ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 1

Abstract— This Micro Electro Mechanical System (MEMS)

device is a 1 dimensional actuator which has a Proof Mass in the

middle and the proof mass has fingers and springs connected to it.

Puncture devices and scalpels and cauterize uses the ultrasonic

vibration, but the ultra-sonic vibration can cause damage to the

tissue and is not a desirable method to use. In this project we found

that by reducing the operating frequency of the needle to

particular frequency less than ultrasonic vibration can reduce

puncture and does not cause significant damage to the tissue. This

MEMS device is designed on the SOG Fabrication Process and the

Circuitry is designed using the Enhancement / Depletion Mode

EDNMOS technique.

I. INTRODUCTION

Numerous procedures in nearly every field of medicine require

the insertion of an access device into a tissue medium along an

axial path. Hypodermic needles, laparoscopic trocars, and other

similar devices require an axial force to be applied by the user

in order to penetrate various tissue layers.

A potential harmful situation arises when the device puncture a

tissue layer and the resistance force decreases suddenly. A force

imbalance is created and causes the device to accelerate further

into the tissue until the user is able to react and decrease the

force they are applying to the device.

Acceleration is directly proportional to applied force, a

puncture device requiring a greater insertion force will have a

greater force applied to it at the moment of puncture, and will

therefore accelerate to a greater degree. If this acceleration is

high enough and the user's reaction time significant, the device

may even advance too far and damage delicate organs. If the

force required to insert puncture access devices can be

decreased, it is likely that epidural anesthesia, needle biopsies,

amniocentesis, and other medical puncture procedures will be

easier to perform and that complication rates will decrease.

The Micro actuator designed to drive a needle like device to

oscillate linearly along its longitudinal axis at audible

frequencies. This concept is intended to lower the force

required to insert a needle and also to oscillate below maximum

frequency.

BLOCK DIAGRAM:

Input for the electronic circuit will be 1V, 150 Hz sine wave,

which in turn feeds 15 V, 150 Hz square wave to MEMS

actuator which vibrates in axial direction. These vibrations are

transferred to medical puncture device to operate in desired

conditions.

DESIGN CONDITIONS:

Figure 1: Insertion Force (N) Vs Penetration Depth (cm)

The insertion force as a function of insertion depth for each of

the 5 parameter configurations that include an input signal

amplitude of 10 v and a 21 gauge needle. In 5 of the 6 frequency

parameter groups, a frequency of 150 Hz results in the lowest

insertion force. In general, 100 and 200 Hz result in the next

lowest insertion force in no discernible order, followed by 250

and 300 Hz resulting in the highest insertion force. At

frequencies around 150 Hz, the needle oscillates with the

greatest amplitude both freely and during tissue puncture,

presumably because the needle system has approached or

reached a resonant frequency. This observation is reflected in

the above graph, where the data set for the 150 Hz configuration

seems to oscillate a significant amount.

Micro Actuator for Medical Puncture Devices

Pavan Kumar Dasari, Leelakrishna Koneti, Sunil Vutukuri, Abdul Baseer and Rakesh Chintala

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 2

II. MEMS DESIGN

A. Calculations

1) Mass of the Structure :

Mass of the whole structure can be calculated by

adding up proof mass and mass of all fingers in the

structure i.e.

Mass of the Structure = Mass of the Fingers + Mass

of Proof Mass.

Mass of the Fingers = Volume of fingers (v) *

Density of silicon = 3.35 * 10^ (-08) kilograms.

Mass of Proof Mass = Volume * Density = 1.17 *

10^ (-07) kilograms.

Mass of the whole structure = [3.35 * 10^ (-08) +

1.17 * 10^ (-07)] = 1.51 * 10^ (-07) Kg.

The total Mass of the Structure is 1.51e-7 Kg

Figure 2: Whole Structure

2) Spring constant K :

Spring width is very important to calculate to make

sure that the structure is actuating and does not break

for the amount of force applied. For the structure to be

moving the spring constant should be set to “1” to get

the minimum required width for the structure to be

moving.

