FINAL REPORT

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Abstract— The report discusses procedures and methods of

fabrication of a designed micro cantilever beam. The cantilever

was fabricated using several techniques and which were carried

out in five main steps. These steps consist of patterning the

sacrificial layer, copper deposition, beam cantilever patterning,

dry etching, and wet etching for releasing the cantilever. Only a

small number of cantilevers were released at the end of the

process due to a number of factors discussed in the report.

Index Terms— Copper evaporation, micro cantilever, wet

etching, dry etching.

I. INTRODUCTION

ICRO and nanotechnology miniaturization has allowed

great achievements in fields such as biomedical,

aerospace, maritime, chemistry, and others. Micro

electromechanical systems (MEMS) are widely used in the

fields mentioned before for the use of sensors, accelerometers

and resonators among others. One of the most common

devices in MEMS is micro cantilevers. These microcantilevers

have several advantages such as ultra sensitivity, ease of

production and low cost [1].

Micro cantilever can be used as the sense element for

atomic force microscopy (AFM), high resolution chemical

analysis [2], and biosensors.

Due to the importance of micro cantilevers, the following

report shows the procedure for its fabrication.

II. MATERIAL AND METHODS

The materials used for the fabrication of the micro

cantilevers were:

Wyko NT 900 optical profilometer (Veeco, USA)

Wyko NT9100 optical interferometer (Veeco, USA)

Veeco’s Vision® analysis software (Veeco, USA)

Acetone (Merck Chemicals Ltd, UK) [3]

Copper beds 2.8 mm 99.99% purity

Thermal evaporator Pcod 1001133651 (Aldrich, USA)

Cas: 7440 – 508 Cu Usa Aldrich,

MicropositTM S1828 G2 positive (Rohmand Haas

Electronic Material, UK) [3]

MF-319 Developer (MicroChem Corp., Norway) [3]

Isopropanol (Merck Chemicals Ltd, UK) [4]

S-1828 G2 Positive Photoresist (Rohmand Haas

Electronic Materials, UK) [5]

UV exposure(Karl Suss)

Long Metal needle (for removing the bubbles)

Photoresist Spinner, Silicon wafer (4”)

Hotplate / Oven

Tweezers

Mask aligner

Mask (master pattern)

Large petri dish/dust proof storage box

Metal Plate( for Carrying the silicon wafer)

Rubber gloves

Safety Glasses

The experiments were performed in clean room where all

the materials and microscopes used are operated. The

fabrication process composed of five main steps: patterning

the sacrificial layer, copper deposition, beam cantilever

patterning, dry etching, and wet etching for releasing the

cantilever. The process can be seen Figure 1.

A. Patterning of the Sacrificial Layer

The silicon wafer was cleaned using nitrogen gas to remove

any contaminants present on its surface. The wafer was then

subjected to a dehydration process. In this process, the wafer

was baked for 2 minutes at 115oC. The wafer was then

allowed to cool down to the room temperature. The wafer was

then placed in a spinner and S-1818 Photoresist was poured

onto the wafer directly from the bottle. The spinner was then

set at the parameters shown in Table 1: Parameters of Spinner.

Table 1: Parameters of Spinner

Sequence Speed

(RPM)

Time (s) Acceleration

(RPM/s)

1 1000 50 100

2 700 10 500

This routine results in a photoresist thickness of roughly 5

µm. After the coating, the wafer was cleaned using Nitrogen

gas to remove any stray dust particles. Once the spin coating is

complete, the wafer was subjected to a soft baking session. In

this process, the wafer was baked for 2 minutes at 115oC.

Fabrication of a Designed Microcantilever

Beam

Salman Mahmood [email protected], Antonio Martellota

[email protected], Ronish Patel [email protected], Amar Romi

[email protected] , Veralia Sánchez [email protected]

M

2

Figure 1: Fabrication Process of Micro cantilevers

Once the post spin baking step was completed and the wafer

was cooled down to the room temperature and was exposed to

UV light using mask aligner for 40 seconds. The mask pattern

used was previously designed by the group using L-Edit

software. Figure 2 shows the mask for patterning sacrificial

layer.

Figure 2: Mask 1

Once the exposure process was complete, the wafer was

removed from the mask aligner and immersed into a developer

solution for 2 minutes at room temperature to remove the

unexposed resist from the wafer. This process also involves

mechanical agitation in which the wafer is continuously shaken

clock wise and counter clockwise in order to increase the

contact area between the wafer and the developer solution.

