FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect MEMS Packaging
Lecture topics Introduction to MEMS packaging concerns: Why MEMS packaging are more challenging than IC packaging Case example: MEMS for petroleum flow measurements FYS4260/FYS9260 2
From another lecture on MEMS packaging: How realistic is this simplified picture on MEMS packaging? DISCUSS! FYS4260/FYS9260 Frode Strisland 3
Realistic MEMS packages usually include: A sensor or actuator interface to the world outside Readout electronics (dedicated ASIC) Careful mechanical design to avoid package stress affecting MEMS elements Must combine MEMS specific features with footprint compatible with circuit boards FYS4260/FYS9260 Frode Strisland 4
MEMS are not ICs! Parallels to IC production are misleading and can be dangerously naive IC development times ~ 12-18 months Well established IC processes, design rules and simulation tools Competitive wafer costs Standardized packaging MEMS design-to-product time lines ~ 3-5 years Processes less standardized Packaging challenges are hughe, and solutions are considered a proprietary advantage Packaging cost can exceed 50% or unit cost FYS4260/FYS9260 Frode Strisland 5
Example: Generic pressure sensor package FYS4260/FYS9260 Frode Strisland 6
MEMS specific packaging issues Some systems must be open towards the environment Pressure sensors, microphones, loudspeaker Chemical and fluidic elements Others need to be hermetically sealed, sometimes with a reference gas and/or an anti stiction agent Accelerometers/gyroscopes Absolute pressure sensors (reference cavity) Oscillators Delicate structures MEMS sensors can sense package induced stresses, which can cause zero offset, drift and nonlinear behaviour FYS4260/FYS9260 Frode Strisland 7
MEMS packaging: No single solution possible FYS4260/FYS9260 Frode Strisland 8
Main package technologies for MEMS Plastic Metal Ceramic FYS4260/FYS9260 Frode Strisland 9
Metal package Metal package is robust and easy to assemble good choice for prototypes Standard TO-type (transistor outline) Can only accommodate fewer than 10 pins. TO-type package remain in use in a few applications Metal packages are attractive to MEMS, especially for microfluidic devices A metal hermetic package is often made of ASTM F-15 (Kovar ), stainless steel is also common Holes are punched, either through the bottom for plug-in packages, or sides for flat packages Metal leads are placed through the holes and beads of borosilicate glass Sensiron STD 60A series pressure sensors FYS4260/FYS9260 Frode Strisland 10
Molded plastic package Not hermetic Two general approaches postmolding premolding In postmolding, the plastic housing is molded after the die is attached top a lead frame (a supporting metal sheet). The process subjects the die and the wire bonds to the harsh molding environment. In premolding, the die is attached to a lead frame over which plastic was previously molded. It is attractive in situations where the risk of damaging the die is high, or openings through the plastic are necessary. More expensive. FYS4260/FYS9260 Frode Strisland 11
Ceramic package Wire bonding establish electrical connectivity between the die and the metal traces on the ceramic header A Cu-Al brazed Kovar seal ring Transparent window consists of a polished Corning 7056 glass fused to a stamped goldnickel-plated Kovar frame Resistance seam welding of the seal ring on the ceramic base to the Kovar glass frame Zeolite getters to ensure longterm desiccation FYS4260/FYS9260 Frode Strisland 12
MEMS vs IC design process IC: Design IC, offer IC in a range of packages MEMS: Co-design MEMS element, readout electronics and package, only offer a dedicated packaged product FYS4260/FYS9260 Frode Strisland 13
Wafer level packaging Wafer-level packaging (WLP) is the technology of packaging an integrated device while still part of the wafer. From a MEMS perspective, it is a question about which functions can be integrated on a wafer level, and what has to be done on a dice level. Can for example a vacuum cavity or smart connector solution for monolithic integration of MEMS and ASIC be realized on a wafer level in order to simplify subsequent packaging? FYS4260/FYS9260 Frode Strisland 14
Wafer bonding: Joining wafers Anodic Bonding Direct Wafer Bonding Fusion Bonding Plasma activated bonding Metallurgical methods for bonding
