Sputtered Tensometric Layers for Microsensor Applications

Transcription

1 Sputtered Tensometric Layers for Microsensor Applications P. KULHA (1), Z.VYBORNY (2), M.HUSAK (1), V.JURKA (2), F.VANEK (3), K. POSPISIL (1) Department of Microelectronics, Faculty of Electrical Engineering, Czech Technical Unversity in Prague Technicka 2, CZ Prague 6, CZECH REPUBLIC Phone: , (2) Institute of Physics Academy of Sciences of the Czech Republic Na Slovance 2, CZ Prague 8, CZECH REPUBLIC Phone: , (3) Institute of Thermomechanics Academy of Sciences of the Czech Republic Dolejskova 5, CZ Prague 8, CZECH REPUBLIC Phone: Abstract: - Design and technology of some types of micromechanical sensors based on tensometric layers sputtered on silicon substrates is described. The aim is to prepare sensors suitable for higher operational temperatures utilizing both available high-quality insulating layers and local reduction of substrate thickness by anisotropic etching. To achieve long-term temperature stability the layers of NiCr, Ta 2 N and CrSi have been studied. Several variants of active layers combined with multilayer electrical contacts were evaluated to prepare successfully the samples Sensors of both onedimensional type (cantilevers for deformation sensing) and two-dimensional one (membranes) have been studied. Simulations using Coventor-Ware software were performed for modelling of properties of the microstructures under study. Key-Words: deformation, tenzometric effect, stress analysis, high-temperature sensor, sputtering 1 Introduction Various kinds of sensors of mechanical deformation have been developed. Those based on resistors implanted into a silicon substrate have frequently a very good deformation sensitivity, linearity of characteristics, low hysteresis, etc. Their resistors are electrically insulated from the substrate using P-N junctions usually, however. Due to temperature characteristics of the junctions their highest operational temperatures are limited to about +130 C only, therefore [1]. Higher operational temperatures (e.g. at least 150 C to 300 C) are important for some sensor application especially in mechanical engineering, power stations, etc. Besides the well known foil sensors also some thin film devices have been tested for such purposes. The sensors using tensometric layers sputtered on electrically insulated thin metallic substrates are known to suffer from a low quality of the insulation, sometimes. Instead of metallic substrates ceramic ones have been more frequently tested. The latter are somewhat brittle (depending also on the temperature regime used during their technology) so they do not have to be deformed excessively. Electrically insulated silicon substrates could be used instead of the ceramics [2]. The monocrystalline silicon is a material with well-defined mechanical parameters. Also the matured processes of microelectronic technology could be utilized to prepare such sensors. Moreover certain special operations like anisotropic etching during the lithography could be used in such a case. It is the aim of present contribution to review the main results of our study of technology and measurement of sensor samples of the latter type. 2 Samples Design and Technology 2.1 Substrates Standard 4-inch diameter silicon wafers (of the thickness 550 μm) polished on one side were used as the starting material. Silicon dioxide (2 μm thick) was prepared by thermal oxidation as the basic insulating layer covered by 75 nm layer of PECVD silicon nitride. Reliability of such electrical insulation of the substrate semiconductor material has proved to be sufficiently high during all the study.

