This is a reverse engineering report of the STMicroelectronics MP34DT01 omnidirectional digital microphone. Details include a full description, tear down analysis and 3D model of the MEMS microphone with cross-sections and SEM images. The reports also includes a full review of the packaging strategy and a description of the sensor assembly process. Furthermore the report has 40 descriptive images, background on the application, performance specifications, interconnect strategies, materials used, EMC strategy description, an electrical schematic, chip attachment means, strengths and weaknesses of the design and links to the patent, data sheets and more.
The report is extremely useful for engineers and business leaders looking to better understand MEMS microphone design, packaging and assemblies processes. It is also beneficial within the MEMS microphone community as a competitive analysis tool.
2. 2
Reverse Engineering of STMicroelectronics MEMS
Microphone
Introduction:
This is a reverse engineering report of the STMicroelectronics MP34DT01 omnidirectional
digital microphone. Details include a full description, tear down analysis and 3D model of
the MEMS microphone with cross-sections and SEM images. The reports also includes a
full review of the packaging strategy and a description of the sensor assembly process.
Furthermore the report has 40 descriptive images, background on the application,
performance specifications, interconnect strategies, materials used, EMC strategy
description, an electrical schematic, chip attachment means, strengths and weaknesses
of the design and links to the patent, data sheets and more.
The report is extremely useful for engineers and business leaders looking to better
understand MEMS microphone design, packaging and assemblies processes. It is also
beneficial within the MEMS microphone community as a competitive analysis tool.
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
3. Table of Contents
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Application
Microphone Description and Key Manufacturer Specifications
Description of Package Design and Function
Description of Package Assembly Process
MEMS Microphone Design
Signal Conditioning Strategy
Electrical Schematic
Review of Product’s Strengths and Weaknesses
Summary of Workmanship Quality
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
5. Application
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Digital, omnidirectional microphones
for mobile phones and tablets,
portable computers and media
players, VoIP, speech recognition,
cameras and video equipment,
gaming equipment and antitheft
systems where limited height and size
is critical and a top acoustical port is
required
STMicroelectronics provides a MEMS
accelerometer and gyroscope for the
Apple iPad but has yet to win a
microphone socket with Apple
STMicroelectronics provides MEMS
microphones for Nokia mobile
phones, HP and Asustek Computer
laptops with 15 million units shipped
in 2011 (Bouchaud 2012)
Apple iPad
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
6. 6
Microphone Description and Key Manufacturer
Specifications
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STMicroelectronics’ omnidirectional, digital MEMS microphone MP34DT01
– Capacitive sensing element and discrete IC for signal conditioning
– Holed cap land grid array (HCLGA) plastic package (3 x 4 x 1 mm) top port
design
– EMI shielded
– SMD and RoHS / ECOPACK compliant
– Pulse Density Modulation (PDM) data output
– Sensitivity: -29 to -23 dBFS (decibels with reference to full-scale digital output)
– Acoustic Overload Point (AOP): 120 dBSPL (Sound Pressure Level)
– Signal-to-noise Ratio: 63 dB (A-weighted at 1 KHz, 1 Pa)
– Power Supply Rejection: -70 dBFS
– Supply Voltage: 1.64 to 3.60 VDC
– Operating Temperature: -40 to 85°C (storage: -40 to 125°C)
– Current Draw: 0.6 mA (typical, short circuit 10 mA max)
– Turn-on Time: 10 msec
Adobe Acrobat
Document
– See technical data sheet for more product specifications
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
7. Description of Package Design and Function
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Holed cap land grid array (HCLGA) package (3 x 4 x 1 mm)
Adobe Acrobat Adobe Acrobat
Top port design with plastic package specifically Bismaleimide Triazine (BT)
Document
Document
Information regarding the design and process are provided in US Patent Applications
US 2012/0153771A1 and 2011/0266640A1 (see pdf’s above)
Bottom
Isometric View
Top Isometric View
Acoustic
Port
BT
Adhesive Seal
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
G5 X G5
Pin 1
Indicator
Metallization
For Mounting
And Electrical
Interconnect
8. Description of Package Design and Function
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Isometric View of Microphone Assembly without Bottom Plate
Microphone Assembly
Bottom Plate
Solder Pads
Solder
Interconnects
MEMS
Microphone
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
EMI Shield
ASIC
9. Wirebond Pads from ASIC to Package
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Gold Wirebonds
Wirebonds start at ASIC
And Continue to Lands
(Next to Solder on
Package Sidewall)
Wirebond
Broke
When
Opening
Wirebond Pad
Lands (Next
to Solder)
ASIC
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
Microphone
10. Gold Wirebonds from MEMS Microphone to ASIC
10
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11. Wirebond Ball Bond on ASIC
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ASIC Circuitry.
