1 Pressure sensors Flashcards
Pressure sensors introduction
Transducers for pressure are among the oldest to be manufactured in Silicon and are devices for which the demand continues to be very high in a very spread range of applications. Specific designs and mounting arrangements vary considerably, ranging from the small, sensitive, catheter-tip transducers used within the heart to the larger, rugged devices needed for industrial process control. Two approaches have come to dominate development in silicon and related materials: PIEZORESTISTIVE and CAPACITIVE.
Difference between piezorestistive and capacitive pressure sensors.
The PIEZORESISTIVE have been the traditional basis for pressure transducers, offering:
-small size,
-excellent linearity over a wide dynamic range,
-moderately high pressure sensitivity and
-relative freedom from hysteresis.
-They measure material stress (translate material stress into an electrical parameter).
The CAPACITIVE ones have been shown to offer:
-higher pressure sensitivity than piezoresistive ones,
-lower temperature sensitivity
-however they require a larger die area;
-are more nonlinear and
-require more sophisticated sensing circuitry;
-They measure an average deflection
Piezoresistive pressure sensor: structure and how it works
STRUCTURE: It consists of a thin silicon diaphragm supported by a thick silicon “frame”, which serves to clamp the edges of the diaphragm. To increase the stiffness of the frame and isolate the diaphragm from stresses coming from the adhesive attaching the sensor to its package, a glass or silicon die is bonded to the back side of the silicon sensor chip. This bonding process is tipically performed at the wafer level before the sensor chips are sawn apart.
OPERATION: When a differential pressure is applied across the square silicon diaphragm, it deflects resulting in a symetric stress distribution with maximum values at the center of each edge of the diaphragm. By using resistors oriented parallel as well as normal to the diaphragm edges, both increasing and decreasing resistance with pressure can be obtained. By connecting opposite-responding resistors in opposing legs of a Wheatstone bridge, full bridge sensitivity can be obtained.
How to integrate a piezoresistor in a device?
Silicon is a piezoresistive material (monocrystalline has a better piezoresistive coefficient than poly).
Locally doping Si with a dopant type opposite to the substrate: generate a p-n junction that will guarantee isolation with proper reverse biasing.
An alternative solution is deposit the piezoresistive material on top of Silicon or on top of the substrate.
Second layout of the piezoresistive pressure sensor to maximize the sensitivity
With pressure applied to the upper surface of the diaphragm, the resulting stress varies from tensile to compressive along a path from the edges towards the center of the diaphragm.
If resistors are all placed perpendicular to the strain field, but a couple is kept under a tensile stress (close to the edges) and the second is kept under compressive stress (close to the center), and the two couple are placed at the opposing legs of a Wheatstone bridge, full bridge sensitivity is achieved.
Two layouts of the piezoresistive pressure sensor to achieve maximum sensibility
1)By using resistors oriented parallel as well as normal to the diaphragm edges, both increasing and decreasing resistance with pressure can be obtained. By connecting opposite-responding resistors in opposing legs of a Wheatstone bridge, full bridge sensitivity can be obtained.
2) If resistors are all placed perpendicular to the strain field, but a couple is kept under a tensile stress (close to the edges) and the second is kept under compressive stress (close to the center), and the two couple are placed at the opposing legs of a Wheatstone bridge, full bridge sensitivity is achieved.
DIFFERENCES BETWEEN THE TWO: In the second not all 4 resistances are close to the edge of the membrane and all 4 resitances are parallel to the stress direction.
Masks in a piezoresistive pressure sensor
The minimal set is 5 masks: 4 on the top side, one on the back side.
1: litographic mask for patterning of the piezoresistor
2: 8 small holes (2 for each resistance): they are vias to allow interconnection
Auxiliary features: alignment marks, sequence control, scribe lines.
Pressure sensors process
1) starting material psub (100) with n+ epitaxial layer.
2) fabrication of piezoresistor by doping: clean surface, ox (generate SiO2 layer for hard mask, spin photoresist, expose, develop, BOE etch, implant/diffuse). MASK 1
3) deposit first dielectric layer and vias: PECVD oxide+ spin PR+ BOE etch, PR strip. MASK 2
4) Generate metallic interconnection: deposit and pattern metal layer (Al+ Cu). MASK 3
5) Lift-off
6) Passivation: PSG deposition
7) Mask 4, PR, expose, develop, etch
8) MASK 5
9) Prepare pyreex glass (relative pressure sensors drilled holes)
10) Wafer bonding frit glass, wafer alignment
11) Electric testint and die marking for successive removal
12) Protect membrane and holes
13) Dice (saw)
14) SIngle die attach on package
15) Wire bond
16) Cap seal
17) Final test
18) Mark
19) Pack for shipment
Capacitive pressure sensors
A capacitive pressure transducer converts the diaphragm deformation, corresponding to pressure,into a change of capacitance, then into output electrical signals such as changes of oscillating frequency, time, charge and voltage.
The change of capacitance DeltaC/C is not linear with respect to deformation or pressure, but the relationship is reproducible. The structure is relatively simple and can be fabricated with conventional micromachining techniques.
The disadvantage is that the capacitance is small (generally 1-3 pF), so the measurement circuit has to be integrated on the chip or specially designed to null the stray capacitance; otherwise the
parasitic capacitance associated with lead wires and bonding pads will generate significant.
interference and noise.
