University of Ljubljana Faculty of Electrical Engineering Laboratory

University of Ljubljana Faculty of Electrical Engineering Laboratory of Microsensor Structures and Electronics Silicon Micromachining for Microstructures Fabrication in LMSE 1

Laboratory of Microsensor Structures and Electronics (LMSE) is involved in research and development of microstructures such as silicon devices, sensors, actuators and microelectromechanical systems (MEMS). Internal properties and external characteristics of these structures are studied using analytical and computer modelling. Technologies available in LMSE allow investigations of new processes in the fields of mask design and fabrication, photolithography, diffusion, metallization, depositions, cleaning, thin film processing, etching, micromachining etc. Based on these activities, research and development of various new microstructures such as photo sensors, pressure sensors, temperature sensors, radiation sensors, sensors for nuclear physics, actuators, nanostructures, various 3 D micromechanical structures and similar, is going on. Research is supported by advanced measurement equipment and characterisation techniques, aided by process and device modelling. A part of research activities is involved in the field of electronic circuit theory, simulations and applications. Harmonic balance is used as a powerful method of analysis for nonlinear dynamic circuits. The team is also engaged with practical solutions in the field of microprocessor aided electronics. It encompasses development of appropriate hardware and software for automatic measurements of electronic and telecommunication equipment. Cooperation with manufacturers of professional electronic equipment is well established. Members of LMSE are collaborating with European universities under the framework of international projects sponsored by the EU commission. LMSE is a free university lab, open for any kind of cooperation with other laboratories and industry. LMSE has a well established cooperation with leading institutions all over the world. LMSE offers complete research and development services in the field of microstructures and electronics, from theoretical analysis to fabrication of test structures, devices and circuits, their characterisation and optimisation. 2

Micromachining in LMS - Overview • Main activities are listed below: ÄCompensation of convex corners ÄBossed diaphragms ÄMicrotips for AFM and field emitters ÄMultilevel microstructures ÄCantilevers ÄAccelerometer microstructures ÄOptical fibre aligning ÄMicromachined reflecting optical mirrors ÄWafer bonding ÄPressure sensor (low and medium range) ÄSmart pressure sensor approach 3

Micromachining in LMS Compensation of convex corners It is well known that in anisotropic wet micromachining of silicon microstructures, fast etching of high-index crystal planes ((411), (212), (323), . . ) inevitably occurs at convex corners (2 -3 times faster etch rate). By utilising different shapes and/or size of compensation structures, we have been able to mitigat this effect to a great extend. This is very important when precise and deep etched microstructures are required (e. g. mesa structures, ridges, . . ) The degree of convex corner undercutting depends also strongly on the wet etchants. Additives like isopropyl alcohol reduces underetching of convex corners. Proposed compensation structures 4

Micromachining in LMS Compensation of convex corners and its applications In case of bossed diaphragm, such as used in low-pressure measurement devices or inertial mass there is the need for proper design of compensation structures that will occupy small footprint and effectively compensate convex corner undercutting to depths beyond 300µm. 5

Micromachining in LMS Microtips for atomic force microscopy (AFM) In AFM , topography, mechanical, chemical and electromagnetic properties of materials are investigated with the highest spatial resolution. A microprobe with extremely sharp microtip as the most vital part is scanned over the surface and the force is detected via different methods. In our microtip realization, phenomenon of undercutting the convex corners is fully exploited for achieving sharp microtips. Mask is laterally underetched, falls off and microtip remain with specific aspect ratio. Further sharpening is obtained via oxidation method. 6

Micromachining in LMS Microtips for field emitter displays Another application of microtip application is for cold cathode emitter tips serving as light sources for displays. For effective light source the angle and sharpness of microtip are important factors affecting the electric field distribution and thus the operating voltage of flat panel displays (FPD). 7

Micromachining in LMS Microtips by isotropic etching and hillocks Isotropic etching spontaneous hillock Summarised results of etched microtips 8