The spring constant is denoted as “K” and is given by

K = (4*E*W*((h) ^3)/ ((L) ^3)

Where “E” is the Young’s Modulus and is equal to 160

for 100 Silicon & “L” is the length of the spring.

Minimum required width for the spring to actuate is

4.13um but according to the design rules width should

be greater than 5um so, we assume the width of the

spring as 10um.

Figure 3: Spring structure.

3) Capacitance :

Capacitance in y direction between the comb fingers

can be calculate by,

Cy = 2*Nf*ἐo*Lf*h /d = 1.06*10^-11 F

Where, Nf is number of fingers, Lf is length of the

finger, h is height of silicon & d is the gap and εo is

the absolute dielectric permittivity.

Figure 4: Capacitance(c) Vs. Displacement (n)

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 3

Figure 5: Tabular Column Displacement (n) Vs.

Capacitance(c).

Resonant Frequency:

F = 1/ (2*pi) √𝑘/𝑚 = 5.28*10^05 Hz {mass of the

whole structure = Kg}

Where, k is the spring constant, pi = 3.14 & m is the

mass of the structure.

4) Pull-in Voltage & Force :

Pull-In Voltage is minimum voltage required to move

the structure in Y direction is given by,

V pull-in = 2*d/3 ((1/ (1.5*Cy) ^ (1/2)) = 0.66797 V

Where, d is the gap between fingers & Cy is the

capacitance in Y direction.

Force Generated =n*εo*h*input voltage^ (2/4.00E-

06) = 2.99E-06 N

5) Displacement, Acceleration & Time for

Movement:

Displacement (X) =d/3= 4.00E-06 m.

Acceleration = Force Generated /Total

Mass=1.9E+01 m/s.

Time for Movement =

(2*Displacement)/Acceleration) ^1/2 = 6.73E-08 s.

III. MEMS OPERATION

A. Structure & Process Flow

The Micro Actuator for Medical Puncture Device is a 1

dimensional actuator which has a proof mass in the middle

connected to the springs with comb drives as the driving unit of

the structure. The comb drives are symmetrical in structure.

Each comb drive has 60 fingers on each side. When the voltage

is applied the structure is set to move in a particular direction

which causes the change in capacitance between the fingers.

The structure is connected to a total of four springs. The main

motive of this device is to decrease the amount of force to insert

needle and model for predicting performance of a hypodermic

needle.

Figure 6 : Silicon Layer after rendering

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 4

The following picture shows 2-D design layout showing all

masks.

Figure 6: 2-D Design Layout Showing All Masks

A. Mode of Movement.

The designed actuator actuates in vertical Y direction at

a frequency of 150Hz providing enough displacement of

1.35um to pierce the exo-dermal tissues, as a result the

insertion force of the puncture device can be reduced.

B. Fabrication Process- Silicon on Glass (SOG)

The devise was fabricated using Silicon on Glass (SOG)

technique.

Figure 10: Cross Sectional View of Step 1

Step 1 coat 500um glass wafer patterned layer of Chromium

with a deposition thickness of 1000A. Spin coat a layer of

photoresist, expose and then develop. Both layers are required

to protect selected regions from being etched away.

Figure 11: Step 2

Step 2 etch glass wafer using hydrofloric acid (HF) untill a

3um recess is formed.

Step 3 deposit 50A cromium and 500A platinum. When the

photoresist is removed the liftoff technique will leave behind

the designed network of bottom electrodes.

Figure 12: Step 4

Step 4 bond silicon wafer to glass wafer at 300-400 degrees

celsius using two electrodes with a potential difference of 1000

volts.

Figure 13: Step 5

Step 5 coat silicon wafer with 5000A of Al.

Figure 14: Step 6

Step 6 Etch away unwanted alumum layer. This particular

desine removes all traces of aluminum not assosiated with the

pads.

Figure 15: Step 7

Step 7 Pattern and etch away the targeted silicon for final

release of structure.

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 5

Measurements

The device was fabricated using 4 masks: metal contact

deposition (PAD), DRIE Etching (DRIE), bottom electrode

metal deposition (METAL), and Recess of the glass wafer

(RECESS).