Once the excess photoresist was removed, the wafer was rinsed

with IPA solution and dried with nitrogen gas. The wafer was

then examined under the profilometer to review the structure.

The thickness of photoresist structures was observed to be

4.6µm.

B. Copper Deposition

The second step of the micro fabrication process involved

the deposition of a thin copper layer through thermal

evaporation. The equipment used for the deposition was a

thermal evaporator by Aldrich.

The thermal evaporation technique for deposition of thin

metal layer relies on the application of high AC current to a

filament of tungsten (high melting temperature) for the heating

and evaporation of metal beds. The target sample of the

deposition is positioned inside the evaporation chamber and

when the evaporation occurs, it is hit by the metal particles

which are propagating in all the directions. A metal layer will

thus be deposited on top of the sample. The growth rate, the

uniformity of the layer and its thickness strictly depend on

parameters such as the current amplitude and frequency, the

exposal time, and the amount of metal used.

A thickness sensor positioned inside the chamber, close to

the sample assists the monitoring of the process. The sensor

does not measure the real thickness of the layer deposited on

the sample but it estimates its actual thickness detecting the

increase of the metal mass deposited. Hence, to ensure proper

achievement of the desired thickness further analysis is

recommended.

In order to avoid the interaction between the vaporized

particles and the molecules of the atmosphere, the entire

procedure has to be conducted in high vacuum condition.

The substrate was positioned inside the chamber of the

machine, on the top side, facing down. A copper bed was

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hence placed on the dedicated indentation of the tungsten

beam at the bottom of the chamber. The process was

conducted for 53:50 minutes and the parameters shown in

Table 2: Parameters for Copper Thermal were used.

Table 2: Parameters for Copper Thermal Evaporation

Current amplitude 130 A

Initial current increasing rate 4.6 A/s

Density 8.93

Tooling 1000

Z-Factor 0.437

Life (%) 39.3

Figure 3 shows the screen of the thermal evaporator

software controlling system. The final achieved thickness

measured by the sensor was of 4.793 kA

Figure 3: Thermal Evaporation Controlling System

C. Beam Cantilever Patterning

The procedure of the beam cantilever patterning can be

split into cleaning of silicon substrate, S-1828 spin coating,

soft baking, UV Exposure, development, hard baking, wet

etching and thickness measurements using profilometer. All of

these steps are discussed in detail as follows:

1) Cleaning of silicon substrate

Cleaning of silicon wafer was performed by just flushing

the wafer with nitrogen gun since one mask was already

fabricated previously; hence, cleaning with water was not

required.

2) S-1828 spin coating

S-1828 spin coating was performed once the wafer

appeared to be clean and dry. To achieve a uniform and

desired thickness of photoresist on wafer it was necessary to

program the spin coater at different steps having different

acceleration. The wafer is then carried in metal plate from hot

plate to spin coater and carefully placed on small disc of spin

coater exactly at the center so that the vacuum under the small

disc of spin coater can firmly hold the wafer. After that, S1828

positive photoresist was poured according to the requirement

but it was recommended to pour a slightly extra quantity than

the required because it was observed that less than required

amount of S1828 significantly affects the thickness of

photoresist. Since S1828 is very viscous liquid, it was poured

at once all and the pouring was started exactly from the center.

The spin coated machine was programmed as shown in Table

3 .

Table 3: Spin Coating Parameters of the S1828 on the silicon

wafer

Steps Spin Speed

(rpm)

Accelerate Time(s) Decelerate

1 2500 500 5 -

2 2500 500 60 -

3 2500 - 5 500

3) Soft Baking

After the photoresist was applied to the substrate, it was soft

baked to evaporate the solvent and increases density of the

film. S1828 is normally baked on hot plate although

convection oven may be used; bake times was optimized since

solvent evaporation rate is influenced by heat transfer. To

obtained best results wafer was heated for 2 minutes at 1000 0C.

4) UV Exposure

S1828 is optimized for near UV (350-450 nm) exposure.

S1828 is virtually transparent and insensitive above 400 nm

but includes high actinic absorption below 350 nm. Over

exposure of the top portion of the resist film may result in

exaggerated negative sidewall profiles or T-topping. The

optimal exposure dose depends on film thickness (thicker

films require higher dosage) and process parameters. Figure 4

shows the mask used for cantilever patterning.

Figure 4: Mask 2

5) Development

The development of S-1828 resists was performed, which

was optimized for use with MF-319 Developer. Immersion,

spray or spray- puddle processes can be used. Other solvent

based developers such as ethyl lactate and diacetone alcohol

may also be used. Strong agitation was recommended for high

aspect ratio and/or thick film structures development process

was carried out for 45 seconds. After development, the wafer

was washed either with isopropanol and then diluted water.