Micromachining in LMS Multilevel microstructures The design of microelectromechanical structures (MEMS) in bulk micromachining is in most cases limited by mask shape and etching anisotropy of single crystal silicon planes. To extend the variety of planes that can be obtained by wet micromachining, much effort in LMS has been directed toward utilising several height levels with minimum set of masks and a combination of mask and maskless etching. 9

Micromachining in LMS Cantilever and bridge microstructures These structures have many applications in sensing and in actuating devices. ØWhen thermopiles are integrated on the cantilever (or bridge), they can detect heat transfer, airflow, etc. ØWhen a piezoelectric layer is deposited on the top of a cantilever or they have integrated piezoresistors, they can sense applied force or vice-versa, they can perform as actuators Resonant frequency of cantilever 15µm thick Si cantilevers t-beam thickness L-beam length E-young modul r-beamdensity 10

Micromachining in LMS Cantilever and bridge microstructures 70 nm thick Si 3 N 4 stress-free cantilevers When residual stress is present in thin free-standing structures such as bridges or membranes, they bend upward or downward, depending on material and/or combination of layers. From the bending curvature and known dimensions of the structure, internal stress S can be determined: where E is Young module, Poisson ratio, ts substrate thickness and tf thickness of thin film producing stress on silicon, R 1 curvature under stress and R 2 is curvature after removing the stress-inducing film. 11

Micromachining in LMS Aluminum cantilevers ØAluminum can play an important role as a masking material in anisotropic etching (usually 5% TMAH-water+1. 5% dissolved Si+0, 6% ammonium peroxodisulfate). By underetching method, cantilevers are obtained. ØAluminum cantilevers were fabricated by above procedure (100µm long, 10µm wide, 0. 45µm thick): 12

Micromachining in LMS Micromachined accelerometer structure with piezoresisitive sensing ØInertial mass is suspended on three hinges. Central one has integrated sensing piezoresistor, while other two make the device more robust against accelerations in other directions. ØUnder the acceleration inertial mass is displaced with respect to the fixed part of microstructure, causing thereby stress in the piezoresistor located in the central hinge. This results in proportional resistance change, which is detected by the outer electric circuitry. ØThe accelerometer microstructure was realised entirely by wet etching processes. Cross-section Top view Bottom view 13

Micromachining in LMS Telecommunications and optical applications Microstructures found various applications also in these fields as single optical components for interconnections between fibres and other active devices, optical benches, passive or active reflecting mirrors, active switches, beam splitters, etc. Optical fibre aligning grooves For precise alignment of optical fibres in case of positioning or interconnections on microoptical benches, where light sources or detection systems can be realised monolithically, microstructures such as grooves are useful. By aid of wet or/and dry etching techniques, different groove structures can be obtained. 14

Micromachining in LMS Silicon crystal planes as reflecting optical mirrors Optical light beam reflectors were realised on different crystal planes, micromachined out of silicon monolithic wafer. Some important planes of interest were: Ø (111) with an angle of 54, 74º and very smooth surface Ø (110) with an angle of 45º with respect to (100) surface, Ø (311) crystal planes, with an angle of 25º toward (100) surface. Single mode fibre guiding 632 nm beam in V groove with 45º mirror 15

Micromachining in LMS Reflected beam angles depending on micromachined crystal planes: Measurements of reflected angles and dispersion of reflected light were performed by photodiode response. Average surface roughness Ra is in the range of 25 nm and the lowest scattering was obtained with (111) crystal planes (cca. 3º) 20º 17º 40º Single mode fibre-NA=0. 1 16

Micromachining in LMS Characterisation setup of reflecting mirror planes by beam image dispersion Results: Beam reflecting images from crystal planes prepared with different etchants: Image shape is proportional to the crystal plane roughness 17