Effects of temperature
The sensitivity of piezoresistors decreases as temperature
increases (due to the injection of intrinsic charge carriers): high doping levels in the resistors can be used to minimize their
temperature coefficients, but with a corresponding decrease in the piezoresistive coefficient.
Any residual stresses due to to the bonding of the constraint layer
or the die attach material which holds the chip to the package may also contribute to temperature-dependent changes in sensitivity and zero-pressure output from the sensor.
Effects of non linearity on capacitive pressure sensors
The non-linearity of response to applied pressure depends upon factors such as the location of the resistors in the strain field and the deflection of the diaphragm.
As the deflection increases towards 10% of the diaphragm thickness, the nonlinearity of the resulting strain increases as the diaphragm moves from the linear plate bending mode to nonlinear membrane deflection.
Center-boss application in capacitive pressure sensors
To achieve high sensitivity and linearity in a wide operation range in a micromachined pressure sensor, one possible design is to use an ultrathin
diaphragm with a CENTER BOSS.
This approach has a couple of advantages:
• an ultrathin diaphragm provides very high sensitivity
• a center boss yields improved linearity (a nearly parallel plate-capacitor is
obtained using the center boss).
The linearity of the load-deflection characteristic improves as the boss
thickness increases. The improvement becomes significant when the boss
thickness is about ten times the diaphragm thickness, because with this
thickness ratio the center boss can deflect uniformly under load.
There is a trade-off in selecting the best solidity ratio ( area,diameter of width of center boss/tot area,diameter of membrane) because it affects both the operating range and the sensitivity. If too small, few benefits derive, while too a high solidity ratio can make the diaphragm too stiff, degrading the sensitivity.
Resonant pressure sensors
Improvement in precision silicon micromachining technology during the past decade have led to a wide range of improved sensing structures, including the use of resonant microbeams as strain transducers.
With resonant sensors, the output is in the form of a robust sinusoidal signal whose
frequency changes with pressure. The output is quasi-digital in the sense that it can be easily converted to digital form using a high-frequency clock.
Vacuum-encapsulated resonant pressure sensors have been realized eliminating the interference from environmental fluids. If a resonator is directly exposed to fluid, the viscosity of the fluid reduces the quality factor of the resonator, and the density change of the fluid alters the resonant frequency of the resonator. The accuracy of a resonant sensor is proportional to the quality factor of the resonator.
The resonator consists of H-shaped dual bridges that are connected to each other at their center points and are vacuum sealed into the same H-shaped microcavity on the surface of the diaphragm. The resonator vibrates in a vacuum environment independent of the external fluid.
The resonant frequency here is modulated by stress induced on the diaphragm by pressure.
The small dimensions of the resonator and the microcavity increase the stress sensitivity to deformation of the diaphragm with applied pressure.
The sensor is formed from singlecrystal silicon. The resonator is heavily doped p-type silicon, and the other areas are n-type silicon.
A second layout is a lateral microcantilever that is facing a fixed electrode. We apply both DC and alternated voltage and generate a variable electrostatic force to make the cantilever oscillate. With a variable electrode in front of the fixed one we have a variable capacitance: we measure a current due to variations of capacitance and voltage. The advantage is that it is a real absolute pressure sensor (not deltaP but P!) and does not require vacuum.
Future trend in pressure-sensing industry
• increased integration
• the development of higher operating temperature sensors (SiC-based for example)
• extended operating range of existing sensors
• improved accuracy and resolution
• more complex structures utilizing micromachining technology
• cost reduction through process improvements and die size reduction
• improved packaging for smaller size, surface-mounted assembly and improved media compatibility
(especially biocompatibility)
High temperature pressure sensors
There is an increasing need for sensors and electronic devices at temperatures beyond 125°C (typical applications are found in the automotive industry, in avionics and space
exploration, as well as in the oil drilling industry, industrial measurement and control systems).
Several approaches have been made to solve the well-known problem of the rising leakage current at high temperatures in conventional silicon devices using isolation by reverse biased pn-junctions. One possible solution is based on SOI wafers (< 250°C).
A promising attempt for high temperature applications is the use of a wide bandgap semiconductor such as silicon carbide (SiC). The excellent electrical and mechanical properties at temperatures far beyond 350°C make it suitable for use as an electromechanical sensor for high temperature applications. However, the rather expensive process technology (and same raw material) is still a drawback for many commercial applications.
Cubic silicon carbide (Beta-SiC or 3C-SiC), heteroepitaxially grown on silicon, gives a
material that combines the high temperature capabilities of SiC with the micromachining possibilities of Si. However, the SiC/Si system shows a major problem: at temperature higher than approximately 250°C the SiC/Si heterojunction starts leaking, which results in a current flow through the silicon substrate.
One approach to get rid of this problem is the deposition of thin Beta-SiC films on SOI substrates: SICOI
By fabricating the transducer elements, in this case the piezoresistors, from Beta-SiC and doing the micromachining with Si, the piezoresistive pressure sensor for high operating temperatures can be produced and standard silicon bulk micromachining instead of the problematic and not yet sophisticated bulk micromachining technology of SiC can be used.