2 Fig. 1. Schematic cross-section of a tensometric structure: 1 silicon substrate, 2 insulating layer, 3- tensometric layer, 4 adhesion layer, 5 gold top layer, 6 region of selectively reduced thickness of the substrate. 2.2 Deposition To ensure good adhesion both tensometric and contact layers were prepared by vacuum sputtering (the magnetron equipment MRC 903M was used). For the tensometric layers three materials promising long-time stability even at elevated temperatures under standard air ambient were chosen: Ni 0,5 Cr 0,5 :N, Ta 2 N and CrSi (cermet). Contact multilayers comprised an adhesion layer adjacent to the substrate surface and covered by gold top layer. A metallic barrier layer (Pd) was optionally used as an interface between them. Two types of contact systems have been tested and found as suitable ones: either Ni/Au (for NiCr based devices mainly) or Ti 0,1 W 0,9 /(Pd)/Au for higher temperature applications. 2.3 Design of Topology The schematic cross-section of a sensor structure illustrated on Fig.1. Both one-dimensional (1D) and twodimensional (2D) types of microsensors have been designed and tested. Some tensometers were incorporated into cantilevers to create bending beams (see Fig.2.). Wheatstone bridge topology was used for the membrane structures (Fig.3.). Si/Si 3 N 4 was used to perform anisotropic etching of silicon. Silicon nitride and silicon dioxide layers deposited also on the rear side of the wafers suited well as the etching masks. The layers have to be annealed after lithography and etching to stabilize their parameters. Extensive annealing tests were performed to find limits of usability of the sensors under study. As a standard annealing the following conditions resulted: 300 C/120m for Ta 2 N, 350 C/120m for NiCr and 550 C/30m for the cermet. 3 Measurement 3.1 Test Chips Sheet resistivity R s of tensometric layers as well as contact edge resistances R ec were measured by Transmission Line Method (TLM) [3], [4] using the test chips incorporated into all the wafers. The same chips were measured several times: both unannealed and after several steps of successive annealing. The chips were mounted into ceramic DIP40 packages for every measurement. The contacts were ultrasonically bonded by gold wires to suppress serial resistances and to avoid any material incompatibility D tenzometric Structures Resistances R T of tensometric resistors, their dependences on temperature and on the chip deformation were measured. The latter was found according to the known deformation of the steel test beam fixed at its one end (near to the position of the resistors of the sensor chip glued onto its surface) and loaded with a mass m applied on its opposite free end. 2.4 Photolitography, etching and annealing Standard photolithography and wet etching of contact and tensometric layers were utilized to prepare the samples. Hot alkaline etchant with optimal selectivity Fig. 2. Two tensometric bridge structures on a silicon wafer prepared to be cut into individual bending beams. Dimensions of the resulting beams: 8 mm x 25 mm, thickness 200 μm Fig. 3. Diced membrane chips: front sides (upper row), rear sides (lower row) see the cross-section on Fig.1. A grid of one milimeter pitch on background.

3 3.3 2D Membrane Sensors Resistances R T of individual resistors of the bridge were measured as well as their dependences on temperature and on the air pressure applied on the front side of the membranes. Those measurements were performed on the chips of the open bridge topology (see Fig.3., upper right sample). For standard applications the closed bridge topology membranes are suitable (the same figure, upper left sample). Fig. 4. A membrane pressure microsensor. The value of the coefficient k of deformation sensitivity (known also as the k-factor or gauge factor) was calculated from the relation: ΔRΤ Δl = k. = k.ε (1) RT l where l and Δ l are the length of the tensometric resistor line and its increment/decrement respectively; ε is the relative deformation caused by the applied force of the load. It holds: ε = 6gLm 2 b h E where L, b and h are the length, width and thickness of the steel test beam respectively; E is the value of Young modulus of the steel material used and g is the gravitational acceleration constant. Temperature dependences of sensor resistances were measured preferrably on free chips (not fixed on a solid surface) to exclude influence of any deformations. (2) 3.4 Measurement Setup The main part of measurement system is data acquisition unit HP 34970A together with 20 channel multiplexer. This configuration allows to measure basic parameters, e.g. individual resistance or bridge output voltage, of plenty samples at the same time. Computer runs measurement routine, controls the climatic chamber, stores and analyzes measured data. 4 Mathematical Simulations 4.1 Software The software package CoventorWare has been used for design of mechanical and thermal characteristics of the structure. The tools enable design, modelling and successive modification of designed MEMS structures. The program enables: drawing of 2D layout and its editing, simulation of production process, generation of 3D model from 2D masks, generation of network by the method of finite elements, solution of mechanical, electrostatic, thermal, piezoresistive, induction, optical, and further simulations. GPIB interface Data Acquisition Unit Climatic Chamber 2x 20ch. MUX S S n Fig. 5. Measurement System Fig. 6. The mesh generated for active tensometric layer of membrane pressure sensor.

