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EETE DECEMBER 2012

DESIGN & PRODUCTS MEMS TECHNOLOGIES A micro-lens array for multiple electron beam lithography tools By Bart Vereecke Optical lithOgraphy has been the patterning technique of choice for many decades in cMOs high volume manufacturing because of the high throughput and relatively low cost of the process compared to electron beam lithography. the 193nm wavelength used in state of the art lithography poses a physi- cal limit to the resolution that can be achieved. Many resolution enhancement techniques - such as multiple patterning schemes and optical proximity corrections - have been applied to push the resolution as far as possible but these methods significantly increase the cost and complexity of both processing and mask making. the 193nm lithography workhorse has been pushed to its physical limits and the 13.5nm EUV litho is expected to take over in the coming years at an even higher manufacturing cost. Electron beam lithography resolution is not limited by opti- cal wavelength. hence intrinsically higher resolutions can be achieved. Moreover, the technique does not require any expo- sure masks. this is why e-beam is widely used for mask mak- ing. E-beam lithography’s major disadvantage for high volume manufacturing is that the writing speed of a single beam writing system is not high enough. The Reflective Electron Beam Lithography (REBL) concept developed by Kla-tencor provides a solution through mas- sive parallelization of the electron beam writing process, hence significantly improving the speed of the e-beam writing process (without need for a mask). It uses a flood incoming electron beam that is decelerated to illuminate a digital pattern generator (DPG) chip. This simplest version of the DPG consists of an ar- Fig. 1: Schematic of the REBL set-up. ray of 1million individual metal plates build on top of a function- al cMOs chip. the chip switches the voltage on each individual four ring electrode can be tuned to achieve optimal focusing of metal plate. Hence either reflecting or absorbing the incident the beam – see figure 2. electrons and thereby splitting the incoming beam into 1million The processing of the lens array was first developed with- reflected beamlets which is the massive parallelization required out the underlying cMOs wafer to prove the functionality and to enable e-beam process to overcome current throughput resolution of the devices. in a second phase the structure was limitations. The DPG patterned electrons are then accelerated, built on top of the controller chip. Manufacturing the lenses on focused and de-magnified towards a wafer. The REBL concept top of existing cMOs circuitry posed many additional technical is depicted schematically in figure 1. however, the metal plate design does not meet the resolution requirements needed to compete with state of the art optical lithography including EUV. Because the lenslet plates are not isolated from one another, the electrostatic fields from individual pixels affect the surrounding pixels, which limits resolution and contrast. DPG performance can be significantly improved by adding electrostatic focusing lenses on top of each individual reflector plate to isolate the individual pixels from one another. Imec manufactured the electron reflection plates and focus- ing system on top of a cMOs chip. the design consists of cylindrical holes with a diameter of 1.4um and spaced only 0.2um apart. the bottom of each cylindrical hole is formed by a metallic reflection plate controlled by the logic circuit under- neath. the cylindrical holes are surrounded by four metallic ring electrodes separated by a dielectric layer. the voltage on the Fig. 2: Operating principle of the lenslet structure: the voltage on the bottom plate controls the absorption or reflection of Bart Vereecke is integration engineer at imec – www.imec.be - the incoming electrons, while the four ring electrodes assist to his focus is on specialty components based on semiconductor focus the electrons. technologies. 34 Electronic Engineering Times Europe December 2012 www.electronics-eetimes.com


EETE DECEMBER 2012
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