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Sensors and packages based on LTCC and thick-film technology for severe conditions P. Ryser, Th. Maeder, C. Jacq, Y. Fournier & G. Corradini Laboratoire de Production Microtechnique Ecole Polytechnique Fdrale de Lausanne EPFL - LPM,

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Sensors and packages based on LTCC and thick-film technology for severe conditions

  • P. Ryser, Th. Maeder, C. Jacq, Y. Fournier & G. Corradini

Laboratoire de Production Microtechnique Ecole Polytechnique Fédérale de Lausanne

EPFL - LPM, Station 17, CH-1015 Lausanne, Switzerland thomas.maeder@epfl.ch, lpm.epfl.ch/tf

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Outline

  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

Outline

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  • 1. Introduction
  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

Introduction

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  • 1. Introduction - thick films

Introduction

Thick-film circuit : series of layers

Screen-printing of layers with a mask Direct dispensing (prototypes)

Each layer comes as a paste:

Functional material (as powder) Organic vehicle: binder + solvent

Materials (usually mineral)

Conductors Resistors : mechanical & thermal sensors Dielectrics …and more!

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  • 1. Thick-film process

Introduction

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  • 1. Thick-film process

Introduction

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  • 1. Thick-film process

Introduction

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  • 1. Thick-film process

Introduction

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  • 1. Introduction - LTCC

What is LTCC?

  • LTCC stands for « Low-Temperature Co-fired Ceramic ».
  • It is an evolution of standard thick-film technology.
  • The ceramic is a silicate material + Al2O3 with outstanding

chemical and thermal stability.

How is it made?

  • LTCC comes as unfired « green » sheet (tape) of various

thicknesses (ceramic powder with polymer binder).

  • Each sheet is shaped & screen-printed with conductive,

resistive, or other pastes.

  • Finally, the sheets are pressed & fired together.

Introduction

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  • 1. LTCC application examples

Introduction

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  • 1. 3-D structuration of LTCC

Introduction

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  • 1. LTCC manufacturing process

Introduction

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  • 1. LTCC manufacturing process

Introduction

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  • 1. LTCC manufacturing process

Introduction

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  • 1. LTCC manufacturing process

Introduction

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  • 1. LTCC manufacturing process

Introduction

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  • 1. LTCC advantages

Tfiring < 900°C --> allows use of silver conductors High-density packaging 3-D structuration Hermetic structures Reliable mechanical, thermal and electrical performance High volume, low cost fabrication

Introduction

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  • 1. LTCC vs. alumina for sensors

> Thermal, low-range mechanical sensors

Introduction

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  • 2. Thick-film liquid level sensor
  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

Level sensor

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  • 2. Thick-film liquid level sensor

Level sensor

Features:

  • Absolute piezoresistive pressure

sensor for continuous immersion in water & fluids

  • 3 thick-film circuits

1. Base 2. Membrane 3. Electronics

  • Membrane side in fluid
  • Electronics & cable protected by

epoxy potting compound

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  • 2. Level sensor - module

Level sensor

Membrane Electronics Membrane free-standing to avoid stress induced by packaging

Epoxy potting Enclosure Cable

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  • 2. Level sensor - membrane

Level sensor

Conductive sealing glass Piezoresistors Coarse offset adjustment (Insulating) sealing glass Cut

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  • 2. Level sensor - seal

Level sensor

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Fluid Epoxy potting

  • 2. Level sensor - seal

Level sensor

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  • 2. Level sensor - conclusions

Level sensor

  • Full media separation achieved
  • Compatibility with fuels & mildly aggressive aqueous

solutions

(critical points : sealing glass & potting compound)

  • Absolute sensor for lower cost & increased reliability
  • Reference cavity through hermetic glass seal
  • Batch production processes
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  • 3. Jet engine AMB sensor
  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

Jet engine AMB sensor

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  • 3. Jet engine AMB sensor (L. Burdet)

Jet engine AMB sensor

  • Active Magnetic Bearing (AMB)

XY position sensor for jet engines

  • Magnetic position sensor - eddy

currents in rotor

  • No mechanical bearings (oil,

wear, cooling,…)

  • Increased reliability
  • Decreased maintenance costs
  • Operation @ 550…600°C in

aggressive gases

  • Thick-film circuit
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  • 3. AMB sensor principle

Jet engine AMB sensor

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  • 3. AMB sensor circuit build-up

Jet engine AMB sensor

Inductor build-up Windings Pads

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  • 3. Kirkendall effect @ high temp.

Jet engine AMB sensor

X OK X Ag lines (c1 & c2) X X OK Au lines (c1 & c2) Ag:Pd pads (c3) Ag pads (c3) Au pads (c3)

  • Only (quasi) identical metals may be in contact!
  • Silver selected for cost & high conductivity

Result of long-term stability (>2’000 hours)

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  • 3. Ag electromigration @ high temp.

