Photosensitive polyimides in semiconductor manufacturing
11/01/1997
Photosensitive polyimides in semiconductor manufacturing
Robbyn Culver, Ultratech Stepper Inc., San Jose, California
The proliferation of compact lightweight electronic devices, such as portable phones and computers, has increased demand for smaller, thinner, and lighter chip packages. At the same time, advanced chips have become larger in area. These factors have posed considerable challenges to chip packaging technologists, particularly in preventing damage to the chip from the package itself, or during the packaging operation.
Stresses in plastic packages cause problems such as cracking of the passivation layer, and deformation of aluminum lines on the chip surface. Stress also leads to parametric shifts in electronic properties within the chip, such as offset voltages, resistance of thin-film transistors, pinch-off voltages, hot electron degradation, and mechanical failure [1]. Another important failure mode due to plastic molding resin is often found in DRAM applications - fillers in the resin can cause point stresses on the surface of the chip that result in single-column-line failure [2]. Figure 1 illustrates this phenomenon, known as "filler-induced stress."
A mechanical buffer layer between the chip top surface and the molding compound of the package prevents damage to the delicate circuitry of the chip. On more mature devices, a liquid plastic compound on top of the chip in its lead frame provides the same buffering. The liquid is cured to form a protective dome covering the circuit. This technique, however, is not applicable to the very low-profile chip packages in use today, and the buffer layer is now often applied and patterned as the last manufacturing step of the whole wafer prior to probe test and scribing (Fig. 2).
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Figure 1. Filler-induced stress.
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Figure 2. Cross section of photosensitive polyimide buffered device.
The semiconductor industry has long been using polyimide materials in applications as diverse as intermetal dielectrics (though this application has not gained wide acceptance due to concerns over metal ion and water contamination), and as an a-ray shielding layer. The widest application is as the stress buffer layer applied after chip passivation, where the polyimide`s mechanical properties make it a reliable protection layer. With the increasing trend toward applying the polyimide to the entire wafer as the last manufacturing step, new challenges have emerged for the lithographer and process engineer.
Photosensitive polyimide as a buffer layer
The polyimide layer on the wafer must be patterned to open windows to the bond pads for subsequent wire bonding. There are two principle techniques in the patterning of polyimide as a process layer. Some manufacturers apply the polyimide to the wafer, and subsequently apply and pattern photoresist and etch the polyimide. This technique is reliable, but adds manufacturing complexity to the process, and therefore cost to the finished device. An alternative is to use photosensitive polyimide, which has been altered chemically to render it photosensitive. When photosensitive polyimide is exposed and developed, the desired pattern is produced without photoresist application and etching.
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Figure 3 a) Nonphotosensitive polyimide vs. photosensitive polyimide process flows. b) Nonphotosensitive polyimide vs. photosensitive polyimide and passivation masking process flows.
John Rose [3] has reported that photosensitive polyimide offers reduced process times and superior results compared with nonphotosensitive polyimide. This translates into further process simplification and manufacturing cost reduction (Fig. 3). Consequently, there has been a rapid increase in the use of photosensitive polyimide in the semiconductor industry.
Challenges in the imaging of photosensitive polyimide
The incorporation of photosensitive polyimide into the wafer fabrication process has led to some profound challenges for the process engineer, arising from the fundamental differences between the film characteristics of polyimide and photoresist (Table 1). At first glance, the specifications for imaging photosensitive polyimide seem very loose, and well within the capabilities of almost any stepper. Large bond pads on the order of 75 ? 75 ?m, with overlay requirements as loose as several microns, are common. However, the thickness of the material adds considerable difficulty to the imaging process. Manufacturers often define relatively small features, such as 15-?m fuse windows in photosensitive polyimide >30-?m thick. This high aspect ratio of more than 2:1 is comparable to those found in more advanced photoresist applications. High numerical aperture (NA) i-line wafer steppers used for the critical layers` wafer fabrication are often not well suited to aligning and exposing the thick photosensitive polyimide layers for the following reasons:
1. Most photosensitive polyimide materials work best with g-line exposure wavelengths. Heavy absorption of i-line leads to poor profile and dimensional control. While CD control for the buffer layer is loose (often >10%), i-line absorption problems can cause severe undercutting of the polyimide due to underexposure, and lead to excessive dimensional control problems. Manufacturers of photosensitive polyimide are developing material with better performance at i-line wavelengths to improve the situation.
