Improving image quality in photosensitive polyimide films
BY PAUL WERBANETH, STEPHEN ROSS, AND MARK ROUSEY-SEIDEL
Flip chip packaging is an established method to facilitate the space-efficient, reliable, low-cost packaging of both very high I/O count devices and high-frequency devices. Polyimide films are widely used in flip chip packaging, either as a final passivation layer placed on top of the standard silicon dioxide or silicon oxynitride passivation films, or to permit an additional layer of electrical interconnect beyond that formed in the wafer fab. Patterning these polyimide films is typically done either with a wet etch using solutions of organic amines, a dry etch process, or a photolithography step using photosensitive polyimide films.
For photodefinable polyimide films, which have the least complicated process flow of all these techniques, image resolu-tion may suffer because of several factors, including poor focus, poor depth-of-focus, and insufficient exposure. This article reviews the uses of photodefinable poly-imide in flip chip packaging, and discusses how an anisotropic plasma descum of polyimide left during the patterning of photosensitive films can improve the image profile before the deposition of the under bump metallurgy (UBM).
Flip Chip Process Requirements
The advantages of flip chip packaging1 are so compelling that industry capacity for flip chip services, such as wafer bumping is still increasing and is forecasted to increase over the next few years, even in the moribund climate pervasive in semiconductor fabrication in 2001.2 Some see the CAGR growth to be 47 percent from 2000 to 2005, driven by memory applications and RF and linear circuits for mobile communication.3
The trend in flip chip processing for increasingly higher pin counts per chip cre-ates a need for decreasing the pitch between I/O pins. As a result, feature sizes of metal bump vias and the metal lines used for bond pad redistribution are becom-ing progressively smaller. In Japan, for example, vias and redistribution features as fine as 10-µm lines and spaces are being used for high pin count circuits.
 Figure 1. a) Oblique and b) cross-section SEM images of a foot left after exposing and develop-ing an image into photosensitive polyimide. |
Organic films are used in flip chip processing much the same way they are used in general wafer fabrication processing. Organic materials like polyimide or benzo-cyclobutene (BCB) are dielectric films with useful mechanical properties that make them suitable as stress buffer passivation layers that improve device reliability by eliminating stresses introduced during packaging operations. As dielectrics, they can also be used to create additional layers of device interconnect beyond the wafer fab. Bond pads can be redistributed from the perimeter of the chip to its interior.
These organic films, which become a permanent part of the packaged integrated circuit (IC), need to satisfy a host of performance requirements. Polymers used in bumping and wafer-level processing must exhibit:4
- Thermal stability at temperatures up to 350°C
- Good adhesion to underlying films like silicon nitride and silicon dioxide
- Good adhesion to itself
- Good adhesion between the polymer and the UBM materials
- Good adhesion between the polymer and the underfill layer
- Low water absorption
- Low shrinkage on curing
- Profiles compatible with subsequent metalization operations.
Applying polymer films and patterning them must also be completed with quick turn-around, low cost process modules.
Photosensitive Polyimides
Three subtractive patterning methods are available for organic films: wet etching with solutions of organic amines using photoresist masks, plasma (dry) etch using photoresist masks, and directly patterning the polyimide or BCB using photosensitive films. There may be several reasons to move away from wet etch processes. For example, improved pattern transfer fidelity5 has driven work on finding dry etch solutions for both small and large polyimide features. Other reasons to move from using photoresist masks and subtractive removal (wet or dry etching) of the organic materials are potential improvements in overall process quality and cycle time. Using photosensitive polyimides to create wafer bumping vias can eliminate more than half the processing steps (applying an adhesion promoter, baking the adhesion promoter, applying a photoresist coat, baking the photoresist, stripping the photoresist) needed to make a feature in the non-photosensitive version of the polyimide film.6
Patterning of Polyimides
Contact/proximity patterning and patterning using 1X wafer steppers are two popular methods for exposing photosensitive polyimides in flip chip processing. The image quality obtained with either technique will be a response of the interaction between the basic laws of optics and the many variables encountered in the flip chip process (such as film thickness, film thickness uniformity, surface topography, pre-exposure bake and exposure tool condition). Even though the feature size for wafer bumping vias is large compared to the transistor gate buried many layers beneath, the thickness of the photosensitive polyimide film (or photosensitive BCB film) makes photolithography process optimization challenging.
