Excimer Laser Ablation – a Novel Patterning Solution for Advanced Packaging

EXCIMER LASER ABLATION – A NOVEL PATTERNING SOLUTION FOR ADVANCED PACKAGING

Ralph Zoberbier SUSS MicroTec Lithography GmbH | Germany

Habib Hichri SUSS MicroTec Photonic Systems Inc | USA

Published in the SUSS report V1 06/2015

E-mail: [email protected] www.SUSS.com EXCIMER LASER ABLATION – A NOVEL PATTERNING SOLUTION FOR ADVANCED PACKAGING

Ralph Zoberbier SUSS MicroTec Lithography GmbH, Schleissheimer Str. 90, 85748 Garching, Germany Habib Hichri SUSS MicroTec Photonic Systems Inc. 220 Klug Circle, Corona, CA 92880-5409, USA

INTRODUCTION EXCIMER LASER ABLATION Photolithography has long been the key TECHNOLOGY patterning technology for structuring organic Excimer laser ablation is a dry patterning pro- materials used in advanced packaging appli- cess, breaking a material’s molecular structure cations like fl ip-chip wafer bumping, electro- and directly etching the desired circuit pattern to plated gold, solder bumps, copper pillar tech- clearly defi ned depths on the substrate, with mi- nologies and redistribution layers. Nowadays, nimal heat affected zone (HAZ). This patterning proximity exposure technologies (mask aligner) technology uses the advantage of the excimer or projection lithography (step and repeat or laser source to emit high energy pulses at projection scanning) are the typical choices short wavelengths. The short wavelength out- to create the features. The continuous trend put on the one hand enables the imaging of of the miniaturization, increasing performance small features but also supports absorption and mobility of electronic devices drive the in many different materials. Depending on the requirements of the chip itself but also its material each laser pulse removes a certain package type. More and more, the photolitho- amount of material. The ablation rates are in graphy process is becoming the limiting factor the 100nm range for polymers and dielectrics to develop cost effective and innovative packa- that are typically used as passivation layers in ge designs that meet the market requirements. the semiconductor backend applications. The technology allows for the fi ne tuning of sidewall In response to this industry challenge excimer angles of the created features by adjustments laser ablation has been adapted to the semi- of the laser fl uence. conductor packaging industry and is now available as a disruptive patterning technology. To address the latest requirements of the advan- This complementary technology offers the ced packaging industry, such as the creation promise of further reductions in manufacturing of feature sizes in the range of 2-5μm com- costs as well as enhancements in chip or bined with an overlay accuracy of less than package performance. This paper provides a 1-2μm, requires a careful selection of the comprehensive overview of the excimer laser right equipment platform. Thus, excimer laser ablation technology, and its level of maturity. It ablation is available on a step and repeat discusses the recent technology developments platform which is a very common platform for and corresponding debris cleaning solutions high-end lithography applications. from a technical perspective.

2 Figure 1 Overview of ablation rates in BCB (Cyclotene 3000) based on different ﬂ uence level [1]

The combination of the excimer laser source and reduction projection optics enables the capability of high resolution imaging with suf- Figure 2 Schematic illustrations of excimer laser ablation of thin fi lms from a polymer substrate fi cient fl uence output to pattern a large area at in a dry one-step etch process once to achieve the highest possible through- put. A reticle defi nes the pattern to be ablated, providing a high degree of pattern fi delity and for a photoresist nor the post-develop and etch placement accuracy. In addition the demagni- processes that accompany a photolithography fi cation of the optical systems generates a high process. excimer laser ablation is suitable for energy level on the substrate side while the ablating a wide variety of polymeric materials, illumination of the reticle itself is still in an ac- thin metals (<600nm), epoxies, EMC’s, nitrides ceptable range to avoid mask damage. Typical and other materials. reticles on an excimer ablation stepper are based on chrome or aluminum on quartz or Excimer laser ablation technology enables the dielectric masks. industry to use new materials that offer better mechanical properties required by the advan- The illumination setup of the optics is typically ced semiconductor packaging industry (low customized based on the application require- CTE and residual stress, and thermally stable). ments to ensure a match of fl uence requirements based on the material to be ablated, A major concern for the adoption of excimer coupled with required ablation fi eld size that is laser ablation is effective and cost effi cient defi ned by die or package sizes. debris cleaning. The laser ablation process itself creates some debris that needs to be As mentioned above, the technology is similar removed. Latest developments show the to traditional UV steppers, however, instead of availability of effi cient post ablation cleaning exposing a photo sensitive material, the mate- solutions. rial is etched directly without the need neither

