Multilayer resist strategies
07/01/2003
IC manufacturers face a number of economic pressures today, not the least of which is the high cost of transitioning to tools with shorter wavelengths or higher numerical apertures. This, in turn, challenges lithographers to maximize the life of present-generation tool sets. Many lithographers are working with chemical solutions to develop innovative ways to reach their goals without accruing capital expense. Some approaches being taken to circumvent these costs involve a shift from conventional single-layer resist (SLR) tactics to multilayer resist (MLR) strategies, such as bilayer, trilayer, and even sacrificial layers.
Bilayer photoresists
Traditional bilayer resist systems consist of a photosensitive silicon-containing top layer and an organic underlayer with good optical characteristics and planarization properties. Bilayer schemes print using thin film imaging with a typical thickness of 100–200nm. Once the features are patterned in the imaging layer (Fig. 1), they are transferred to the underlayer using an O2/N2 reactive ion etch. This transfer forms a thin layer of SiO2, which enables the high selectivity of the thin imaging layer to the organic bottom layer (Fig. 2a).
The underlayer will provide antireflective properties and act as a good etch-resistant mask for the substrate etch. Typical underlayer thicknesses range from 400–1000nm and allow for excellent planarization over local topography and high- aspect-ratio (A/R) etch masks to the substrate. Planarization of the substrate topography by the underlayer increases the focus margin.
Top-imaging materials consist of 5–20% silicon, depending on the platform used. Photoresist vendors currently offer two groups of silicon-containing bilayer systems. The first group consists of norbornenes — maleic anhydride alternating copolymer (COMA) or (meth)acrylate resists in which silicon has been substituted in the polymer's pendent groups. The second group includes siloxane/silsesquioxane platforms (SSQ), which have silicon in the main polymer chain.
 Figure 1. 193nm COMA-based bilayer system showing advanced lithography (130nm iso; trench 120nm; 1:1 L/S). |
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Because of the pendent group location, there is a greater potential for the silicon in COMA and acrylate-based polymers to cleave and outgas during exposure. Resist companies are taking extensive measures to minimize this outgassing and reduce potential contamination and damage to costly optical elements in the stepper/scanner [1].
 Figure 2. a) A standard bilayer scheme; b) a trilayer scheme; and c) the use of a DUO underlayer. |
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Originally designed for work at the 157nm wavelength, the transparency of SSQ polymers have made them an obvious choice for lithographers as they work to extend the life of existing tool sets. Because of the porosity of SSQ polymer matrixes, there is the potential for poisoning from outside amine contamination during any sort of post-exposure delay [2]. This challenge can be minimized through unique resist formulations. Low-molecular-weight compounds, or strong photo acid generators with good quantum efficiency, can promote diffusion of the acid after exposure, which can minimize these post-exposure delay effects.
Underlayer materials are organic and designed with reflectivity control, planarization control, and etch selectivity to the top layer and final substrate in mind. In any bilayer resist system, it is critical to use an optimized underlayer for the pattern transfer of the imaging layer. Because this layer is used to transfer the pattern into the substrate, etch selectivity is critical.
It is important that underlayer materials have good via-filling properties for dual damascene integration schemes (Fig. 3) and that the underlayer can be easily ashed away after etch transfer with a simple O2 process that does not damage the substrate. Optimizing the interaction between the top layer and underlayer is critical for good adhesion and to prevent intermixing with the imaging layer. Phenolic and naphthalene-type resins are typical materials used for these layers because of their high etch resistance and reflection control.
Though sacrificial layers are not considered a conventional MLR process, their use is similar to the underlayer of either a bi- or trilayer resist scheme. Sacrificial layers consist of siloxane-based polymers and dye that help with reflectivity control at the desired exposure wavelength. Sacrificial underlayers, such as DUO, have the ability to fill vias and provide a planar reflectivity control layer for lithography patterning (Fig. 2c).
Some attributes of a sacrificial underlayer are good compatibility with the photoresist-imaging system, absorption at 193nm wavelength, similar etch selectivity to the low-k dielectric, and good gap-filling properties.
