Basic Lab Report

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BASIC LAB REPORT

M104ETCHING OF SEMICONDUCTOR

GROUP 104

Hafid Suharyadi [email protected]

Torben Waldmann [email protected]

7 December 2012

FACULTY OF ENGINEERING

KIEL UNIVERSITY


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Table of Contents

1. Introduction....................................................................................................................................1

2. Theory..............................................................................................................................................1

3. Experiment .....................................................................................................................................1

3.1 Equipment and Materials....................................................................................................1

3.2 Procedure.................................................................................................................................2

4. Results .............................................................................................................................................3

5. Discussion ................................................................................................................................... 10

6. Summary...................................................................................................................................... 11

7. References ................................................................................................................................... 11


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1. IntroductionRecent update

aim

2. TheoryPolar-nonpolar relation of H2O and Copper

Contact angle based on hidrophobil, hydrophobic

Microscope work based on reflectivity

3. Experiment3.1 Equipment and Materials

For every procedure, deionized water and air jet, were used to clean the

sample. The optical microscope was used to analyze the sample surface.

a) Polishingi. Solution consists of 60 ml HF (48% aq. solution), 100 ml HNO3

(65% aq. solution), 60 ml CH3COOH (96% aq. solution).

ii. A thermometer.iii. 4 pieces of unpolished silicon wafer, 2.5x2.5 cm2.

b) Defect Etchingi. SECCO solution consists of K2Cr2O7 (1.452 g), H2O (33 ml), and

HF (67 ml).

ii. A piece of polished silicon.

c) Structuringi. 70 g KOH pellets.

ii. 40 ml Isopropanol.iii. Two thermometers.iv. Two pieces of polished silicon.


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3.2 Procedurea) General

After submerging the samples in the etchant solutions, those were

rinsed in deionized water and dried with air jet. The pictures of silicon

surface were photographed by the optical microscope in various

magnifications.

b) PolishingFour pieces of unpolished wafer silicon, without overlap, were polished

by immersing those in an etchant, which was thermally isolated and

consisted of HNO3 and HF with CH3COOH as a solvent. Two main

steps are oxidative reaction with HNO3 (equation 1-4) and dissolution

reaction of SiO2 with HF (equation 4-5). This procedure aimed to

remove the layer of samples close to the surface, and to produce a shiny

and flat surface of silicon. This was maintained for 8 min 30 s. A

thermometer was used to monitor temperature of solution.

c) Defect EtchingThe areas near defects of polished surface were investigated. One of

polished silicon pieces was used and put into the SECCO solution for 5

min. The sample was cleaned with DI water and dried with an air jet.

d) Pyramid EtchingIn this experiment, two of remaining samples were used, in which one

sample was put in KOH solution by treated in room temperature (20C)

and and another was in 80C. KOH solution with two thermometers

were used and two magnetic stirrers were used to stir the solution.

Finally samples are washed in DI water and dried by the air jet again.


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4. Resultsa) Unpolished Sample.

The smoothness or roughness of the silicon surface is determined

qualitatively by the optical microscope image [1]. Before polishing, the

pictures of samples show some roughness surfaces. The layer of silicon

may contain contaminants that will be removed by etching process. At

the figure 1 and 2, one can see that very grainy and coarse structure is

observed.

Figure 1. The optical microscope image of unpolished sample,grainy structure, 50x.


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Figure 1. The optical microscope image of unpolished sample,

grainy structure, not all planes in-focus, 1000x.

b) Polished-etched SampleThe quality surface of samples after polishing determined from

microscope images is pointed to the influence of etching solutions

on their surfaces. As explained by reactions on equations 1-5, the

etching mechanism is performed by chemical reaction that occurs to

form the volatile compounds and atoms, which are desorbed from

the sample surface. At this experiment, one can see the flat surface

and mirrored face from these four samples. It is expected that the

silicon in native or SiO2 amorphous has been consumed partiallyduring SiO2 growth. Based on figure 3, a roughness on atomic scale

is reduced. On the figure 4, with using higher magnification, one can

see that no roughness is visible. Some dark areas appeared are

created by microscope lens which is not clean.

Since the temperature was increasing for 15.2C (17.3C - 32.5C), one

could expect that the state of the products was exothermic, relating to


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oxidation process. At the last of this step, gas brown that relates to the

formation of NO2 (equation 1) was observed.

Figure 3. The optical microscope image of polished sample, still

micro-rough surface, 50x.

Figure 4. The optical microscope image of polished sample, the best

achievable focal contrast at 1000x.


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c) Defect-etched SampleIf the silicon defects are found, one should carefully identify them.

In general, the crystalline defects delineated by SECCO give

elliptical etch pits [2,3]. This method is applied in failure analysis of

wafer fabrication.

The main advantages in using SECCO is a rapid chemical etching

method to delineate crystalline defects on silicon wafer, in well-

defined etching pits with elliptical shape, and fast and simple in

preparing the solution [2,3].

At this part of experiment, it is not easy to identify the defects. One

factor is that etching process possibly does not remove the substrate

layer completely, or not well polished. From figure 5, three black

dots are appeared on the silicon surface. The red circle (close view at

figure 6) indicates the area near defects on the surface.

r

Figure 5. The optical microscope image of defect etched sample with

1000x.