3 ADVANCED PACKAGING APPLICATIONS AND TECHNOLOGY TRENDS Today a wide variety of advanced packaging thus, are ever more sensitive to any thermo- technologies exists to meet the requirements mechanical stress. Different non-photo sensi- of the semiconductor industry. The leading tive materials with a better match are available advanced packages, including chip-on-chip, for quite some time but could not be patterned wafer-level packages, chip-on-chip stacking, at the required resolution, required profi le and embedded IC, all have a need to structure at an acceptable cost level. thin substrates, redistribution layers and other package components like high resolution vias. Finally cost pressure of the total package is The consumers constant push for higher func- typically addressed in the semiconductor tionality on smaller and thinner end devices, industry by a transfer of technologies to a like smart phones or tablets drives the need for larger, next generation substrate format. While next generation packages with fi ner features at the wafer based packages are limited to the increasing reliability of the package. In addition, largest wafer scale of 300mm, fan out wafer cost considerations become more and more level package technologies offer the scalability important to survive in the competitive land- to larger substrate sizes in a panel format. scape for all parties within the supply chain, To sum it up, a very attractive technology from chip manufacturer, assembly and test to alternative to photolithography would be a the consumer end device manufacturer. There- technique that can directly structure non-photo fore, the industry desperately strives for innova- PIs, PBOs and epoxies at high resolution tive approaches to lower manufacturing costs combined with panel compatibility. coupled with enabling technologies that meet the challenging technical requirements. Excimer laser ablation technology now provides that alternative while also delivering several Specifi c needs in the patterning area are to other advantages. overcome current resolution limitations that are caused by the today‘s available photopolymers. They are limited in the supported resolu- EXCIMER LASER ABLATION IN ADVANCED tion and via wall angle, even though the today‘s PACKAGING APPLICATIONS imaging photolithography technologies would The most promising advanced packaging theoretically support even higher performance. application that could benefi t from the inherent In addition some polymers or dielectrics such technology difference using laser ablation is via as polyimides (PIs), PBOs and epoxies that drilling in polymers to create openings on top of are used as passivation layers remain in the metal pads for electrical interconnection. While package and will fi nally also impact the package the polymer is patterned by the laser energy, reliability. There is a signifi cant coeffi cient of the relatively thick metal pad actually acts as thermal expansion (CTE) mismatch between a natural ablation stop layer. Additional key these materials and the chips. Furthermore, advantages are the ability to pattern a large higher density packages have bigger thermal area fi lled with thousands of vias enabled by loads, exacerbating this problem. As a result, the mask based patterning technology and the this CTE mismatch can cause issues such as provided high laser energy level. 5μm vias or stress damage to low- dielectrics and wiring smaller can be created in both traditional layers, which are also getting thinner and, photopolymers and new materials.

4 Figure 3 SEM image of a 5μm via in BCB at 650mJ/cm2, 30 pulses [1]

A complete wafer level package based on Figure 5 Cross section of two layer PBO patterning by SUSS [2] excimer laser ablation was completed and ELP300 with plated copper RDL and solder bump tested by a cooperation of SUSS MicroTec and FlipChip International. One major advantage of POST-ABLATION CLEANING the excimer laser is the capability of building The ablation process of these photo and non- stacked redistributed layers of 2 and more at photo sensitive materials using the excimer lower cost than photolithography tools. laser usually generates debris that will need to be removed during and/or after ablation. Excimer laser tools are always equipped with a debris cell that would remove any loose debris generated during the ablation process. Hence, in most case, one need to subject excimer processed wafer or substrates to a cleaning process depending on materials to remove any residual debris that was not removed during the ablation. Various cleaning processes exist that match the corresponding materials and ablation process. However, besides an effective cleaning process, cost effi cient solutions are required by the market to allow the adoption of excimer laser ablation for cost sensitive Figure 4 SEM view of two layer PBO patterning by SUSS applications in the fi eld of advanced packaging. ELP300 [2] These cleaning technologies have been developed over the last years and promise very good effi ciency that is required to introduce excimer laser ablation into high volume production. The debris formed during the ablation process must be removed after the process, using an effi ciently designed debris cell. The debris cell