Figure 7: Mask Pattern for Electrode Layer

Fingers that overlap silicon where restricted to no greater or less

than 50um. The width of each finger and the gap between each

finger was restricted to 10um by 20um respectively. This allows

for at least moderate bonding to take place between the silicon

and the glass wafers even in overlapping regions. The smallest

dimension for any electrode deposited was 35um which far

exceeds the minimum requirement of 5um given to us in class.

A greater then 5um distance between the drawn glass anchor

and the main body of the electrode needed to be greater than

5um to prevent possible unwanted shorts between unconnected

components. The electrodes seen with no fingers attached act

as a protective barrier, these where placed to prevent the silicon

wafer from bonding with the glass wafer underneath.

Figure 8: Mask Patterned for Silicon DRIE Etching

SILICON

Silicon structures that come in contact with the pads on the top

most layer where made at least 10um wider and longer then the

pads deposited. This will prevent possible over etching by

DRIE from reducing the size of the designed pads. Due to minor

inconsistencies between pads and testing equipment small

changes in pad size can mean the difference between a possible

contact point and a missed connection. Gaps between silicon

structures must be greater than 3um. Smaller gaps will prevent

the DRIE from etching all the way through stopping your

structure from being completely released. The minimum width

of silicon structures is 5um as stated in lecture notes. Any

smaller then this and you run the risk of the DRIE over etching

the component away entirely. Ideally all structures should be

designed slightly thicker then you actually want them to be. The

openings over bottom electrodes need to be less than 5um. Any

greater width will allow the DRIE to etch away the bottom

electrode along with the silicon layers above it. A gap greater

then 3um between comb drive structures must be maintained in

order to prevent unwanted contacts between them. One’s

contact has been made normally negligible coulomb forces are

strong enough for structures of this size to prevent them from

separating again.

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 6

Figure 9: Mask Pattern for Glass Recess

The recess is used to release the designed structure from the

glass wafer. Any contact will prevent the structure from moving

as intended. As shown everything was released except for 4

anchors and the surrounding pad support structures. All recess

areas must be connected to one another to allow for pressure

equalization during the DRIE process which can be as little as

several milli-Torr. The outer perimiter is used to separate one

class project from others that will be fabricated on the same

wafer. Any contact between dies will create several

unintentional electrical shorts.

Figure 10: Default Pads

The pads where left unchanged to allow for continuity

between SOG and ED/MOD designs as well as between

universties and different class projects.

C. Simulations

Simulations for the SOG portion of the project where

completed using Coventorware. Two types of simulations

where needed to be run. The first was to confirm correct

mechanical movement. The second was to confirm the

capacitance between key structures within the device. This was

achieved by meshing the silicon layer of the device using

tetrahedral blocks in linear mode. No other meshing shape was

giving the proper meshing in the corners of the structure. For

model analysis ones the mesh was made the proof mass was

restricted in movement in the x and z axis as expected in the

case where the supports function properly. The anchors where

restricted from movement in all directions since they will all be

bonded directly to the glass wafer. Using modal analysis we

were able to generate several examples of the device

functioning as expected. One such example is shown below

after a high degree of forced exaggeration to help clearly show

its motion.

Figure 11: Modal Analysis

In the electrical analysis the proof mass was charged to +15

volts to get the displacement of 1.35um.

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 7

Figure 122: MEM Electrical Analysis

As seen in the table below the capacitance between the fixed

fingers and proof mass is 16.1pF and Fixed left 1 capacitance

equal to fixed right 2 capacitance and also fixed left 2 is equal

to fixed right 1 capacitance.

Figure 133: Table with Simulated Capacitance

E. Simulation problems

Do to the limitation of the available computers on campus

simulations where not able to be done on the final design of the

project but rather at an early simpler point. After many

simulation attempts it was evident that rounding the edges of

the silicon layer creates complications within Coventorware

requiring far more computational power to arrive at a solution.

