Photoresist strip has traditionally been a low technology process step, but is becoming increasingly more complex with
the migration to ultra-shallow junctions, 3D structures, double patterning, and high-mobility channels. At junction
depths of a few tens of nanometers, surface effects become increasingly important. Small changes to surface conditions
can affect junction resistivity, junction depth, and dopant activation.
Advanced high-resolution chemically amplified resist can be problematic when used as an implant mask. Ion beam
induced chain scission and photoacid generation can lead to thermal instabilities during the resist strip process. Multilevel
resist structures can be difficult to remove and rework and high aspect ratio 3D structures can require near infinite
selectivity during the strip processes. This paper will summarize the issues and offer options for solutions.
In order to increase the etch resistance of photoresists, patterned resist features are stabilized by UV/Bake process. This process provides increased etch resistance to the film by densifying the resist. It is understood that resists is crosslinked during the UV/bake process and as such, better crosslinked resist, typically have better plasma etch resistance. However, historical UV/bake curing of Deep UV resists can cause loss of Cd control because of the excessive shrinkage of the film. Resist shrinkage has been reported to be due to loss of free volume. A major source of free volume los in deep UV resists is the deprotection reaction and the evaporation of the protecting groups. A significant reduction in film shrinkage is observed in UV/bake curing of deep UV resist films under ammonia or other amine purge gases. It is known that deep UV chemically amplified resist, are sensitive to environmental contaminants such as ammonia and other amines resulting in 'T-topping' or other forms of image degradation. However, during the UV/bake curing of deep UV resist images, under amine purge, it appears that the amines act to neutralize the photogenerated acid, thereby preventing the deprotection of the acid labile groups, and thus, results in a significant reduction in shrinkage. In this paper we demonstrate that the major role of amines in shrinkage reduction, during the UV/bake cure process, is not entirely prevention of deprotection of the acid labile groups alone. An additional mechanism is proposed to explain further significance of ammonia and other amines in film shrinkage reduction during the UV/bake cure process.
In spite of the comparatively modest level of effort devoted to ion projection lithography, the results obtained so far indicate that the technology is highly promising. Accordingly, a $36M program has been launched in Europe to develop a full field, IPL process tool.
The error budget allotted to a lithographic mask is generally only a small fraction of the critical dimension of the device features. Consequently, ion projection lithography in the sub-0.13 micrometers technology regime will place large demands on image placement accuracy, a component of which is mask distortion. During the design stage then, it is desirable to identify those intrinsic loads which distort the mask pattern from its intended shape and, ultimately, to reduce those distortions to an acceptable level. This paper assesses the in-plane distortions (IPD) due to gravity as a function of the mask's geometric parameters. The optimal mask geometry is identified by minimizing the IPD function.
Ion projection lithography (IPL) is analogous to an optical wafer stepper except the exposing photons have been replaced by high energy, light ions. In the IPL machine being developed by the Advanced Lithography Group (ALG), a silicon stencil mask is `illuminated' by a broad area beam of hydrogen or helium ions. The ions pass through stencil mask openings and enter a multi-electrode electrostatic lens system which projects a demagnified image of the stencil mask onto a resist coated wafer substrate. Demonstrated IPL performance is covered. Independent calculations of the novel ion-optical column of the ALG prototype tool show less than 15 nm distortion over a 20 mm X 20 mm field, and indicate that even larger fields are possible. This machine will utilize standard optical, off-axis, wafer alignment and a precision laser interferometer controlled X-Y-stage. This combined `pattern lock' will enable the ALG prototype to achieve overlay requirements necessary for 0.15 micrometers geometries. The Advanced Lithography Group project for constructing the prototype ion projector is discussed.
High performance focused ion beam systems utilizing liquid metal ion sources are performance limited due to the chromatic aberration of the optical elements and the finite energy spread of the ion source. Some concepts are presented to reduce the chromatic and spherical aberrations utilizing axially symmetric electrostatic optical elements. Unlike paraxial optics, in off-axis optics the individual aberrations of each lens element interact. This allows the total system aberrations to be minimized by optimizing lens parameters. Fifth order raytracing is used to determine the off-axis aberrations of a two lens system. The aberrations are then calculated as a function of the inter-lens spacing and are shown to have distinct minima. By optimizing the lens acceptance angle and the lens spacing off-axis chromatic aberration and the geometric aberrations (including spherical aberration) may be reduced to values below that achievable using paraxial optics at the same beam current. Novel coaxial optical lens designs are presented which also offers the possibility of chromatic aberration correction and may provide improved performance over conventional optics for special applications.