2. The thickness of the photosensitive polyimide (10-40 ?m) requires that the stepper have a large depth of focus to maintain imaging acuity. This requirement is often at odds with high resolution i-line steppers, which have a high NA (typically =0.55).
3. The nonuniformity and thickness of the material often result in difficult and poor alignment for i-line steppers whose alignment systems have been optimized for photoresist of =1 ?m. Wafer targets below polyimide are often difficult to detect, and produce variable alignment signals due to the nonuniformity of the overlying material. Laser-based alignment systems often fare poorly since interference effects create highly variable alignment signals, even across one wafer. Broadband illumination alignment systems are more robust: interference effects tend to be averaged across the continuum of the alignment spectrum.
4. The complex optics of i-line steppers can result in a relatively high amount of scattered or stray light impinging on areas where it is not intended. This is a particular problem if the mask is predominantly clear field. Many photosensitive polyimide products are negative acting because of the chemistry of the material. Thus, the bond pad apertures are chrome islands in a clear field, resulting in a mask which is >99% clear. In a stepper where the stray light is high and bounces around the optics, unintentional exposure of the photosensitive polyimide in the bond pad areas can occur, leading to scumming and yield degradation.
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Figure 4. Buffer mask appearance.
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Figure 5. Post-cure SEMs of a through focus test in 20-?m-thick photosensitive polyimide from vendor 1.
Very often, i-line steppers have relatively low wafer plane illumination intensity due to the narrow spectrum of the lamp output used and to absorption loss in the lens. This can lead to low wafer throughput when exposing photosensitive polyimide at doses of up to 1 J/cm2 (Fig. 4). i-line reduction steppers, then, may not be the best choice for exposing photosensitive polyimide. An ideal stepper for this application would have the characteristics shown in Table 2. Recently, steppers optimized for exposing photosensitive polyimide have become commercially available. Below are some results from the evaluation of such a stepper.
Evaluation of a widefield g-line stepper for photosensitive polyimide exposure
Using a g-line broadband stepper, several commercially available g-line photosensitive polyimide materials have been characterized. The stepper used is based on the patented 1? Wynne-Dyson lens design using broadband g- and h-mercury lines, including the wavelength continuum from 390 to 450 nm. This efficient optical design consists of five elements and delivers very high intensity light at the wafer plane. It has a numerical aperture of 0.26 and a partial coherence of 0.6.
Figure 5 shows post-cure results of a through focus test in a 20-?m-thick (pre-cure) photosensitive polyimide from a vendor, while Fig. 6 gives results in a much thicker material (40 ?m) from another vendor. In both cases, excellent sidewall angles are evident.
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Figure 6. Pre-cure results in 40-?m-thick (pre-cure) photosensitive polyimide from vendor 2.
Summary
There is a rapid increase in the use of photosensitive polyimide in the semiconductor industry. Because its thermal and dielectric characteristics are compatible with the requirements of semiconductor devices, photosensitive polyimide has been used as an interlevel dielectric, an a-ray shielding layer, and a stress buffer.
Of these, the stress buffer application dominates, driven primarily by the requirements of thin profile packaging. In this application, photosensitive polyimide improves yield by minimizing local stress, and also lowers cost, reducing the number of process steps. The improved sidewall angle of photosensitive polyimide films offers yet further cost savings by allowing consolidation of the passivation and polyimide lithography steps into one process level.n
References
1. C. Schuckert, et al., IEEE/Semi Advanced Semiconductor Manufacturing Conference, p. 72 (1990).
2. Horie, Kazuyuki eds., Photosensitive Polyimide - Fundamentals and Applications, Technomic Publishing Co. Inc., 1995.
3. John D. Rose, "A Practical Comparison of Photosensitive and NonPhotosensitive Polyimides Used as a Buffer Coat," OCG Microlithography Seminar, Interface `94 Proceedings, p. 269-287, 1994.
Robbyn Culver received her BS degree in metallurgical engineering and materials science from the University of Notre Dame and her MS degree in materials engineering from Purdue University. She is a senior product manager of semiconductor systems at Ultratech Stepper Inc. Prior to this, she served as Ultratech`s senior engineering program manager of development engineering. Culver has more than eight years of professional experience in semiconductor engineering and management, with an emphasis in advanced packaging technology and wafer fabrication. She holds three US patents. Ultratech Stepper Inc., 3050 Zanker Rd., San Jose, CA 95134; ph 800/222-1213, fax 408/325-6444.