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MFA1M Lab 1 Report: A Practical Study On LeicaOptical Microscopic System

Lars Karlot Stubberud Bratrud, Haakon Karlsen and Bao Ho Dang

Abstract—In this lab work, extensive functionalities of Leicaoptical microscopy system is investigated, using a set of RFMEMS Switch samples. The practical work focused mainly onviewing, capturing and measuring dimensions of the switches atvarious magnification level, with the aid of Leica ApplicationSuite software system. Dark field microscopy was also reviewedin this paper.

Index Terms—Microscopy, Numeric aperture, Depth of focus,Depth of field, Dark field microscopy

I. INTRODUCTION

OPTICAL microscope, invented in late 1500s, is an opticaldevice that helps extend the ability of human eyes when

viewing microscopic objects, things whose size smaller thanthe resoltion of eyes. Soon after its appearance, optical micro-scopes were intensively used in various fields of science andtechnology like biology, chemistry, medicine and afterwards,microelectronic, MEMS.

In this project, the optical microscopy system to be usedis Leica system, including Leica microscope and Leica ap-plication suite (LAS) software. The case study, a set of RFMEMS Switches, is investigated and measured using the LASsoftware.

In the next section we will describe the scientific materialand method in optical microscopy. After that, the practical re-sults of the lab work is presented, following by our discussionon these result.

II. MATERIAL AND METHOD

A. Equipments

Our equipments include a wafer with RF switch test pattern,Leica DM 4000 M with camera attachment and computer withLeica Application Suite installed.

B. Formulas

Relationship between Depth of focus and numericalapertureNA:

Dfocus = 1.12λM2

(NA)2

√n2 − (

NA

M)2 (1)

, where n is the refractive index of the environment betweenthe lens and image plane.Relationship between resolution s and NA

s =0.61λ

NA(2)

C. Method

Part 1: Getting started.The first part of the lab was to basically get acquainted withthe microscope. It was stressed that we should be able to seeclearly through both oculars as this gives us somewhat of adepth perception making us able to distinguish details better.To do this, first we set the objective with least magnification(2.5x) in position and tried to see through the eyepiece. Therecommended procedure for this is as follows.1. With right eye, look through the right eyepiece and bringthe specimen into sharp focus.2. Then, with left eye, view the same specimen and rotate theleft eyepiece tube until the object is brought into sharp focus.Do not use the focus dial.We also experienced that it was necessary to adjust the lengthbetween the eyepiece tubes to get a correct image.The next step was to set the beam splitter to 50/50, that is,half the intensity of the light goes to the eyepieces and theother half to the camera attachment. This is very helpful forwhen acquiring an image on the computer, as the softwareexperiences a 1-2 second lag which makes it frustrating to doinitial adjustmen of the focus. It is important to know that thereare a difference in the focus between the image the camera seesand what we see in the eyepiece. We now had the possibilityto acquire images from the camera and do non-live analysisthrough the ”Analysis” tab of the software; like measuringdimensions, angles, etc.Part 2: Measuring.In the next part we did different measurements of the testspecimen. The specimen was a test pattern of RF switchesconsisting of bridges of different lengths (indicated next toeach bridge, 150 to 750 µm) over transmission lines. Thebridges and transmission lines are made of gold and thetransmission line center conductor is covered with an isolatinglayer of Silicon-Nitride which has a blue color.1. Length and width of all the bridges.2. Length and width of the center transmission line.3. Angle of the metal-edge at the ancors.4. Height-distance from top on bridge to top of the nitridelayer.5. Height-distance from top on bridge to top of the groundconductor.For the length and width measurements we used the objectivewith 2.5x magnification.To set the measurement tool as precise as possible we usedthe Shift button on the keyboard to bring up a magnificationof the area around our mouse pointer. The measurements were

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annotated to the image and saved on the computer. The focuswheel on the side of the microscope had a measuring scale inthe micrometer area that we could use for measuring heightsby first bringing e.g. the top a bridge in focus then the top ofthe nitride layer in focus and read of the height on the focuswheel scale. For height measurements we used the objectivewith a 100x magnification, as this gives us a shallow depth offocus which in turn gives us a more precise measurement.

III. RESULT

A. Field of view

When using the smallest magnification,x2.5, the whole teststructure can be viewed, and the field of view is 4605x3419µm. When using magnification x100 the field of view is115x85,5 µm. (see Figure 1 below)

(a) 2.5x objective

(b) 100x objective

Fig. 1: Field of view when using x2.5 objective (above) andx100 objective (below).

Magnification check to see if the objectives gives the correctmagnification:

length of picture with x2,5 objectivelength of picture with x100 objective

=4605

115≈ 40 (3)

width of picture with x2,5 objectivewidth of picture with x100 objective

=3419

85.5≈ 40 (4)√

area of picture with x2,5 objectivearea of picture with x100 objective

=4605× 3419

115× 85.5≈ 40

(5)

B. Measuring bridge length L1

The length L1 of various bridge is measured and thencompared with the actual (given) data, as seen in Figure2 and Table I. We see that the data seem correct althoughthere is a factor of 10 between the measured and actual data.This is because we had zoomed the sample appearance beforeaquiring the image which we measured. This fault gave a goodexperience in using Leica LAS software.

Fig. 2: Length of RF MEMS Switches.

TABLE I: Measured and actual length L1

Measured length L1 Actual length L11819µm 200µm2554µm 250µm3057µm 300µm3483µm 350µm4102µm 400µm4373µm 450µm4953µm 500µm5573µm 550µm6038µm 600µm6501µm 650µm7043µm 700µm7514µm 750µm

C. Lateral measurements of bridge and nitride layer

The results are in Table II below.

TABLE II: Lateral measurements of bridge and nitride layer

No. Bridge width W1 Angle V1 Nitride width W2 Nitride length L21st 80.69µm 38.6◦ 220.63µm 2501.96µm2nd 80.69µm 40.3◦

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D. Height measurementsWe measured (actually approximated) the distance Z1 from

nitride layer to top of the bridge and Z2 from top of groundplane to top of bridge. The results are in Table III below.

TABLE III: Height measurements

No. Z1 Z21st 1µm 1µm

2nd 1µm 1µm

3rd 1µm

E. Dark field modeDark field mode illuminates edges and impurities which can

be seen in the Figure 3 below.

Fig. 3: Length of RF MEMS Switches.

IV. DISCUSSION

From equation (1), we can see that the Depth of focus isnearly inversely proportional to the square of NA, so for 100xobjective (NA = 0.9), we have much short range of Depthof focus comparing with 2.5x objective (NA = 0.07). That iswhy we can use 100x objective to approximate the height ofour switches.

Since Z1 is equals to Z2, we may say that there is nogap between the SiN layer and the bridge. However, theremay be posibility that the gap is so small comparing with ourfocus dial’s resolution (1µm). It means that for example, ifthe gap is in order of 0.1µm, it is not measureable with ourapproximation. Also, there is a probability that our sample hasbeen abused at some time, as it was missing all the cantileversand we found some piece of skin laying on one of the bridges.

Another lesson from the lab is that we should be awareand careful when using the LAS software. A mistake inorder of manipulating the software can leads to a degree ofinaccuracy in the result, as can be seen with our bridge’s lengthmeasurement.

REFERENCES

(No reference so far)