The longest test was run for 6 hours at which point we needed

to leave the lab and where unable to continue. As seen below,

the effects can also be seen at the meshing stage. Before

rounding fewer points where plotted by Coventorware then can

be seen after rounding even when selecting equal block sizes

with the meshing default tab.

Figure 14: Failed mesh settings

IV. EDNMOS Design

Figure 15: BLOCK DIAGRAM OF EDNMOS.

1V ac voltage is given to the input of the opamp with a gain

approximately equal to 15. There is a current mirror in the

input stage which is explained in the later part of the report.

Trans-conductance is defined as the ratio of current variation

at the output to the voltage variation at the input, it is denoted

as gm and is given by

gm = Change in input current/change in output voltage.

Integrator stage maximizes the available gain.

Output stage consists of a load where we measure the

output voltage and currents.

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 8

LT SPICE CIRCUIT AND SIMULATIONS

Figure 16: LT Spice Circuit.

The Opamp has four different stages of operations which are

Input stage.

The biasing current ID5 for the differential pair M0, M1 is

derived using the current mirror of enhancement devices, M4,

M5 biased with depletion device M6. The advantage over using

a single self-biased depletion device in M5 position is the

arrangement has a lower minimum saturated drain voltage at

the drain of M5, resulting in larger common mode range while

retaining full gain.

𝐴𝑉0 ≅2

𝛾√

(𝑊𝐿⁄ )0

(𝑊𝐿⁄ )2

𝐼0

𝐼2(𝑉1 + 2∅𝐹)

Where, 𝐴𝑉0is the overall gain,(𝑊𝐿⁄ )0,(𝑊

𝐿⁄ )2 is the

width to length ratio of transistor 0, and transistor 1

respectively.𝑉1 Is the output voltage of the transistor 1, 2∅𝐹

is part of the MOS threshold voltage which corresponds to the

voltage drop across the depletion region under the gate at the

onset of strong inversion. The relation between I3 and I7 is

compromise between gain and slew rate.

Figure 17: Input Voltage

Level shifter stage.

The level shifter is designed so that V0=Vg1=Vg8 as a DC bias

condition. This voltage is set at VDD-|VTD|, as desired for

maximum gain, by design of U8 and U10.

𝑉𝑔10 = 𝑉𝐷𝐷 − 𝑉𝑇𝑆 [1 − √1 + (𝑉𝑇10

𝑉𝑇8)

2

∗ 𝛽 (10

8)]

Where 𝑉𝑔10 is the gate voltage of the transistor 10, 𝑉𝐷𝐷 is the

circuit driving voltage =15v,

𝑉𝑇8&𝑉𝑇9 Are the threshold voltage of transistors 8 &9,

𝛽 (10

8) geometrical ratio of transistors 8&9.

By choosing proper 𝛽 value the quantity within the brackets

can be made equal to 1 thus achieving𝑉9 = 𝑉𝐷𝐷 + 𝑉𝑇𝐷,

which is desired operating point for M2 and M3.

The voltage Vg10 is transferred to the gate of transistors 9 and

13. The dynamic series resistance of the level shifter must be

low to avoid phase shifts of the signals passing through the

level shifter.

Figure 18: Output of the Level shifter and Input of the

Integrator

Integrator stage

The transistors 14 and 17form a cascaded inverting amplifier

with higher output impedance than we would obtain from the

transistor 14. Transistor 17 is a load device as the transistor 12

provides a constant part of drain current of transistor 14,

which increases the trans-conductance without increasing the

current which flows through transistor 17, this increases the

gain available which is given by

𝐴𝑉 =2

𝛾√(𝛽 (

14

17)

𝐼𝐷14

𝐼𝐷17

(𝑉𝑂 + 2∅𝐹))

Where, 𝐴𝑉 is the available gain of the integrator, 𝛽 (14

17)is

the geometrical ratio of the transistors 14 and 17,

𝐼𝐷14& 𝐼𝐷17are the drain currents of the transistors 14 and 17

respectively, 𝑉𝑂 is the offset voltage of the integrator which is

equal to𝑉𝐷𝐷, 2∅𝐹is part of the MOS threshold voltage which

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 9

corresponds to the voltage drop across the depletion region

under the gate at the onset of strong inversion

The gain transistor U14 must have a large trans-conductance to

achieve high gain and to minimize the undesirable effects.