5 consists of a metal confi nement that surrounds Other solutions were proposed for during and the excimer laser beam during the ablation post ablation debris removal such as second process. This confi nement is connected to a va- exposure if the same initially ablated surface of cuum pump to pull any particles and fragments dielectric material to intermittent pulses of UV formed through the photodecomposition of di- laser having alternating high and low fl uences electric material (photo and non photo polyme- scanned in a direction away from the initial scan ric materials) by the laser beam. The removal of area [5]. CO2 laser was also applied to clean debris during ablation is not suffi cient enough black deposits formed during the excimer laser to eliminate any residuals that will adhere to ablation of polyimide in Air [6]. The 10.6μm CO2 surface of the dielectric materials and would radiation was strongly absorbed in the debris need other means to remove and prepare the but only weakly absorbed in polyimide thus ablated pattern to the subsequent processes enabling the clean removal of the debris with- such as plating or deposition of another dielec- out any damage to the polyimide. tric layer or epoxy molding compounds. We have addressed the post ablation cleaning Multiple approaches were suggested in the at SUSS MicroTec using other approaches that literature to clean post excimer laser ablation will be specifi c to each material since there is debris such as the use of Nd:YAG laser [3,4]. This no “one size fi ts all“ cleaning solutions for post technique is useful for specifi c pattern density ablation debris removal. Our goal is to provide where there is uniform distribution of thin layer alternative cleaning solutions to our customers of debris at dielectric surface. We have che- that will either fi t with their existing infrastruc- cked this technique and confi rmed its validity ture and/or provide innovative techniques that using our picosecond DPSS laser (532nm). add value at low cost of ownership. Figure 1 shows the cleaning of post excimer laser ablation debris of PBO materials. We can see clearly the differences between post ablati- O2 PLASMA CLEANING on and cleaned areas. O2 plasma was well adopted in many advanced packaging fabs for patterning or resist stripping. The O2 plasma was used in conjunc- tion with either Helium (He) or Argon (Ar) gas to improve the plasma effi ciency. In addition, multiple tools from different tool vendors were used to process wafers with O2 plasma. We have used the O2/He or/and O2/Ar plasma to clean the post excimer laser ablation debris generated and redeposit back on the surface of the ablated dielectric material. Most of the O2 plasma cleaning process will remove a thin layer of the dielectric with the debris that ranges from 200 to 600nm that would be com- Figure 6 Post excimer laser ablation cleaning using pensated in the initial coating process. picosecond DPSS laser (dark area: post ablation, clear area: post debris cleaning)

6 7a 7b

Figure 7a Post ablation of HD8930 showing debris around Figure 7b Post O2/Ar plasma cleaning of post ablation the patterned feature debris for HD8930

The criteria of success is high removal of deb- cleaning the debris with minimal material re- ris with minimal removal of dielectric materials. moval. Figure 7a (post ablation) and Figure 7b Our plasma tool is supplied by Plasma Etch (post cleaning) shows the results obtained Inc. We processed coated wafers with PBO from debris removal on HD8930 using O2/Ar (HD8820 and HD8930) [7] materials through plasma cleaning process. Figure 8a (post our excimer laser tool then exposed these ablation) and Figure 8b (post cleaning) shows wafers to O2/He or O2/Ar plasma process to the efﬁ cient removal of post ablation debris remove the post ablation debris. The results from HD8820 using O2/He plasma cleaning were very impressive and we were able of process.

8a 8b

Figure 8a SEM view of post via ablation of HD8820 showing Figure 8b SEM view of post O2/He plasma cleaning of post debris around the patterned feature ablation debris for HD8820

7 SACRIFICIAL LAYER 9a The search for a cost effective solution to remove the post ablation debris leads us to look into formulating a sacrifi cial thin water soluble fi lm that is capable of absorbing the UV light during scan ablation. This sacrifi cial layer would be easily removed with water spray after the ablation process. The removal of the sacrifi cial layer will carry with it all the debris redeposits during ablation on top of it. The literature indica- ted the existence of other sacrifi cial layers that were used for the solid state or CO2 laser whe- Figure 9a Post ablation of HD4104 showing debris around re only one feature at a time was ablated [8,9] the patterned feature redeposit on top of the sacrifi cial layer (Emulsitone EMS4611 or HogoMax from Disco). Hence some of these materials were not able 9b to withstand the excimer laser scan process where a full die was scanned in one time.

To solve this issue, we have formulated our own thin sacrifi cial layer using water soluble UV ab- sorbent polymer to be able to be ablated with the dielectric. Figure 9a shows the amount of debris post ablation surrounding the vias and trenches patterned into HD4104 [7]. Figure 9b shows pattern cleaned after the removal of the thin sacrifi cial layer with the debris redeposits using water spray. Figure 9b Post water spray cleaning showing removal of the sacrifi cial layer with the post ablation debris that was on top