BT Fragments
From Opening
Microphone Asm.
20 micron OD
Gold Wire Used
Gold Ball.
Wirebond Pad
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12. Stitch Bond Over Gold Ball on MEMS Microphone
12
Stitch Bond.
MEMS
Microphone
Back Plate
BT Fragments
From Opening
Microphone Asm.
Gold Ball.
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13. Cross Section of Microphone Assembly
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Copper Via
Lead-free Solder
Interconnects
Electrical
Isolation Groves
Back
Chamber
Adhesive
Seal
Bottom Plate
Side
Walls
ASIC
First
Metallization
Layer
Second
Metallization
Layer EMI Shield
Front
Chamber
MEMS
Microphone
Acoustic Port (0.4 mm diameter)
Top Plate
Soft Die
Attach
Thin Metal Film
Seed Layer
Note: Second metallization layer is also solder and wirebond pads
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
14. Description of Microphone Assembly
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BT is used for the external packaging as it is easily routed to desired size and
geometry using high volume, low cost processes.
The first and second metallization layers create a grounded EMI shield, minimizing
the effect of electromagnetic disturbances, that surrounds the majority of the
package. Only the area between the side wall and bottom plate (area of adhesive
seal) does not have the EMI shield present (vertically).
The acoustic port leads to the front chamber of the microphone. Front chamber
geometry optimization provides improved sensitivity to the acoustic sensor. The back
chamber is above the microphone in the cross section image and fills the reminder of
the package providing negligible resistance to the air flowing through the holes in the
back electrode / plate of the MEMS microphone.
Both the ASIC and MEMS microphone are attached using a soft adhesive.
An adhesive seal located between the bottom plate and package side walls
surrounds the periphery of the device and seals it from the environment. The seal
may flow into the electrical isolation groves.
Lead-free solder is used to make electrical interconnect between the package and the
bottom plate.
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
15. Description of Package Assembly Process
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The first metallization layer is applied to the top plate and core plate that will
eventually become the package side walls
An adhesive layer is applied to one side of the core plate over the first metallization
layer
First Metallization Layers
Top Plate Assembly
Core Plate Assembly
Adhesive Layer
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
16. Description of Package Assembly Process
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The top and core plates are bonded
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A blind hole is cut forming side walls up to the first metallization layer on the top plate
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
17. Description of Package Assembly Process
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Seed and second metallization layers added
Seed Layer
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Second Metallization Layer
Acoustic port fabricated
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
18. Description of Package Assembly Process
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Isolation groves fabricated using a diamond-saw cutting tool
Package cleaned
Isolation Groves
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19. Description of Package Assembly Process
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MEMS Microphone and ASIC attached to the top plate with soft adhesive
Chips wirebonded to each other and the package (not shown)
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20. Description of Package Assembly Process
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Adhesive dispensed
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21. Description of Package Assembly Process
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Bottom plate is assembled to the package with lead-free solder to electrically couple
the package to the bottom plate and form an environmental seal
Completed devices are diced (all preceding process steps are in panel form)
Dicing Line
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
Dicing Line
22. MEMS Microphone Description and Function
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STMicroelectronics collaborates with and sources their microphones from Omron
Corporation
The MEMS microphone is a capacitive based acoustic
sensor with a polysilicon membrane that flexes with applied
acoustic pressure from sound. The diaphragm is anchored
to the handle wafer with silicon dioxide posts at the four
corners and freely suspended else where around its
circumference.
A second fixed electrode (also polysilicon) completes the
capacitor and is housed in a thicker silicon nitride back
plate. Both the rigid electrode and the back plate are
proliferated with holes to allow air flow to minimize the
acoustic impedance as the distance between the plates
oscillates. The holes also provide the etchant access to
remove the sacrificial layers during assembly. Acoustic
hole OD is ~18 micons.
