STMicroelectronics MEMS Microphone -- Reverse Engineering Analysis

STMicroelectronics
MEMS Microphone
Reverse Engineering Analysis

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Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com

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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.


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


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STMicroelectronics Omnidirectional Digital
Microphone
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Part Number: MP34DT01

STMM ro p ho ne
ic
US Pe nny


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


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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


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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


G5 X G5
Pin 1
Indicator

Metallization
For Mounting
And Electrical
Interconnect

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Isometric View of Microphone Assembly without Bottom Plate
Microphone Assembly

Bottom Plate
Solder Pads

Solder
Interconnects

MEMS
Microphone


EMI Shield
ASIC

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


Microphone

Gold Wirebonds from MEMS Microphone to ASIC
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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


Stitch Bond Over Gold Ball on MEMS Microphone
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Stitch Bond.

MEMS
Microphone
Back Plate

BT Fragments
From Opening
Microphone Asm.

Gold Ball.


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

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.


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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


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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


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Seed and second metallization layers added

Seed Layer

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Second Metallization Layer

Acoustic port fabricated


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Isolation groves fabricated using a diamond-saw cutting tool
Package cleaned

Isolation Groves


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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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Adhesive dispensed


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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


Dicing Line

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.


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.


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


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Back Plate
Front Chamber

Cross Section
of Microphone
EMI Shield
Soft Die Attach
BT Substrate


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Acoustic Holes
In Back Plate

Mechanical Stop
to Prevent
Electrode Contact

Minor Debris
from Opening
Assembly


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Close Up of
Microphone Corner


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Flexible
Electrode

Fixed Electrode
and Back Plate

Electrode
Ends Here

Electrode Interconnect
To Wirebond Pad


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Solid Model of MEMS Microphone


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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


SiO2

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Enlarged Image of Cross Section

Flexible Electrode

SiO2

Anchor

Silicon Nitride
Back Plate


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Confirmation that Silicon Nitride Was Used for Back
Plate
Note: Similar analysis was done for several
materials in the design but is not shown.


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


Electrical Schematic
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Fixed Electrode
Interconnect

1
4

5

4

1

3

2

5
3
2
Vias

5
5

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Flexible Electrode
Interconnect
1.
2.
3.
4.
5.

Power Supply
Left / Right Channel Select
Synchronization Input Clock
Left / Right PDM Data Output
Ground


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


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


STMicroelectronics MEMS Microphone -- Reverse Engineering Analysis

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