4 Cut A Cut B Fig. 7. Stress analysis of square membrane computed by MemMech solver. 4.2 Finite Element Method The first step of any finite element simulation is to discretize the actual geometry of a structure using a collection of finite elements. The number of elements used in a particular mesh is referred to as the mesh density. The next step in physical design is mesh generation. CoventorWare uses finite element and boundary element techniques for solving the differential equations of each physical domain in the problem. The differential equations are solved by descritizing the 3-D model into a mesh, which consists of a number of elements, each with a specified number of nodes. The example of generated mesh is shown at Fig MemMech solver MemMech is CoventorWare s mechanical solver, which computes displacement and stress results. The user applies the boundary conditions set in solver windows, and the solver generates output that may be viewed in a tabular form or rendered in three dimensions over the structure s domain in the Visualization tool. 3D model and stress analysis of square membrane are shown in Fig.7. Fig. 8. Stress analysis of square membrane computed by MemMech solver. Maximal membrane deflection is 2.62 μm, when load 0.3 MPa is applied (Fig.8.). Fig.9. shows stress curve along membrane in two different cuts. Maximal stress is almost 80 MPa in the central cut. 5.2 Deposited Layers and Films The results of measurement of parameters of both asdeposited and annealed layers reviewed in Tab.I. For unannealed samples the sheet resistivity is Rs0 (given in Ohms in the table) and the contact edge resistance is Rec0 (given in Ohms.cm there). Those parameters after standard annealing (its conditions specified in section II- 4 higher) denoted as 1. The parameters after additional annealings (under standard air ambient) labeled as 2, the annealing temperature and time used given in the last column of the table. The results after a specific annealing (under forming-gas Ar+5%H 2 ambient) denoted as 3 there. 5 Results 5.1 Simulation Results Results from mechanical simulations show the stress distribution in Si membrane. Maximum stress is close to fixed edge and around the membrane center. Tensometric elements are located exactly to hi-stress areas to achieve maximum resistivity change (Fig 7.,9.). Fig. 9. Stress analysis of square membrane computed by MemMech solver.

5 Fig. 10. Typical dependence of the resistance R T on the deformation load m (sample Ta 2 N #533, measurement at T = 25 C) D Tensometric Structures A typical dependence of resisitivity of a tensometric structure on its deformation (during the test on the steel test beam) illustrated on Fig.10. The values of k-factors of the Ta 2 N based samples exceeded those of NiCr:N by about on 20%. Their highest values found on NiCr:N tensometric layers were k 3 depending also on the level of nitrogen doping. The values of temperature coefficient (TCR or α) of resistivity were (10 to 30).10-6 deg -1 and about (- 170).10-6 deg -1 for NiCr:N and Ta 2 N layers respectively D Membrane Structures Typical dependences of resisitivity of individual resistors of the bridge on applied pressure (and therefore on membrane deformation) at different ambient temperatures shown on Fig.11. The set of curves representing pressure dependences of the bridge output voltage U B (when the bias U b = 5 Volts was applied) given in the Fig Discussion 6.1 Deposited Layers The basic electrical parameters of the deposited layers depend on their thickness, on technology used and on its material choice. The values of sheet resistivity Rsc (see Tab.I.) are influenced by the conductivity of the top gold layer mainly and they depend on its thickness in the first approximation. A thick Au layer (1.3 μm see the section I-2 higher) was used for TiW/Au contacts, whereas it was sufficient to use the thinnest gold layers for Ni/Au ones. Contact edge resistances were the lowest for Ni/Au contacts on NiCr:N tensometric layers. The results of TLM measurement showed linear characteristics of TiW/Au contacts on Ta 2 N layers. Fig. 11. Typical dependences of resisitivity of individual resistors of the bridge on applied pressure at different ambient temperatures (sample NiCr #03A). Extensive tests have been carried out also on contact systems to cermet layers. Good results were obtained for CrSi-NiCr/Au systems. Their highest operational temperatures (about 300 C) are limited by the contact, however. TLM characteristics of CrSi-TiW/Au systems were nonlinear for unannealed cermet samples. 6.2 Annealed Layers Sheet resistivity Rs changed in a different way for the samples NiCr and Ta 2 N as a result of their parameters stabilization by annealing (labeled as 1 - see Table I). The temperature used for NiCr samples stabilization should not exceed about 350 C, as degradation of their contact resistances was observed yet during 400 C annealing tests. Electrical parameters of Ta 2 N layers are less stable at temperature interval 200 C to 300 C compared to NiCr samples under study. 6.3 Simulations Because of crystallographic anisotropy of selective etching square-shaped membranes had to be prepared. A lower symmetry stress distribution under deformation inside such deformed membranes (illustrated in Fig.7.) rather differs from that for circular membranes. Results of mathematical simulations have been used for the membrane sensors design and interpretation of measurement results. 6.4 Design of Topology Two variants of topology of contact layers have been studied to prepare tensometric structures (see Fig.2.). Either their areas were limited to expanded contacts only (the lower structure on the photograph) or they were used also to electrically short all other parts of the tensometric layer except those of entire longitudinal and transversal lines (the upper structure there). The latter variant may be advantageous (e.g. to ballance individual resistors of the bridge) provided sufficiently high quality contacts are used.