 Figure 2. The local environment for O2 plasma processing of polyimide films. |
One of the classic image defects introduced by photolithography is a “foot” at the base of the feature. Figures 1a and 1b show field and cross-section SEM images of a foot left after exposing and developing an image into photosensitive polyimide. (The same kind of foot can be present in thick photoresist films, as well as in photosensitive BCB.) Adjusting exposure conditions, including exposure energy and focus, can alleviate footing, but even the best efforts to completely eliminate it may fall short.
Classically, descum processing in plasma reactors has been a way to modify sidewall profiles, correcting for profile errors induced by problems with photo exposure (or errors in focus) when producing photolithographic images in photosensitive films. In the descum operation, a reactive plasma is used to remove the foot, either as a batch operation acting on many wafers at one time or by pro-cessing the wafers through a single wafer tool. Plasma descum continues to find acceptance in new applications where photolithography encounters difficult challenges and, as such, is well-suited for improving flip chip image quality. A real challenge for plasma descum, though, is to remove the unwanted foot formed at the base of the photolithographic image, and to remove other surface residues, without significantly changing the critical dimensions of the image.
Plasma Etch of Polyimide Films
Polyimide films are readily etched in plasma reactors using sources of oxygen or fluorine (or mixtures of these reactants). Using an electron cyclotron resonance (ECR) plasma source,7 for example, the etch rate of polyimide in a pure oxygen plasma is directly related to the amount of reactive species present, with rates of 500 to 900 nm/min obtained in the ECR tool. The anisotropy of etch profiles seems to be related to the directionality of ions impinging on the wafer surface during etch, with higher ion bombardment conditions resulting in greater anisotropy. Other studies have also shown that the etch rate of polyimide in oxygen plasmas is dependent on the number of reactive species reaching the wafer surface per unit time, and that adding small concentrations of CF4 to oxygen-containing plasmas can increase polyimide etch rates.8
The absolute etch rate of the polyimide film is not critically important for polyimide descum applications (because only a few hundred nanometers of film need be removed); the ability of the descum to be perfectly anisotropic, though, is. An opti-mized plasma descum step should remove material only from horizontal surfaces (like the long sloping foot), which leaves the critical dimension of the feature unchanged.
How can plasma descum be made anisotropic? By considering the local envi-ronment within a polyimide feature on a wafer immersed in a reactive plasma environ-ment (Figure 2), it is possible to follow the movement of the two important reactive components of the plasma: electrically neutral chemi-cally reactive radicals and electrically charged ions (which may also be chemi-cally reactive).
In a plasma generated with pure O2 as the feed gas, it's expected that oxygen atoms are the electrically neutral radical species, and O2 (or to an extent atomic oxygen) are the ions.
The motion of electrically neutral (0) and electrically charged particles (+) in plasma reactors differs significantly. Assuming again the case where the plas-ma is generated with pure O2, oxygen molecules and oxygen radicals will dif-fuse throughout a plasma descum reactor with an essentially random motion, gov-erned primarily by their temperature and the number of collisions they experience within the reactor (walls, other gas species and wafer surfaces). At a micro-scopic scale, it is likely that oxygen radi-cals will encounter not just the horizontal surfaces of the polyimide images – they will also strike and react with the vertical sidewalls of the feature. Descum per-formed using only oxygen radicals will result in isotropic removal of polyimide material, with equal removal rates on horizontal and vertical surfaces (and along any angled surfaces).
 Figure 3. A reactive ion etching diode plasma reactor that can be used for anisotropic descum processes for flip chip applications. |
The path followed by ions is different. The motion of ions will have some thermal component like the radicals described above, and they will be scattered when the ions collide with other gas species or with solid surfaces. However, the ion motion will mostly be governed by the ions responding to electric fields within the plasma reactor. These electric fields create accelerat-ing forces, attracting ions from the central volume of the reactor.
To reduce unwanted etching of polyimide from feature side-walls and to enhance etching of polyimide from horizontal sur-faces, the ion bombardment component of the etch must be strengthened. There are several ways to do this. Operating the plasma reactor at low pressure will typically reduce the number of oxygen radicals (PV = nRT) and will also increase the mean free path, the distance plasma species travel before colliding with some other gas phase component. Ions traveling down to the wafer surface (if there is some accelerating force attracting them) will be more likely to strike on horizontal surfaces if they encounter fewer collisions diverting them off-axis.