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Table 4: Thicknesses before photoresist etching

Wafer

Anchor

Thickness(µm)

Cantilever

Thickness(µm)

1 2.74 6.75

Figure 5: Width and Length of Cantilever

6) Hard Baking

Post exposure baking was performed to selectively cross

link the exposed areas of the film. S1828 photoresist was

readily cross linked and can result in highly stressed film.

Therefore, to minimize stress heating of the wafer, post

exposure baking was performed in at 1000 C for 1 minute.

7) Wet Etching

Wet etching of the wafer was performed to remove copper

from wafer using alkaline solution of acetone. Alkaline

solution is compatible with the most of the metal resist,

relatively high etch rate typically 2-2.5 mils/min and is

environmental friendly and less undercut and sideways etch.

The etching was performed until etched copper was optically

visible on the wafer.

8) Thickness Measurements using Profilometer

The different wafers were coated by photoresist and the

thickness of photoresist on the wafers was measured using

profilometer. Figure 6 and Figure 7Error! Reference source

not found.Error! Reference source not found. shows the

thickness obtained from the wafer. Table 4 shows the

thickness measurement with photoresist before etching.

Error! Reference source not found. shows the width and

length of cantilever.

Figure 6: Thickness of Anchor profile from cantilever

Figure 7: Thickness of cantilever profile

D. Dry Etching

The dry etching process was performed using reactive ions

etch. The wafer used was directly dry etched to remove the

sacrificial layer. The wafer was exposed under O2 (48sccm)

plasma in RIE chamber for dry etching. Other etching

parameters were high RF power of 300W and low pressure of

100mTorr. The exposure time was 20 min. In this step, high

power atoms are bombarded at low pressure in plasma state.

The wafer was inspected under the profilometer after the

first etching exposure. It was observed that although plasma

etched the photoresist from top and inside of the cantilevers a

significant amount of resist was still present under the

cantilever beams.

The wafers were subjected to a second exposure of plasma

for 20 min. After second exposure of plasma, it was observed

that the resist was still visible under the beam; in addition,

some of the beam structures were damaged due to high power

plasma particles.

Figure 8 shows the profilometer image of cantilever after

second exposure with small quantity of photo-resist. It was

possible to observe that high power bombardment of plasma

atoms damaged the metal layer on the wafer. Figure 9 shows

the cantilever structure damaged due to RIE plasma during the

release process.

It was shown that power and pressure were the main factors

for the optimization of dry release process, variation of power

and pressure from high to low and vice versa can change the

plasma behavior inside the chamber.

A third exposure of plasma was given again to the wafers with

different parameters to optimize the RIE release process. In

this case, the wafer was exposed under O2 (48sccm) in plasma

in RIE chamber, RF power of 100W and high pressure of

500mTorr. The exposure time was 10 min. The power and

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pressure parameters of the RIE tool were changed from high

power to low pressure to high pressure as it created an

isotropic behavior of the plasma instead of anisotropic

behavior, which was observed after all the exposures. Table 5

summarizes the parameters of RIE.

The low RF power generated a weak bombardment of

plasma atom with high pressure. The sample was then studied

under the profilometer and it was observed that the photoresist

was cleaned from top and inside the beam, but there was still

some photoresist under the beam.

Figure 8: Cantilever profile after second RIE exposure

Figure 9: Cantilever profile after third RIE exposure

Table 5: Parameters of RIE

Anisotropic Isotropic

1st

exposure

2nd

exposure

3th

exposure

Pressure

(mTorr)

100 100 500

Power

(W)

300 300 100

O2

(Sccm)

48 48 48

Time

(min)

20 20 10

E. Wet Etching for Cantilever Release

Firstly, some measurements of the cantilever were

performed. An interferometer was used on which the wafer

was placed and the Vision computer program was started for

the analysis of the surface. The intensity window was used to

take the required measurements, obtain proper focus and

adjust light intensity. The fringe with the strongest intensity

was selected according to the appearance of red pixels, which

are at the maximum, though the measurements were taken

without the red pixels as shown in Figure 10.