Anodic bonding Anodic bonding is a wafer bonding process to hermetically seal glass to either silicon or metal without introducing an intermediate layer. This bonding technique is also known as field assisted bonding or electrostatic sealing Example of chip made from glass-silicon-glass anodic bonding FYS4260/FYS9260 Frode Strisland 16
Anodic bonding geometry In anodic bonding, an electrical field is applied to cause ion drift in the (borosilicate) glass which in turn causes bonding Scheme of anodic bonding procedure. The top tool works as a cathode and the chunk as anode. The process parameter are bond voltage U B, current limitation I B and bond temperature T B. Source: http://en.wikipedia.org/wiki/anodic_bonding FYS4260/FYS9260 Frode Strisland 17
Anodic bonding process Ion drifting in bond glass influenced by electrostatic field.[8] (1) Formation of depletion zone (gray) through Na+ drifting. (2) Drift of O ions in the depletion zone. Source: http://en.wikipedia.org/wiki/anodic_bonding FYS4260/FYS9260 Frode Strisland 18
Direct wafer bonding: (Fusion bonding) Direct bonding describes a wafer bonding process without any additional intermediate layers. The bonding process is based on chemical bonds between two surfaces. Important that joined surfaces have low roughness level (at atomical levels e.g. 3-5 Å) and are planar FYS4260/FYS9260 Frode Strisland 19
Metallurgical approaches to wafer bonding Soldering/bracing approaches can be used to join wafers. The most common example is the use of deposited gold on wafers, following by an annealing step. The minimum in the liquidus line in the Au-Si phase diagram is at 363 C for 19 at % Si. FYS4260/FYS9260 Frode Strisland 20
Advanced packaging: 3D packaging with thru silicon vias TSVs used by stacked DRAM-dice in combination with a High Bandwidth Memory interface. Courtesy Shmuel Csaba Otto Traian under CC-BY-SA4.0 FYS4260/FYS9260 Frode Strisland 21
Advanced packaging: Highly integrated MEMS components 9 axis Inertial Measurement Unit from InvenSense with digital output 3 axis accelerometer 3 axis gyroscope 3 axis magnetometer Signal processing Control of package stresses very challenging in small dimensions FYS4260/FYS9260 Frode Strisland 22
Advanced packaging: Monolithic integration Monolithic 3D ICs are built in layers on a single semiconductor wafer, which is then diced into 3D ICs. There is only one substrate, hence no need for aligning, thinning, bonding, or through-silicon vias. Cross section of a pixel consisting of CMOS backplane and MEMS mirror (16 µm pitch). From: http://www.ipms.fraunhofer.de/en/memsfoundry/micromechanics/technology-modules/monolithic_integration.html FYS4260/FYS9260 Frode Strisland 23
Intro to case: Downhole petroleum flow measurements Motivation: Know and control petroleum flow from individual zones in branched oil wells FYS4260/FYS9260 Frode Strisland 24
Application specific constraints High temperature (175 deg C) High pressure (up to 1000 bar) Hostile environment (highly corrosive) Inaccessible Relatively high cost acceptable FYS4260/FYS9260 Frode Strisland 25
How can we measure fluid flow in a pipe? For example: Rotary sensors (measure propeller spin frequency) Heat transfer sensors (thermal mass flow sensor) Velocity measurements, e.g. by using Dopper-based methods. Pressure change through a flow constriction (venturi) FYS4260/FYS9260 Frode Strisland 26
Bernoulli flow sensor Bernoulli equation for incompressible flow: p V 1 1 + 1 2 1 2 1 ρ V v = p V + 2 2 1 2 ρv v 2 2 2 p 1 p 2 A = 1v1 A2v2 In constrictions, the speed goes up, and the pressure goes down. p = 1 2 A1 2 1 1 (1 2 p p = ρv A2 2 ) FYS4260/FYS9260 Frode Strisland 27
Measurement geometry dp Pressure inlets FYS4260/FYS9260 Frode Strisland 28
Sensing principle for MEMS element? Piezoresitive? Capacitive? FYS4260/FYS9260 Frode Strisland 29
30 Recall: What is piezoresistivity? Some materials change electrical conductivigy when stretched due to change in shape (metals) and/or deformations in their crystal lattice Silicon has a large resistance change as a function of mechanical tension The resistance change is often measured using a Wheastone bridge configuration
Piezoresistive appraoch Torsion sensitive axis FYS4260/FYS9260 Frode Strisland 31
Silicon element principle used WHY? FYS4260/FYS9260 Frode Strisland 32