Jet engine AMB sensor

  • Shorting of windings

after long-term operation at high temperature

  • Degradation only
  • bserved if voltage

applied

  • Creation of Ag

conductive paths identified at pads

  • High-temperature

Ag electromigration!

  • No problems if covered

Pad Pad Short

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  • 3. Ag electromigration - mitigation

Jet engine AMB sensor

  • Need modern migration-

resistant cristallising dielectric!

  • Use of resistive guard

pad or ring to cancel electric field around pad also possible

  • Extensive long-term

qualification of these solutions still needed!

glassy dielectric cristallising diel. Resistive guard pad / ring

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  • 3. Jet engine AMB - conclusions

Jet engine AMB sensor

  • Control of AMB (Active Magnetic Bearing) achieved
  • Operation of thick-film circuit demonstrated @ 600°C !
  • Electromigration of silver & Kirkendall effect problematic
  • Electromigration stopped with
  • dielectric
  • resistor (conductive) guard ring
  • Kirkendall effect avoided by using only pure Ag for

conductors & cables

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  • 4. LTCC hydrostatic high-pressure sensor
  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

LTCC hydrostatic high-pressure sensor

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  • 4. LTCC hydrostatic high-pressure sensor
  • Ceramics: limitations for tensile stresses
  • In principle “unlimited” pressure allowed

LTCC “hydrostatic” sensor concept:

LTCC hydrostatic high-pressure sensor

High pressure Ambient pressure

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  • 4. LTCC hydrostatic sensor module
  • Principle : LTCC “stick”

High pressure (active) resistors Ambient pressure (reference) resistors

  • Circuit: 4 LTCC layers (DP 951 & DP 2041)

Lids Core layers: measurement bridge

  • Resulting module

LTCC hydrostatic high-pressure sensor

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  • 4. Hydrostatic sensor packaging & test jig
  • Epoxy adhesive feed-through
  • Sensor test jig:

Carrier O-ring seal Sensor Tightening nut Complete assembly

LTCC hydrostatic high-pressure sensor

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  • 4. Resistor in hydrostatic pressure

Vertical:

  • E(resistor) 70 GPa

Horizontal

  • E(LTCC) 110 GPa
  • E(Al2O3) 320 GPa

LTCC advantages:

Higher signal More isostatic stress (smaller E difference) Easy hermetic sealing (buried resistors)

LTCC hydrostatic high-pressure sensor

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  • 4. Expected response vs. pressure

Very high achievable signal In theory limited by packaging only (feed-through)

3 mV/V Includes 50% reduction due to half-bridge design

signal = 1 2S 2ES

L + T + T 1

( ) 1+ w ( )

[ ]

w = 1+R 1 R ES ER 1 2R 1 2S 1

  • LTCC : Al2O3

3:2

LTCC hydrostatic high-pressure sensor

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  • 4. Resistor & bridge offset values

(0) 78 ± 33 mV/V 7 ± 21 mV/V Offset (10 k) 3.6 k ± 6% 24 k ± 6% Resistance (nominal) Co-fired

  • pen

Co-fired buried Type

Values affected by interactions with LTCC Offsets OK

LTCC hydrostatic high-pressure sensor

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  • 4. Measured sensor response
  • Linear & hysteresis free behaviour up to 1’200 bar
  • Response corresponds to expected value

Results

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  • 4. Limitations & potential issues
  • Problems with some sensors (sudden drift or death)
  • Cause: voids or local stresses at feed-through ?

LTCC hydrostatic high-pressure sensor

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  • 4. Comparison with other results

Very good match with published results on same resistors

Dziedzic et al., IMAPS-PL conference, 2003 DP 2041 on alumina Calculated

LTCC hydrostatic high-pressure sensor

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  • 4. Hydrostatic sensor - conclusions

Studied “hydrostatic” LTCC high pressure sensor Model for resistors under hydrostatic pressure Sensor successfully tested at > 1’000 bar Some quality issues High response agrees very well with predictions

LTCC hydrostatic high-pressure sensor

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  • 5. LTCC microreactor
  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

LTCC microreactor

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  • 5. LTCC microreactor

Micromixers/reactors

A simple LTCC micromixer

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  • 5. LTCC microreactor
  • Avoid long, narrow & windy cuts!