CO2 SNOW CLEANING

Another post ablation cleaning solution was 10a also developed using the CO2 snow [10]. It consists of exposing the surface of the ablated dielectric to CO2 snow spray to remove the debris redeposit during ablation. This technique was used to remove post seed layer removal and also post via and redistributed layer ablation using excimer laser. Figure 10a shows the surface of the dielectric post seed layer removal ablation process. Figure 10b shows the surface of the dielectric materials post CO2 snow spray where most of the post ablation debris was removed. Figure 10a Post ablation of HD8820 showing debris redeposit around the patterned feature

8 10b References

[1] Michael Töpper, Karin Hauck, Lukas Weituschat, Mario Schima, “Laser Ablation of Thin-Film Polymers for a Sub-5μm Via Technology“ SUSS Technology Forum Semicon Europa 2014 [2] Jim Zaccardi, Guy Burgess, Theodore Tessier, Matt Souter, “Dielectric Laser Via Drilling for Next Generation Wafer Level Processing” IMAPS Device Packaging Conference, March 2014 [3] Jianhui Gu, Puay Khim Lim, and Pean Lim,“Method for Cleaning Debris off UV laser Ablated Polymer, Method for Producing a Polymer Nozzle Member Using the Same and Nozzle Member Produced Thereby” US Patent application US2003/0052101 A1, March 20, 2003 [4] Jianhui Gu, Puay Khim Lim, and Pean Lim, “Nd: YAG Laser Cleaning of Ablation Debris From Excimer-Laser-Ablated Polyimide”, Proceeding SPIE 4595, Photonic Systems and Applications, 293 (October 29, 2001) [5] Kevin j. McIntyre, “Method of Cleaning Laser Ablation Debris”, US Patent 5,257,706, November 2, 1993 [6] J. Koren, and J.J. Donelon, “CO2 Laser Cleaning of Black Deposits Formed During The Excimer Laser Etching of Polyimide in Air”, Applied Physics B, Vol. 45, Issue 1, pp45-46, Figure 10b Post CO2 snow spray cleaning showing January, 1988 signifi cant removal of the post ablation debris [7] HD Microsystems, www.hdmicrosystems.com [8] Emulsitone Chemicals LLC, www.emulsitone.com The post ablation cleaning solutions proposed [9] Disco Corporation, www.Disco.co.jp above are only few to address specifi c ma- [10] Eco-Snow Systems, www.Eco-snow.com terials and process conditions. To achieve higher effi ciency of cleaning, it would be re- . commended to either use O2 plasma, sacri- Ralph Zoberbier graduated in Precision Engineering and fi cial layer, DPSS, or to combine two solutions Microsystems Technology from the University of Applied Sciences in Nuremberg. He joined SUSS MicroTec in 2001 when infrastructure and cost allow such as as R&D Project Manager. In 2005 he became International CO2 snow followed by O2 plasma or CO2 Product Manager Aligner. ionized water spray followed by O2 plasma. From 2010 – 2014, he led the Aligner Product Management team as Director Product Management. With the acquisition of Tamarack Scientifi c Inc. his responsibility was extended by complementary projection lithography and laser process SUMMARY technology. In 2014 he was appointed to General Manager Exposure and Laser Processing. Excimer laser ablation was demonstrated to Ralph gained his MBA degree in Entrepreneurship at be a high potential patterning technology that Louisville University, Kentucky. meets today‘s but also the future requirements Habib Hichri joined SUSS MicroTec on October 2013 as Engineering Applications Director for advanced packaging platforms in patter- in Corona CA, USA. He has been focusing on improving customers interaction through quick ning of polymers and dielectrics. In addition to turnaround of the demos work for excimer laser ablation and providing cleaning solutions the patterning technology and material qualifi - for post ablation. He also works with customers on developing applications for copper pillars and traces using SUSS lithography scanner tools. Before joining SUSS MicroTec, Habib cations, effi cient cleaning methods have been spend about 12 years with IBM Semiconductors Research and Development Center in East developed to support the introduction and ad- Fishkil, NY where he worked as lead process integration engineer for microprocessor (IBM), games and communications chips. He later was promoted to a management position within option of the technology in high end manufac- IBM on process development in lithography and Dry Reactive Ion turing technologies. Further developments will Etch in the front end of line area for microprocessor fabrication. continue to open up new application areas to Prior to joining IBM, Habib was a senior research engineer responsible for design, develop, scale up, and commissioning/ support the ever increasing requirements of start-up of specialty chemicals products production at Occidental industrial and consumer electronics. Chemical Corporation, in Buffalo NY. Before Joining Occidental Chemicals, Habib held a post Doctorate research position on polymer processing and thin fi lm photovoltaic at the University of Delaware. Habib holds Master and PhD degrees in Chemical Engineering from the Claude Bernard University at Lyon, France and an MBA degree from the State University of New York at Buffalo.