A barometric vent is created between the diaphragm,
handle wafer and silicon nitride back plate around the
diaphragms edges. This equalizes changes of ambient
pressure on each side of the diaphragm.
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
23. MEMS Microphone Description and Function
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The front chamber’s double angled geometry is fabricated using a TMAH wet etch
process. This enables a larger chamber to be formed than a straight angled design
seen in wet etched pressure sensors.
The wirebond attachment on the MEMS microphone is a gold stitch on a gold ball.
This was likely done to create a more robust joint. Depending on the metallization
layers making up the wirebond pad, STMicroelectronics may have had some trouble
with the robustness of using a stitch bond alone. Gold ball bonds have better
adherence to substrates than stitches and the gold ball provides a thick, robust
substrate for the gold stitch forming a stronger bond.
Mechanical stops protrude from the silicon nitride back plate towards the diaphragm
to prevent the diaphragm from making contact to the fixed electrode. This serves two
purposes; it prevents 1) the diaphragm from sticking to (not able to return to normal
position) the fixed electrode when excess voltage is applied due to electrostatic
attractive forces and 2) a similar sticking phenomena due to surface tension when
moisture enters the cavity and the electrodes come in contact.
A gold guard ring appears to be used to minimize the effects of the non uniform
electric field at the capacitors edges thus improving its performance.
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
24. MEMS Microphone Images and Description
24
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25. MEMS Microphone Images and Description
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Anchor Point
of Diaphragm
MEMS
Microphone
Minor Damage
From Cleaning
Fixed Back Plate
(see through)
Guard Ring
Wirebonds
Polysilicon Electrode
In Back Plate
End of Polysilicon
Electrode in Back Plate
Diaphragm
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
26. MEMS Microphone Images and Description
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Back Plate
Front Chamber
Cross Section
of Microphone
EMI Shield
Soft Die Attach
BT Substrate
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27. MEMS Microphone Images and Description
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Acoustic Holes
In Back Plate
Mechanical Stop
to Prevent
Electrode Contact
Minor Debris
from Opening
Assembly
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28. MEMS Microphone Images and Description
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Close Up of
Microphone Corner
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29. MEMS Microphone Images and Description
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Flexible
Electrode
Fixed Electrode
and Back Plate
Electrode
Ends Here
Electrode Interconnect
To Wirebond Pad
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
30. MEMS Microphone Images and Description
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Solid Model of MEMS Microphone
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31. MEMS Microphone Images and Description
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Cross Section Showing Diaphragm, Anchors and Vent
A
Front Chamber
Section A - A
A
Acoustic Holes
Fixed Electrode
Barometric Vent (Green)
Flexible Electrode
SiO2
Anchor
Silicon Nitride
Back Plate
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
SiO2
32. MEMS Microphone Images and Description
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Enlarged Image of Cross Section
Flexible Electrode
SiO2
Anchor
Silicon Nitride
Back Plate
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
33. 33
Confirmation that Silicon Nitride Was Used for Back
Plate
Note: Similar analysis was done for several
materials in the design but is not shown.
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
34. Signal Conditioning Strategy
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Discrete ASIC
provides signal
conditioning
Pulse Density
Modulation (PDM)
data output
No glob top present
Bare die used
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
36. Review of Product’s Strengths and Weaknesses
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Strengths
– Packaging fabrication approach extremely flexible to accommodate changing
designs with minimal tooling cost while incorporating a robust EMI shield
– Uses high volume, low cost processes for fabrication
– Small envelope with room to go smaller (but not the smallest on the market)
– Eliminates the need to attach wirebonds below the ASIC’s top surface where
room is limited
Weaknesses
– EMI shield is not present around solder joints but there is an alternate
configuration in the patent that addresses this issue (should it be a problem)
– Wirebond pads and solder joints are in close proximity on same land and care
must be taken to ensure wirebonds are not compromised
– Significantly more process steps than traditional metal can approach and hence
has the potential for lower yields
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
37. Summary of Workmanship Quality
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Approximately 10 microphone assemblies were inspected at varying levels
Workmanship quality was high for all of the device areas inspected
MEMS microphone and ASIC soft attachment to the substrate is one area that
showed less consistency than other processes but it is not expected to result in
performance issues
Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com