Figure 19: Output of Integrator

Output stage

A broad band unity gain stage with high output impedance is

desired. Gain accuracy is not critical, so we designed the

circuit with (W/L) ratio to be unity in order to get high output

impedance. Also we do not need the circuit to be driven

continuously, so we chose to excite the circuit by a square

wave of 14v magnitude, in order to achieve perfect output

signal for the actuator. The output stage has both an

enhancement mode NMOS and a Depletion mode NMOS

transistors, where in the transistor U19 acts as a feedback

resistor resulting in desired trans-resistance characteristics,

when the transistors U18 and U19 are geometrically identical

the overall stage gain is approximately equal to unity.

Figure 20: Output of Opamp.

Figure 21: Comparison of the input and output signals.

Gain calculated:

𝐴𝑉𝑜𝑢𝑡=

𝑉𝑂𝑈𝑇

𝑉𝐼𝑁=14.4 v

V. TESTING

a. SOG testing

Figure 22: SOG chip on probe station

Capacitance measured are as below,

C1 = 183.6 pF

C2 = 280.0 pF

C3 = 169.0 pF

C4 = 277.0 pF

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 10

The test process of the SOG die part of the accelerometer id=s

completed using DC voltage bias for actuation and

corresponding movements of the silicon parts from the die is

recorded to see how good the sensitivity is.

Based on the expected displacement, the capacitance changing

between the proof mass and fixed fingers are given by figure 4

above.

b. EDNMOS testing

Test Die Measurements:

Figure 23: EDNMOS chip on probe station

Summary:

SOG : Capacitance measured is too high from the calculated

values. Device was not working as expected. Possible

reasons could be,

1. Shorting of fingers on one side as we can see tiny dots

on fingers in the figure 21. Because of this, even the total

ECE 6450 ADVANCED MEMS, DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING, WESTERN MICHIGAN UNIVERSITY 11

capacitance while testing was recorded as 393 pF instead

of 829 pF.

2. Stiff spring, which holds the proof mass from moving.

3. We were not able to test the system in desired

conditions.

EDNMOS:

As we are driving the EDNMOS circuit with +15V and

-15V DC supply we are getting an output as level shifted

square wave with an offset of 14V. The possible reasons

would be,

1. Design issues while layout designing.

2. Manufacturing issues due to low

L/W (6.8/6).

Vi. References.

[1] Nikolai D.M. Begg, Alexander H. Slocum. Audible

frequency vibration of puncture-access medical devices.

journal home page: www.elsevier.com/locate/medengphy

2013

[2] Kataoka H, Washio T, Chinzei K, Mizuhara K, Simone C,

Okamura AM. Measurement of the tip and friction force

acting on a needle during penetration. In Presented at the

Proceedings of the 5th International Conference on Medical

Image Computing and Computer-Assisted Intervention-Part I.

2002.

[3] Okamura AM, Simone C, O’Leary MD. Force modeling

for needle insertion into soft tissue. IEEE Transactions on

Biomedical Engineering 2004; 51:1707–16.

[4] Davis SP, Landis BJ, Adams ZH, Allen MG, Prausnitz

MR. Insertion of micro-needles into skin: measurement and

prediction of insertion force and needle fracture force. Journal

of Biomechanics 2004; 37:1155–63.

[5] Slocum A. Precision machine design. Michigan: Society of

Manufacturing Engineers; 1992.

[6] Daniel senderowicz, David A. Hodges, Paul R. Gray. High

performance NMOS Operational Amplifier. December 1978

[7]] Gholamhassan Lahiji. Course Information and ED/MOS–

SOGPROCESS[Online].Available:

https://ctools.umich.edu/access/content/group/824a4bbe-e9e8-

45f2-92af-cb072d42bc97/Lectures/Lec%20_%205.pdf