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Figure 6. Close-up view of defect area three black dots.

d) Pyramid-etched SampleThe pyramidal structures in term of geometric allow light to be more

easily coupled into the silicon and absorbed into the solar cells. The

structuring step was done by removing the damage on the surface and

producing straight-up pyramid on a surface. This method is using

alkaline solutions with considering its higher etching rates. However,

due to the anisotropy of single crystalline silicon, etching rates in

alkaline solution varies with the different crystal orientation. Etching

rate ratio of crystalline planes in [110]:[100]:[111] is approximately

160:100:1 at room temperature[4].

Atom arrangement at the silicon crystal [111] plane is the tightest one.

There is only one free covalent bond per atom at the interface and other

three bonds in inter-layer integrity. In contrast to this plane, the

difference is less tight arrangement of silicon atoms on the [110] crystal

plane with two free covalent bonds at the interface, thus those are

preferentially dissolved. Macroscopically, the silicon surface is attacked

at the random places in an inverse-pyramidal manner, as the etching

occurs only at the tip of this inverse-pyramid, the structure is

maintained but grows in size.

As the temperature increases, the etching rate and will increase and the

pyramids formed will increase as well. The phenomena can beexplained by the chemical kinetics and the Arrhenius Law.


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()

where v is the rate for the chemical reaction, Ea is the energy activation,

R is the Boltzmann constant, k is the reaction rate constant, k0

is the

constant, T is the etching temperature, and C and D are the reaction

orders in respect to OH-and H2O respectively [5].

The diffusion rate (the transportation rate of reactants to the surface or

products from the surface) can be enhanced by stirring to remove the

silicate solute from the silicon interface. Therefore, by applying

sufficient stirring, the spontaneously formed oxide film can be removed

[4,5].

The figure 7 shows the surface of sample that is etched in KOH

solution at 20C. The surface of this sample is slight rough and no

pyramids are found in this sample.

Figure 7. The optical microscope image of etched sample in KOH

solution, time 30 min, temperature 20C, 200x.

In contrast to the sample of figure 7, the pyramid structures appear

on the surface that is etched at the 80C, as showed in figure 8. One


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can see that the morphology of silicon surface change visibly. The

pyramid structure with various sizes of square base forms randomly

on the surface. This result agrees with the report [5] in which a

tendency for forming pyramids on silicon wafer surface is indicated

at 50C rather than at 25C. Figure 9 shows the close view of

pyramid structure on silicon surface.




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5. DiscussionThe roughness of the silicon surface is determined qualitatively by the optical

microscope image. In the unpolished sample (fig. 1 & 2), a very grainy and

coarse surface is observed. The effects of several solutions as etchant and

temperature to the morphology of silicon surface in this experiment are

described below.

After polishing, the surface of sample is changed. With low magnification, the

roughness on atomic scale (fig. 3) is reduced visibly. It is expected that the

substrate containing SiO2 amorphous has been consumed partially during SiO2growth. Otherwise the flat surface (fig. 4) is appeared by higher magnification

due to a good reflectivity and low micro-roughness of the sample. The reaction

state is described as an exothermic leading to the increase of temperature.

At the defect etching, an area expected as a defect area is presented by

figure 5. The three black dots on the silicon surface (in red circle of the fig.

6) may be interpreted as a kind of defects that are appeared after removing

very thin layer of substrate.


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The different etching temperatures lead to the different morphology of silicon

surface. At the higher temperature, the forming of pyramid on the silicon

surface is appeared visibly. The size distribution is more than inhomogeneous;

as there were no etch seeds, in order to preferenciate certain spots. In contrast

to the sample treated at room temperature, the surface is slight rough and no

pyramids are found. This difference can be explained by considering the rate of

chemical reactions as a function of the temperature.

6. SummaryThe wet etching accompanied by the morphology change of silicon surface has

been studied. A morphology change after polishing has been observed for

removing a substrate from the silicon surface. For the defect etching part, the

area with three black dots may be expected as the preferentially etched areas

near defects. A further work is important to observe this defect area precisely.

At structuring, the pyramids with inhomogeneous size have been observed

visibly on the silicon surface treated at the higher temperature, in contrast to

the sample treated at room temperature.

7. References[1] Lab instruction M104, TF CAU Kiel.

[2] R.B. Heimann, M.B. Ives and P. Zaya.Influence of surface films on the

development of pits during etching silicon. Journal of Crystal growth 57

(1982) 48-56.

[3] http://silicon.tf.uni-

kiel.de/matwis/amat/def_en/kap_6/backbone/r6_1_1.html#_4

[4] Seidel, L. Csepregi, A. Heuberger. Baumgrtel, Anisotropic etching of

crystalline silicon in alkaline solutions. J. Electrochem.Soc. 137 (1990) 36123626.

[5] Yuan Fulong, Guo Yongfeng, Liang Yingchun, Yan Yongda, Fu

Honggang, Cheng Kai, Luo Xichun. Micro-fabrication of crystalline

silicon by controlled alkali etching. Journal of Materials Processing

Technology 149 (2004) 567572.

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