the spray developer for developing the dry fim photo resist. 880

4 Figure 6 shows the developing results with fixed location of the chemical spray. The uniformity of whole wafer after developing is not good and only center of the wafer has good developing results. On the contrary, the edge of wafer is under-developing, this is due to the chemical spray fixed at the center. Figure 8. The optical microscopy pictures with optmized exposure dose and developing parameter. Figure 6. Shows the optical microscopy pictures with various exposure doses after developing. Due to the thicker dry film photoresist, the 1%wt. Na 2CO 3 developer is hard to penetrate through during the developing process. Therefore, mild agitation is necessary. However, if the shaking force is too large as shown in Figure 7 using mega-sonic assisted develop, it may cause over developing and resulting in bigger features size than desired. Only when the Si wafer substrate is rotated with a fixed chemical spray that a narrow developing range can be obtained. Figure 9. Cross-sectional SEM images using optmized dry film photoresist developing parameter: (a) wafer center and, (b) wafer edage. After dry film developing, on the bottom vias of the dry film also have some residues. Therefore, O 2 descum to remove the dry film residue on the bottom vias have been performed. High temperature of over 50 C O 2 descum has been used to clean the residue. The results show in Figure 10 (a) that the residues can be removed on the bottom vias. However, the oxidation had occurred on the Cu seed and the dry film photo resist will changed in colour. It will effect the Cu plating. Therefore, with the decrease in descum temperature of lower than 50 C, the residue can be removed and without oxidizing the Cu. The results show in Figure 10 (b). Figure 7. Shows the optical microscopy after developing with megasonic. Therefore by using a sweeping motion of chemical spray and rotated Si substrate at the same time, it is observed that a wider developing range and reduced developing time can be achieved. Figure 8 shows the developing results with the moving location of chemical spray and rotated substrate. The developing uniformity is good and the whole wafer can be fully developed. After a substrate is taken out of the developer, it should be immediately cleaned by DI water in 30 to 90 seconds in order to remove the residual developer. The last step is drying the substrate by N 2 to remove the residual water. Figure 10. Shows the optical microscopy pictures with various O 2 descum temperature (a) over 50 C and (b) lower 50 C. C. Dry film Photoresist Stripping Evaluation A developed region also called pattern can be used as the electroplating mold or etching mask. After plating or etching process is finished, the dry film is no longer needed and has to be stripped off. The stripper is usually DMSO & TMAH 881

5 based are used with the operating temperature of 70 o C. The dry film photoresist dipped into the chemical solvent which was heated to strip. The stripper will penetrate into the interface between the dry film and the substrate, make it easily to be stripped or dissolved. Finally, after DI water cleaning and drying with isopropyl alcohol, dry film is fully removed. For dry film photoresist stripping, the chemical temperature plays an important role. If the chemical temperature is high, the stripping time will be shortened. When the chemical temperature is lower than 50 C, the stripping time is over 30 mins. On the contrary, the stripping time is less halt time when the chemical temperature increases over 50 C. Therefore, the entire surface of the wafer keeps high temperature (over 50 C) during whole stripping process. After chemical and temperature treated, the large area of the dry film can be removed. However, the dry film swelling occurs between the Cu pillars which show in Figure 11 and Figure 12. The swelling residues are difficult to be removed by using chemical force only. When increased the stripping time to 30 mins, the swelling residues is difficult to be removed. stripping using chemical and mechanical force. Not only larger area but also in between Cu pillar features the dry film photoresist residues were cleaned and shows good stripping performance. Figure 13. The dry film photoresist stripping results combined with chemical and mechanical force. D. Cu Pillars Plating & PR Stripping Results 12 inch Si wafer with Ti/Cu seed layer after dry film photoresist developed is later send for Cu pillar plating process. In order to plate the Cu pillars height target of about 200 μm, the plating has been performed including chemical pre-plate- cleaning of the seed layer. Cu pillars have been plated using the optimized Cu pillar plating parameters. The whole plating duration is about 4 hours. Cu pillars plating followed by dry film photoresist stripping and seed layer etching. The results show in Figure11. About 200 μm height Cu pillars (aspect ratio = 1.33) were plated on the wafer and no dry film photoresist residues entrapped in between Cu pillars. Figure 11. The optical microscopy images with chemical force after stripping. Figure 14. The fabricated results of Cu pillars using dry film photoresist: (a) top optical image, (b) tilted image, and (C) SEM image. Figure 12. The SEM of Cu pillar without remove the swelling residues. Therefore, need to additional physical force to support to remove the swelling dry film residues. Use dual fluid nozzle to remove of dry film photoresist swelling residue. Figure 13 shows the top view of Cu pillars after dry film photoresist V. CONCLUSION Double lamination was used to achieve 220 μm thick dry film photoresist. Different exposure doses was evaluated to determine the optimum dose and various developing methods were used to develop the thicker dry film photoresist. Optical inspection and X-section were performed on the sample wafers. Based on the X-section results, the optimum dose should be in the range of 300 to 400 mj/cm 2. Cu plating was performed on developing wafer 882

6 and dry film photoresist was stripped using spray developer tool. Some of the important results are summarized below: 1. Good performance of die developing process and cleaning after developing has been successfully achieved. The residues at bottom of via were removed using the descum process. 2. The good CD uniformity and vertical resist profile after developing have been provided. 3. The dry film resists stripping process has been successfully developed. Shorter stripping time and no dry film residue near the Cu pillar were observed after stripping process. 4. Fabrication of copper pillars with 200 m in height which have the copper pillars with flat top surface and that are free of voids and defects can be achieved. ACKNOWLEDGMENT Thanks to all our colleagues who provided insight and expertise that greatly assisted the research. Also special thanks to TOHO s team for assistance with dry film photoresist developing and stripping processes for Cu pillar fabrication. REFERENCES [1] TDK components, The customer magazine October 2012, A new dimension in miniaturization [2] S. W. Ho, S. A. Sek, B. L. Lau and V. S. Rao, " Process and challenges of ultra-thick spin-on photoresist", IEEE Electronics packaging Technoha Conference, pp.1-5, (2015). [3] H. Watanabe and H. Honma, "Direct Nickel Plating Copper Circuits Using DMAB as a Second Reducing Agent", 1998 IEMTM Proceedings, pp [4] K. H. Uang, K. C. Chen, S. W Lu, H. T Hu and S. H. Huang, "The Reliability Performance of Low Cost BumPing on Alundnum and Copper Wafer", IEEE Electronics packaging Technoha Conference, pp , (2000). [5] S. K. Kang, D. Y Shih, K. Fogel, P Lauro and M. J. Yim, "Iflterfacial Reaction Stodies on Lead (Pb)-Free Solder Alloys", IEEE Electronic ComPonents and Technology COthence, pp (2001). [6] Lorenz H, Paratte L, Luthier R, de Rooij N F and Renaud Ph Low cost technology for multilayer electroplated parts using laminated dry film resist, Proc. Eurosensors IX & Transducers 95 (Stockholm, 1995) 883