Trilayer systems
While bilayer systems contain a large percentage of silicon and a photo-imagable top layer, trilayer systems introduce a standard 193nm photoresist and a silicon-containing middle layer to serve as the masking layer during pattern transfer.
Most middle layers consist of spin-on-glass, organo polysiloxane polymer, or silicon dioxide chemical vapor deposition films [3]. Middle layers typically contain 7–15% silicon and have very low optical densities at 193nm, making them quite transparent. Due to the transparency, middle layers rely on the underlayer for reflectivity control properties.
Lifting, footing, or undercutting of the resist pattern can be eliminated through good matching of properties between the middle and imaging layers. It is possible to optimize this interface through the use of thermal acid generators to minimize the diffusion of acid from the exposed resist to the middle layer.
 Figure 3. Via-filling properties of a JSR carbon-based underlayer over various pitch densities. |
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The addition of a silicon-containing middle layer increases flexibility in the resist system by allowing lithographers to choose any commercially available SLR specifically designed for the necessary layer. Trilayer resist schemes can completely eliminate outgassing because the silicon is in the middle layer, and, therefore, not exposed to any exposure tool optics.
Because of high silicon content, the imaging layer and the middle layer for a trilayer system have a selectivity of ~7:1. The transfer etch of the imaging layer into the middle layer uses a standard CF4/O2/Ar etch chemistry and can be easily controlled to provide anisotropic profiles. The transfer of the middle layer into the bottom layer utilizes an O2/N2 etch. As in bilayer systems, the underlayer etch has a selectivity of ~10:1 and takes advantage of the SiO2 mask formed during the transfer. Because there are two separate transfer etches (top to middle, and middle to bottom), etch engineers are able to use negative biasing two separate times and take pressure off the imaging aspects of any resist system with respect to CD reduction.
Additional processing steps and potential rework complications necessary for the silicon-containing middle layer pose additional concerns when using a trilayer system as opposed to an SLR. It is often necessary for the middle layer to be coated on a separate track than the imaging layer because of the need for a high bake temperature and because most spin-on-glass materials use different solvent systems than standard photoresists.
Additional advantages
There are many other inherent benefits to MLR platforms. As low-k dielectrics become more integrated into device technologies, resist poisoning becomes more of a concern. MLR schemes allow for a larger barrier between the low-k dielectric and the photoresist than SLR systems. Most SLRs use an organic or inorganic BARC that is only 50–80nm thick, while MLRs range from 600–1000nm of material separating the low-k dielectric and the imaging layer. This larger barrier consistently reduces or eliminates poisoning that results from amines leaching up from the dielectrics.
MLRs also have a reduction in A/R of the imaging layer. As lithographers push A/Rs to >3:1, issues such as pattern collapse plague process windows. By implementing MLRs, lithographers can send systems to their etch group that allow for final pattern transfers of aspect ratios >10:1.
Conclusion
While the economic challenges for the IC industry persist, actionable solutions can reduce costs within a production environment. MLR strategies offer lithographers relatively inexpensive options to meet both economic and development needs. Both bilayer and trilayer approaches will extend the use of 193nm resists and provide solutions to the 90nm node and beyond.
Mark Slezak, JSR Micro Inc., Sunnyvale, California
Acknowledgments
The author would like to thank Tsutomu Shimokawa, Takashi Chiba, Hikaru Sugita, Ray Hung, Jeff Smith, Vic Marriott, and Mark Dennen of JSR for their contributions. DUO is a trademark of Honeywell Corp.
References
1. J. Meador et al., "193nm Multilayer Imaging Systems," SPIE, 2003.
2. R. Hung et al., "Development of SSQ-based 157nm Photoresist," Proceedings Photopolymer, 2002.
3. M. Hussein et al., "A Novel Approach to Dual Damascene Patterning," Proceedings IITC, 2002.
For more information, contact Mark Slezak, technical manager, lithography, at JSR Micro Inc., 1280 N. Mathilda Ave., Sunnyvale, CA 94089; ph 408/543-8800, fax 408/543-8996, e-mail [email protected].