6 TABLE I PARAMETERS OF THE CONTACT AND TENSOMETRIC LAYERS Material sample # contact Rsc Rs0 Rec0 Rs1 Rec1 Rs2 Rec2 Rs3 Rec3 Anneal..2/3 NiCr : N 515-I Ni/Au C/ 2h NiCr : N 515-II Ni/Au C/ 13h NiCr : N 534 Ni/Au pilot sample Ta 2 N 533-I TiW/Au C/ 2h Ta 2 N 533-II TiW/Au C/ 2h Ta 2 N 516-I TiW/Au C/ 11h Ta 2 N 516-II TiW/Au C/ 12h Ta 2 N 5/17X TiW/Au pilot sample 6.5 Operational Temperatures 150 C T 200 C: Sensors based on both Ta 2 N and NiCr layers could be used well for this temperature interval. The former should be perhaps preferred because of slightly higher obtainable values of deformation sensitivity coefficient k. 200 C T 300 C: Those temperatures could be too high to apply sensors using Ta 2 N layers as reliable devices with longer-term stability of their parameters. Structures based on NiCr layers represent a good choice in such a case. T > 300 C: From the three materials studied only cermet layers could be applied as a promising solution. Contact system of good electrical and mechanical parameters and a reliable long-term stability has to be developed first, however. Deposition of the contact layer on the tensometric one without interruption of vacuum used or some kind of chemical activation of the cermet layer surface prior the contact deposition could be tried as possible ways how to solve the problem. Fig. 12. The set of curves representing pressure dependences of a NiCr bridge output voltage U B (when the bias U b = 5 V was applied) at different ambient temperatures (25 C to 200 C). 7 Conclusion Electrically insulated surfaces of silicon wafers are suitable substrates for sputtered tensometric layers. Microsensors both of one-dimensional (bending beams) and two-dimensional (membranes) topology have been designed, successfully prepared and measured. The best deformation sensitivity was achieved on Ta 2 N layers whose highest operational temperatures T are 200 C to 250 C. The layers of NiCr:N are suitable for temperatures T up to 300 C with only slightly reduced sensitivity. Mathematical simulations are useful to facilitate both the sensor structures analysis and their design optimization. Acknowledgement: This research was supported by Grant Agency of the Czech Republic, grant project of Reg. Nr. 102/03/0619. References: [1] M. Husak, P. Kulha, J.Jakovenko, Z.Vyborny, Structures of Cantilever with Implanted Strain Gauge, Proc. 35 th Int. Symp. on Microelectronics IMAPS, Denver, CO, USA, p , Sep [2] P. Kulha, M. Husak, Z.Vyborny, F. Vanek, Study of Piezoresistive Implanted and Thin-Film Layers for Using in Microsensors, Proc. 14 th MicroMechanics Europe Workshop MME2003, Delft, The Netherlands, p.61-64, Nov [3] J. Hampl : Mereni kontaktnich vlastnosti kovovych vrstev na planarnich soucastkach z GaAs, Slaboproudy obzor, No.8, 1987 (in Czech) [4] H.H. Berger: Models for Contacts to Planar Devices, Solid-State Electronics, Vol. 15, pp , 1972 [5] CoventorWare, Users Manual