Dicsussion points: Can you see what type of microstructuring technology has been used? Where are the pressure ports HTASIC Is a reference capacitor actually needed? Can you see what type of wafer bonding technique is used? FYS4260/FYS9260 Frode Strisland 33
Silicon element principle Could we drop a silicon reference capacitor and do as follows? Yes! Would give a smaller sensor element. However, would need: External reference to compare with (which would then not be affected by changes in the same way as te measurement membrane) Package stress more likely to affect package FYS4260/FYS9260 Frode Strisland 34
Sensor terms Accuracy (nøyaktighet): How close a reading is to a standardized reference Sensitivity (følsomhet): What are the smallest changes possible to detect? Zero-point offset: What is the reading when it was supposed to be zero signal Drift: Change with time or other parameter Hysteresis: Change in reading between + - and - + Linearity: To what extent the sensor output is a constant times the measured parameter FYS4260/FYS9260 Frode Strisland 35
How do we connect pressure connectors to the silicon element? A tube! What material, and how can we attach it? FYS4260/FYS9260 Frode Strisland 36
"Plumbing": Attach a tube to one pressure inlet Adhesion layer, for example NiCr High temperature solder, for example 95Pb5Sn Adhesion layer Kovar (low CTE nickel iron alloy) tube FYS4260/FYS9260 Frode Strisland 37
The realized element connected to a Kovar tube And now how can we connect to this electrically? FYS4260/FYS9260 Frode Strisland 38
Electrical connection: Wirebonding What wire metal should we use? (Pads are in aluminium) FYS4260/FYS9260 Frode Strisland 39
Wirebonding wire: Aluminium. But decision also depends on terminals outside the element The sensor element as we see it will be exposed to several hundred bars. Electronics should not be exposed to more than a few bar. How can we work around this? FYS4260/FYS9260 Frode Strisland 40
We need an electrical feed-thru! Steel Glass Plated Kovar Low pressure electronics compartment The sensor element will be in an environment of ~700 bar, hot petroleum and saline water. Is that ok? FYS4260/FYS9260 Frode Strisland 41
No! We need to protect the sensor element from the hostile environments The sensor element can be exposed to silicone oil, which is relatively incompressible and does not affect electronics. Still, we need separation membranes FYS4260/FYS9260 Frode Strisland 42
The solution Feedthru Sensor element Kovar tube Separation membranes Can you figure out why the two membranes are of different size? FYS4260/FYS9260 Frode Strisland 43
Why membranes need to be different size: Thermal expansion effects CTE of steel ~ 10 ppm/k CTE of silicone oil ~ 100 ppm/k Want to allow membranes to be slightly bent outwards all the time. Why should the membranes be bent outward and not inward? "Large" volume "Small" volume FYS4260/FYS9260 Frode Strisland 44
Measurement results 5 Sensititity better than 1 mbar Temperature drift less than <2.5% FSO Common mode pressure drift < 2.5 % FSO Can withstand more than 15 bar static differential pressure Ratio C mem / Cref [FSO] Ratio Cmem/Cref [%FSO] 2,5 0-2,5-5 0 50 100 150 200 Temperature [C] 5 2,5 0-2,5-5 0 200 400 600 800 1000 Absolute pressure, [Bar]
Test of differential pressure Measured differential pressure Dp [mbar] 450 400 350 300 y = 1,1027x - 79,811 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 450 500 Estimated differential pressure FYS4260/FYS9260 Frode Strisland 46
Final implementation Several primary sensor collected in a commen housing: Absolute pressure Differential pressure Temperature Capacitance Acoustic sensor All signals transferred on a common "local bus".
Learning points MEMS packaging often more demanding than IC packaging: IC packaging standardized vs MEMS packaging solution customised and can be a competitive advantage Development of MEMS element, readout electronics and packaging cannot be done independently Numerous examples of fancy MEMS elements that turned out to be impossible to package Knowledge on MEMS packaging is valuable: The cost of packaging often much more than the sensor element cost FYS4260/FYS9260 Frode Strisland 48
END OF LECTURE Any questions? This presentation is made for FYS4260/FYS9260 teaching purposes, and is not intended for publication elsewhere.