Positive Photoresist Polymerization
Through Pulsed Photomagnetic Curing

THE DEMANDS THAT VLSI PROCESSING has placed on resist patterning systems goes beyond good quality high resolution images. These high quality critically dimensioned resist patterns must now maintain their characteristics while being subjected to severe subsequent processing conditions. Previously, resist patterns were used primarily as stencil masks for wet chemical etching of oxides and metals. The emerging technologies seldom require wet chemical masking, and the resist must now be able to maintain its integrity during high dosage of ion bombardment and exposures to a wide variety of ionized gases.

Almost all positive photoresists are Novolak resin based materials with sensitizers, solvent systems, and other additives such as hardeners and adhesion promoters 11]. The type and ratios of these components determine in part, the temperature at which each resist will begin its plastic flow, and the ability of each to resist erosion. Depending on the manufacture and type, plastic flow will begin between 120 and 180. degrees C. Once flow begins, a rapid increase in critical dimensions occurs as the temperature is increased until an upper limit is reached. After reaching this upper limit small increases in critical dimensions will occur with increasing temperatures.

Unfortunately, many processes, such as plasma etching and ion implantation, elevate the resist surface temperature in excess of their flow point. Not only does this cause errors in critical dimensions, but outgassing from the resist can occur. The pressure burst associated with the outgassing will cause automatic systems to abort their process cycles, causing lost process time. This is especially true for high dose ion-implanted wafers that are processed on systems which have inadequate wafer cooling. Even systems equipped with efficient cooling chucks can still have outgassing which disrupts the resist surface. This type of outgassing occurs when large amounts of hydrogen are liberated from the hydrocarbon molecules. The high energy of the bombarding ions causes polymer carbonization resulting in the release of hydrogen [2].

Therefore, it is necessary to find a technique that will increase the temperature at which plastic flow is initiated, and has the potential to harden the resist surface to retard erosion and hydrogen liberation. Techniques such as the PRIST (Photo Resist Image Stability Technique) 131 and deep UV curing have been offered as methods to allow positive resist to be baked beyond its normal flow temperature prior to ion implantation or plasma etching 141. These methods have had limited success while increasing process times.

The following tests indicate that the Pulse Photomagnetic Curing (PPC) meets the requirements of retarding plastic flow and improving ion implant protection. The required treatment time is less than a few minutes per wafer, and when used prior to ion implantation the postbake cycle may be eliminated, causing a reduction in the total process time.

Acknowledgement

None of this work could have been accomplished without the assistance of Louis and Richard Panico of the Xenon Corporation, who made the pulsed Photomagnetic Curing process available. I would also like to thank D. Michael Mack of the Eaton Corporation for his role in making the Nova 160 ion implanter available for the high dose implant test and Joseph Bulger of ADS for his resist preparation assistance.

References

1. W.S. DeForest, "Photoresist Materials and Processes," McGraw-Hill, N.Y. , 1975, pp.47-60.

2. T.C. Smith, "Wafer Cooling and Photoresist Masking Problems in Ion Implantation," Motorola MOS Group/Advance Product R&D Lab, 1982. To be published.

3. W.H.L. Ma, "Plasma Resist Image Stabilization Technique (PRIST) Update," Proc. Submicron Lithograpghy, vol. 333, pp. 19-23, SPIE, Bellingham, WA, 1982.

4. R. Allen, M. Foster, Y.T. Yen, "Deep U.V. Hardening of Positive Photoresist Patterns," J. Electrochemical Soc., vol. 129, p.1380, 1982.