 Figure 4. a) Oblique and b) cross-section SEM images of photosensitive polyimide features that have been treated with oxygen descum chemistries in an RIE diode reactor. |
A reactive ion etching (RIE) diode plasma reactor is well suited for anisotropic descum processes (Figure 3). In this kind of single wafer plasma reactor, the wafer sits on an electrode powered by a radio frequency (RF) generator; the top electrode of the diode (two electrode) reactor is grounded. Having the wafer electrode here be the smaller surface area plate of the parallel-plate diode, and having it be powered, creates relatively greater ion bombardment on the wafer electrode.9 This particular plasma reactor configuration is the classic RIE reactor.
Polyimide Image Quality
SEM images of photosensitive polyimide features that have been treated with oxygen descum chemistries in an RIE diode reactor are shown in Figure 4. Note that these are the same features shown before descum in Figure 1. The ragged foot that had been present at the bottom of the photosensitive polyimide has been completely removed by the descum process without materially affecting the feature size. The polyimide removal rate is between 100 and 200 nm/min, and can be increased or decreased as needed. The ratio between etch rates on horizontal surfaces, like the sloping surface of the polyimide foot, and those on vertical surfaces (the feature sidewall) is approximately 4:1 (Figure 5).
Images created in photosensitive BCB and images made with thick photoresist both react to plasma des-cum the way photosensitive poly- imide images do. The 4:1 ratio between the horizontal and vertical etch rates holds for BCB and for thick photoresist the way it does for poly-imide. Etch rates during descum for all three films are nearly identical. The imaging problems encountered with photosensitive BCB and with thick photoresists are no different from the examples presented for photosensitive polyimide. They may be improved through the application of plasma descum in an RIE etch tool as readily as photosensitive polyimide features.
Conclusion
Flip chip processing is an increasingly important part of IC packaging. Thick organic films like polyimide or BCB are applied during wafer bumping to create additional layers of metal interconnect on finished die, to relocate bond pads from the perimeter of the chip to its interior, or as stress buffer layers. Photodefinable polyimides or BCBs have the simplest processing schemes for creating these features, but they may suffer from image quality problems as a result of the difficulty in directly imaging patterns into thick photosensitive films. A plasma descum step performed in an RIE diode tool can effectively remove the foot created in photosensitive polyimide without significantly altering the critical dimension of the original image, thus improving image quality in the photosensitive films used for flip chip packaging.
AP
References
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- Amkor Technology, Wafer Level Packaging 2001, data courtesy of Prismark and BPA Consulting Ltd.
- Dan Scheck, “BCB resins for bumping and wafer level packaging applications,” Proceedings of the Ultratech Wafer Level Packaging Seminar, Hsin-Chu, Taiwan, September 2001.
- J. Munoz and C. Dominguez, “Dry development of photosensitive polyimides for high resolution and aspect ration applications,” Journal of Vacuum Science and Technology B 13(6), 1995, p. 2179.
- Stephen Hall and Craig C. Schuckert, “Single mask wafer overcoat process using photodefinable polyimide,” Solid State Technology, October 1999, pp. 95-96.
- W.H. Juan and S.W. Pang, “High aspect ratio polyimide etching using an oxygen plasma generated by electron cyclotron resonance source,” Journal of Vacuum Science and Technology B 12(1), 1994, p. 422.
- V. Vukanovic, G.A. Takacs, and E.A. Matuszak, “Plasma etching of organic materials. II. Polyimide etching and passivation downstream of an O2-CF4-Ar microwave plasma,” Journal of Vacuum Science and Technology B 6(1), 1988, p. 66.
- Brian Chapman, Glow Discharge Processes, John Wiley and Sons, New York, 1980, p. 158.
Paul Werbaneth, regional marketing manager, Stephen Ross, senior field process engineer, and Mark Rousey-Seidel, manager of customer applications, can be contacted at Tegal Corp., 2201 South McDowell Blvd., Petaluma, CA 94954; 707-765-5608; Fax: 707-773-3015; E-mail: [email protected], [email protected], and [email protected].
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