Micromachining in LMS Low temperature bonding of silicon wafers (<400ºC) In order to bond wafers successfully attention was paid to the following steps: Øsurface preparation - to obtain particle- free and hydrophilic surface. Cleaning of silicon surface has a great impact on surface chemistry and topography. After RCA cleaning dry wafers were immersed into hot nitric acid (HNO 3), allowing growth of a few monolayers of fresh hydrous chemical oxide, increasing roughness to Ra=10 -12 nm. AFM after RCA Ra=10 -12 nm hydrophobic surface a >50º after RCA cleaning hydrophilic surface a <10º after forming hydrous chemical oxide 18

Micromachining in LMS Øprebonding at room temperature - two wafers were put into intimate contact in cleanroom ambient at room temperature. We initiate bonding by locally pressing the centre region from the top, thus enabling bonding phenomenon to propagate radially. By doing this, we actually help to accommodate the two surfaces that suffer from nonflatness, through elastic deformation process via attractive Van der Waals forces. Mating of rough surfaces via elastic deformation (zip) Bond interface chemistry Si Si interface between two wafers 19

Micromachining in LMS Øbond annealing (strengthening) - transformation of silanol to strong siloxane bonds takes place at elevated temperature. Bond strengthening was performed in the range from 80 C to 400 C, in different ambients. Bonding efficiency was characterised by quantitative analysis of tensile strength of bond. Overall reaction across two hydrophilic surfaces: Si-OH + OH-Si Si-O-Si +H 2 O covalent bonds 20

Micromachining in LMS Bond quality characterisation Voids at the bonding interface reduce the bond strength. Their origin could be trapped ambient gas, particle or gaseous by-product from interface reaction. Cross-section of bonded interface IR transmission investigation is performed by IR camera, model PTC-10 A. By this method only larger area defects can be recognised. Void Bonding interface Voids 21

Micromachining in LMS Differential pressure sensor Four p-type resistors are diffused into the silicon membrane and connected in the Wheatstone bridge configuration. Membrane is realised by bulk micromachining in 33% KOH etchant at 80ºC and has thickness of 23 2µm. Silicon thin membrane (25µm) Diffused piezoresistors 22

Micromachining in LMS Differential pressure sensor characteristics 23

Micromachining in LMS Advanced smart pressure sensor Smart sensors are the leading edge in advanced sensor applications. R&D activities in this field are taking place in LMS. Ø The developed smart pressure sensor, in excess of standard features uses a special calibration algorithm which minimises the offset voltage impact and compensates temperature dependencies. Ø The starting point of calibration is a raw pressure sensor without any offset or temperature compensation. Ø The calibration procedure also eliminates sensor nonlinearity. Ø Full-scale pressure range is totally adaptable to the user’s requirements Smart pressure sensor with digital temperature compensation and in-system calibration. 24

Laboratory of Microsensor Structures and Electronics (LMSE) Phone: (+386 1) 4768 303 Fax: (+386 1) 4264 630 http: //paris. fe. uni-lj. si/lms/ Head: Professor Dr. Slavko Amon Staff: Professor Dr. Slavko Amon Professor Dr. Igor Medič Assistant Professor Dr. Žarko Gorup Assistant Professor Dr. Andrej Levstek Assistant Professor Dr. Drago Resnik Assistant Professor Dr. Danilo Vrtačnik Senior Lecturer Niko Basarič, M. Sc. Researcher Uroš Aljančič, M. Sc. Researcher Matej Možek, M. Sc Technical Collaborator Matjaž Cvar Technical Collaborator Marijan Žurga Phone: (+386 1) 4768 + Ext. E-mail: slavko. amon@fe. uni-lj. si igor. medic@fe. uni-lj. si zarko. gorup@fe. uni-lj. si andrej. levstek@fe. uni-lj. si drago. resnik@fe. uni-lj. si danilo. vrtacnik@fe. uni-lj. si niko. basaric@fe. uni-lj. si uros. aljancic@fe. uni-lj. si matej. mozek@fe. uni-lj. si matjaz. cvar@fe. uni-lj. si marijan. zurga@fe. uni-lj. si 25 Ex 35 32 32 84 30 30 33 30 30 30 27