Figure 10: Fringes in the Interferometer

Figure 11: measurement of the cantilever before wet-etching

The resulting thickness of the cantilever was ranging

between 2.5 and 5 µm, whereas the resulting thickness of the

anchor was 220.2 nm (Figure 11). Error! Reference source

not found. illustrates the measured readings of the cantilever

for the x and y profile. Figure 13 shows a capture of the wafer

using the vertical-scanning interferometry (VSI) that uses

multiple wavelengths of light and enables step measurements

up to eight millimeters [6]. Finally a 3-D profile was obtained

using the 3-D plot and interactive display as depicted in Figure

14Error! Reference source not found..

After all the measurements were taken, it was observed that

the release of the cantilevers was not achieved yet. Therefore,

a final step of wet etching was performed. The etchant used

was Acetone. Given the high difficulties in controlling the

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process and the short times of exposure required, the wafer

was divided into four pieces and different wet etching times

were adopted (see Table 6). Short etching times could lead to

an incomplete release, whereas long etching times could over

etch the structures and destroy them. Nevertheless, the main

risk of a wet etching of these structures was the collapse and

stiction on the substrate after the etching.

Table 6: Wet Etching Time

Wafer Piece Time (s)

1 60

2 40

3 25

4 7

Figure 12: the measuring of the cantilever for the x and y profile

After the wet etching was performed, the pieces of wafer

were immediately dried with nitrogen gas. The nitrogen gas

was not blown straight onto the wafer, but a soft towel paper

was put as a mask to avoid damage to the wafer. Figure 15

shows the release of one cantilever.

Following the release of the cantilever, measurements were

taken again. The optical profilometer was used first to perform

the measurements and Figure 16 was obtained. Then, the

interferometer was used to obtain more measurements as

shown in Figure 17 where the x-profile shows a resulting

measurement of 0.6051mm and a z-profile (anchor thickness)

of 0.2481 µm. Finally, Figure 18 shows the release of the

cantilever with the VSI mode.

Figure 13: VSI Image of Wafer

Figure 14: 3D profile

Figure 15: Release of Cantilever

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Figure 16: Measurement after Wet Etching with Profilometer

Figure 17: Measurement after Wet Etching with Interferometer

Figure 18: Release of the Cantilever VSI mode

III. RESULTS AND DISCUSSION

Figure 15 shows the final structure of the designed

cantilever. Among few hundreds of cantilevers designed

above the silicon substrate, just a couple of structures were

properly released and produced. The development time for the

wafer that produced the released cantilever was 7 seconds.

Figure 15 shows on the right a cantilever that was released and

on the left a cantilever that was not. It can also be seen that the

released cantilever is characterized by residual stress coming

from the various processes that have been conducted. This

stress caused the bending of the structure of roughly 45°.

The reason why only few cantilever were properly produced

relies on the difficulties that occurred during the releasing of

the structure. The decision of using dry etching first, rather

than wet etching was taken to reduce the risk for the structure

to collapse after releasing. This collapse would have occurred

due the surface tension of the liquid used. However, even

though the dry etching was performed three times, it did not

bring to a proper release of the structures and wet etching was

hence performed.

IV. CONCLUSION

A micro cantilever beam on based silicon wafer was

successfully produced. Several steps were performed for this

fabrication: a sacrificial photoresist layer was coated and

patterned through photolithography above the silicon wafer; a

thin copper layer was deposited with thermal evaporation

technique, a further photoresist layer was coated and patterned

for the wet etching of the copper layer; a dry etching process

was performed three times for trying an optimal release of the

structure avoiding stiction; a final wet etching process was

hence performed after having observed that the cantilevers

were not properly released after dry etching.

However, in the initial design few hundreds of cantilevers

were present. The most of the structures were damaged or

collapsed due to the stiction occurring during the final wet

etching process. Suggestions for a future fabrication would be:

design the cantilever with bigger holes and increasing the time

of the dry etching and use oxidation layer as a sacrificial layer.

V. REFERENCES

[1] A. Vasudev, "Microcantilever-Based Sensor," NNIN REU

2, Research Accomplishments, pp. 98-99, NNIN REU

2006 .

[2] B. Hubert, A. Nishimoto, M. Ottensmeyer, D. Robinson

and B. Romo, "Process Development for the Fabrication of

a Piezeresistive Microcantilever," MIT Media Lab, 1999.

[3] CHEMCUT, "Process Guideliness for Alkaline Etching,"

2002.

[4] MERCK, "Safety Data Sheet for Isopropanol," Merck

Chemicals, Nottingham, 2006.

[5] "Safety Data Sheet S1828," Rohmand Haas Electronic

Materials, Coventry, 2006.

[6] M. Zecchino, "Wyko Optical Profilers: Most Repeatable

Step Measurements," Veeco Instruments Inc.