Micromixers/reactors

Fabrication of complex LTCC fluidics

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  • 5. LTCC microreactor

top bottom Mengeaud, EPFL 2002

  • “Zig-zag” mixer
  • Preserves

integrity of LTCC layers

Micromixers/reactors

Complex mixer by cutting

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  • 5. Chemical microcalorimeter (1/7)

Willigens, 2005

Features:

  • DIL-24 package
  • Global heating track
  • Inlet pre-heating zones

(meanders)

  • Thermally insulated

reaction zone

  • Microcalorimeter with

calibration heater

  • Separate flow sensor for

each inlet

Micromixers/reactors

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  • 5. Chemical microcalorimeter (2/7)

Micromixers/reactors

Fluidic layout Electrical layout

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  • 5. Chemical microcalorimeter (3/7)

Preheating zone Flowmeters Reactor

Fluidic layout

Micromixers/reactors

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  • 5. Chemical microcalorimeter (4/7)

Flowmeters Reactor <- General heating + DIL-28 package

Electrical layout

Micromixers/reactors

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  • 5. Chemical microcalorimeter (5/7)

Micromixers/reactors

Global layers 5 LTCC layers Exit channel Flow meter Pre- heating Reaction zone

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  • 5. Chemical microcalorimeter (6/7)

Micromixers/reactors

Flow meter response

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  • 5. Chemical microcalorimeter (7/7)

HCl + NaOH H2SO4 + NaOH Acid-base calorimetry

Micromixers/reactors

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  • 6. LTCC gas viscosity sensor
  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

LTCC gas viscosity sensor

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  • 6. LTCC gas viscosity sensor
  • Purpose: identification of gas mixtures
  • Simple sensor principle: time relaxation

Viscous gas

LTCC gas viscosity sensor

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  • 6. LTCC gas viscosity sensor
  • Purpose: identification of gas mixtures
  • Simple sensor principle: time relaxation

Viscous gas Thin gas

LTCC gas viscosity sensor

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  • 6. LTCC gas viscosity sensor

LTCC gas viscosity sensor

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  • 6. Gas viscosity sensing principle
  • Pressure differential generated by heating / cooling
  • Pressure measured by membrane sensor
  • Pressure relaxation through meander

LTCC gas viscosity sensor

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  • 6. LTCC for membranes & meanders
  • Small spacings
  • Intricate layout

LTCC gas viscosity sensor

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LTCC sintering Graphite burnout

  • 6. Cavities by graphite fugitive phase
  • Graphite burns
  • ut shortly

before LTCC densifies.

  • Spacing can be

controlled by heating rate.

LTCC gas viscosity sensor

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  • 6. Heater module

LTCC gas viscosity sensor

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  • 6. Heater module & thermal model

LTCC gas viscosity sensor

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  • 6. Heater module characterisation
  • Good correspondance with model
  • Change = increase of air conductivity with temperature
  • Some positive temperature drift at low power

LTCC gas viscosity sensor

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  • 6. Heater module transient behaviour
  • Very fast resistor response time (1 s)
  • Heating faster than cooling due to TCR
  • Additional delayed response due to lid, etc.

LTCC gas viscosity sensor

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  • 6. Membrane-base thermal resistance
  • Thermal resistance depends on gas thermal conductivity

and membrane displacement due to pressure.

  • Relaxation time depends on gas viscosity only.

LTCC gas viscosity sensor

  • Good match between

thermal modelling (dots) & results (plain lines)

  • Nonlinear behaviour over

large displacement

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  • 6. Sensor cycling in air

LTCC gas viscosity sensor

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  • 6. Sensor cycling in air : heat & cool
  • Pressure relaxation time (Wm) 30-50 s

LTCC gas viscosity sensor

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  • 6. Relaxation of pressure
  • Inverse exponential behaviour for small P
  • Good reproductibility (10 cycles)
  • Air vs. helium : viscosity = 8% ; time (sensor) = 9%

LTCC gas viscosity sensor

Heating Cooling

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  • 6. Gas viscosity sensor - conclusions
  • Measurement of viscosity & th. conductivity
  • Viscosity resolution ca. 1-2%
  • Capactive version will be investigated for better

resolution & less thermal drift

LTCC gas viscosity sensor

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  • 7. Conclusions & outlook
  • 1. Introduction - thick-film technology & LTCC
  • 2. Liquid level sensor - “flip-chip” thick-film technology
  • 3. Jet engine AMB sensor - thick-film @ high temperature
  • 4. Hydrostatic high-pressure sensor - immersed LTCC
  • 5. Chemical liquid microreactor - LTCC fluidic modules
  • 6. Gas viscosity sensor - LTCC membranes & -channels
  • 7. Conclusions & outlook

Conclusions & outlook

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  • 7. Conclusions & outlook
  • Proven materials (automotive, aerospace & telecom)
  • Very good thermal & chemical stability
  • For both sensor & packaging application
  • Many promising development areas:
  • MEMS packaging & interconnects @ high temperature
  • Interconnects for high-temperature (sensor) electronics
  • LTCC structuration for micromechanical devices
  • LTCC fluidics

Conclusions & outlook

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Merci

Thank you !

Conclusions & outlook