Holger Moritz

Optical single layer lift-off process

The lithography of the metal wiring layers is becoming the most confining technology in the era of VLSI (very large-scale integration), as more and more circuits have to be wired on the chip itself. The two competing technologies are subtractive etch (wet or dry), and additive metal lift-off. As lift-off needs no etching, it inherently offers cost and density advantages. It, however, requires an undercut photoresist profile. These undercuts can be achieved with an image-reversal process. The paper describes such a reversal process, especially tuned for lift-off applications. The result is a simple single-layer lift-off technology with excellent image quality.

Method of making conductive paths through a lamina in a semiconductor device

On the layer 12 a mask 3 corresponding to the desired pattern of holes 15 is provided with via openings 14 having overhanging walls. The layer 12 is selectively etched with a method where the etching attack takes place vertically to the layer surface, and wherein the mask 3 is thinned simultaneously, so that holes 15 are obtained having a cross-section increasing toward the mask 3. If subsequently material 16 for filling the holes 15 is applied in a blanket deposition these holes are completely filled when the material 16 has the same thickness as the layer 12 although the openings over the holes are decreasing with increasing thickness of the material 16. The layer 12 consists preferably of an insulation material, the mask 3 of positive photoresist, and the material 16 of a metal.

Contrast transfer function measurements of deep ultraviolet steppers

The aerial image contrast is a key parameter for the evaluation of the performance of optical projection systems. State‐of‐the‐art deep ultraviolet steppers and some i‐line steppers were investigated for comparison by measuring the contrast transfer function, i.e., contrast versus linewidth. Examples are shown how specific weaknesses of an imaging system like astigmatism and sensitivity from internal reflections can be quantified with this technique. For comparison the contrast transfer functions were also calculated, assuming ideal, purely diffraction limited image transfer of the optics, and with the numerical aperture and partial coherence as parameters. Interestingly none of the steppers meet the imaging performance which can be expected from theory.

Aerial image studies of an advanced deep-UV exposure system

Abstract A method to measure directly the aerial image of arbitrary patterns, imaged by a stepper optics in a very thin layer of photoresist, is presented. Using this technique we characterised the optical properties of an advanced Deep UV stepper. Measured and simulated aerial images differ significantly, which can be attributed to imperfections of the projection optics. The major goal of this study was to find out the aerial image related limitations of lithographic process windows for patterns with 0.35 μm and 0.25 μm ground rule. Experimental data for several types of illumination (conventional and oblique illumination techniques) were obtained., and the aerial image induced offsets between the widths of isolated and grouped lines studied.

Robust and environmentally stable deep UV positive resist: optimization of SUCCESS ST2

By optimization of SUCCESS ST2 an environmentally stable and robust deep UV positive resist has been derived where the generally encountered problems related to chemical amplification resists, the formation of T-tops and linewidth changes during delay have been solved. The previously reported chemistry, protected poly-p-hydroxystyrene and SUCCESS type sulfonium salts, has been proven to be insensitive to airborne contaminations. In this paper the optimization of processing conditions, based on thermal analysis is reported. With the derived conditions linewidth changes during delays could be minimized and excellent performance obtained. At the IBM lithography test center in Boblingen an integrated photosector, consisting of equipment, materials, process and control philosophy, was balanced and 0.25 micrometers pattern can routinely be resolved using an ASM-L DUV stepper (NA 0.5).

Aerial image analysis of quarter-micrometer patterns on a 0.5-NA excimer stepper

Applying a method for direct on stepper measurement of aerial images, the quarter micron performance of an advanced deep UV excimer stepper is analyzed. The aerial images are compared to corresponding simulation results as well as data obtained on an aerial image measurement system (AIMS). The study includes three different mask types: standard chrome on glass, an attenuated and an alternating phase shifting mask. Substrate effects as well as laser spectral purity effects have been measured. In general, the data show that simulation and AIMS data represent an upper limit for the aerial image contrast, which may be degraded significantly under real world imaging conditions. Use of a bottom anti reflective layer and a new laser with improved spectral bandwidth result in optimum aerial images close to those measured on AIMS, but they are still worse than simulation results. Some examples of quarter micron resist profiles and process windows in a deep UV resist are shown that were obtained under these optimized conditions with an excimer laser stepper.

Resist thickness influence on depth of focus - A key question for advanced i-line and DUV lithography

Abstract Motivated by a dramatically shrinking depth of focus as device geometries approach 0.25 um and the increasing importance of resist thickness as part of the focus budget, this paper summarizes experimental depth of focus results obtained at various resist thicknesses with different resist types.

Performance data of an environmentally stable deep UV positive resist

Abstract In this paper we describe the lithography performance of a chemically amplified positive DUV resist named “SUCCESS-ST2” from BASF [2]. The resist consists of three components. A polymer component which provides good etching resistance, a molecular component which combines the functions of the photoacid generator and the dissolution change agent and a contrast enhancement additive. Specifically time delay effects, process window, printing quality over poly-Si steps and etch performance are investigated.

Increasing the defocus latitude of a deep UV system

Abstract This paper shows how the defocus latitude of a deep UV (DUV) system depends on the thickness of the photoresist in two ways: by thin film interference, and by the bulk thickness. Results of experiments and simulations are presented. Since both resolution and defocus latitude are expected to depend on the bandwidth of the laser, we here report the first tests to evaluate the merits of laser bandwidth narrowing down to FWHM = 1.2 pm on the imaging capabilities of a 0.5 Numerical Aperture (NA) DUV stepper. Especially for 0.25 μm lithography, this is found to be advantageous.

I-line photoresist evaluation for A 0.5 micron technology

Abstract The lithographic performance of IBM Spectralith, Hoechst AZ 7110, and Hoechst AZ 5214E in image reversal mode was evaluated on a Nikon body 7 0.45NA i-line stepper. All 3 resists are capable of resolving 0.45 μm lines and spaces, printing linearly down to 0.45 μm with zero bias, and show at least 1.5 μm depth of focus. The photospeed of Spectralith and AZ 7110 is around 150 mJ/cm2, and their exposure latitude is 32 %, for 0.5 μm lines/spaces. AZ 5214E in image reversal mode is 3 times faster, prints 0.5 μm patterns to nominal at only 45 mJ/cm2, and also shows a significantly wider exposure latitude of 52 % (again for 0.5 μm patterns). In conclusion a 0.5 μm technology can be run on todays i-line steppers with these resists.