Anisotropic etching of silicon in a complexant redox alkaline system

Sensors and Actuators B 58 (1999) 438–449

Anisotropic etching of silicon in a complexant redox alkalinesystem

Carmen Moldovan a,*, Rodica Iosub a, Dan Dascalu a, Gheorghe Nechifor b

a National Institute for R&D in Microtechnologies, P.O. Box 38 160, R72225, Bucharest, Romaniab Faculty of Industrial Chemistry, Politehnica Uni6ersity of Bucharest, 1 Polizu, Bucharest, Romania

Received 14 September 1998; received in revised form 2 February 1999; accepted 8 February 1999

Abstract

This paper presents the results from the investigation of the chemical anisotropic etching of single-crystal silicon �100� in thefollowing solutions: KOH, K3[Fe(CN)6] 0.1 M, K4[Fe(CN)6] · 3H2O 0.1 M, KNO3 0.1 M and or complexant added. Thecomplexants added in KOH solution were: calix[4]arenes, phenols and ether dibenzo-18-crown-6. The reaction mechanism, theetch rate, the roughness and the hillocks are analysed. The results allow us to use the redox system and/or the organic complexantsto monitor the etching process, to obtain a smooth silicon surface with increased etch rate and to utilize the usual mask materialresistant at the new etchants. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Anisotropic etching; Redox system; Organic complexants

1. Introduction

After many years of research and work in the an-isotropic etching field, the anisotropic etching of single-crystal silicon in alkaline solutions became a keytechnology for micromachining due to the strong de-pendence of the etching rate on the crystal directionand the boron concentration.

Increasing attention of anisotropic etching for crys-talline silicon has been paid after recognizing its capa-bilities for micromachining 3D-structures.

The silicon etching in basic medium presents anenormous interest motivated by the encountered appli-cations in sensors and microsystems field. Microme-chanical structures fabricated using bulkmicromachining concepts are typically constructed ofeither silicon crystal or of composite materials de-posited or grown on the silicon surface. Wet anisotropicetching is a process of preferential directional etching ofmaterial using liquid source etchants.

The most frequently used etchant is potassium hy-droxide for many reasons: an important etch rate, a

strong basic character, the strong dependence of theetching rate on the crystal direction and the boronconcentration.

The following aspects are interesting for the research:The hydrogen generation rate from etching solution,the determination of the silicon etch rate, the roughnessof the surface.

Up to now, two aspects of the silicon etching havebeen accepted by everybody [1,2]:(a) The general reaction:

Si+2H2O+2HO−�SiO2(OH)22− +2H2;

(b) The intrinsic etching process steps:1. diffusion of the reactant molecules through the

boundary layer to the silicon surface;2. adsorption of reactant molecules on the solid sur-

face of silicon;3. surface reaction;4. reaction product desorption;5. diffusion of by-products back across the boundary

layer into the bulk of the solution.Step 3 permanently concentrates the researchers’ at-

tention and the intimate mechanism is not yet com-pletely understood.

Our contribution consists in introducing special com-pounds in a classic anisotropic etching system (KOH/

* Corresponding author. Fax: +40-1-2301553.E-mail address: [email protected] (C. Moldovan)

0925-4005/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII: S 0 9 2 5 -4005 (99 )00124 -0

C. Moldo6an et al. / Sensors and Actuators B 58 (1999) 438–449 439

water) which allows us to monitor the etching process,to increase the etch rate and to improve the surfacequality.

We added at the KOH solution a redox system:K3[Fe(CN)6] 0.1 M, K4[Fe(CN)6] · 3H2O 0.1 M, KNO3

0.1 M [3] and or a complexant (ether-crown,calix[4]arenes, phenols).

2. Reaction mechanism

Considering the general mechanism presented in [1,2]we accept the following reactions:

(1)

We suppose the following reaction is the most probableto occur:

(3)

We assume that the protons from the aqueous solutionaccept the electrons appearing during the process. Theprotons exist in the aqueous solution due to theequilibrium:

HOH+HOH ? HO− +H3O+

This equilibrium, tending towards the left because thevalue of pH (pH\12), it participates at the mediumreactions. The continuous change of place of the equi-librium to the right can be explained by hydroxide ionsconsumption in the reactions with the silicon surface andthe reaction of the protons (H3O+) with the free elec-trons:

H3O+ +e−�H2O+1/2H2

2.1. The role of the complexants and the redox system

The complexants can contribute to the etch rate, both,by accepting the electrons from the medium or byhydroxide ions consumption (4)

(4) The phenol type ion may intervene in the solvabilityof the silicic acid still at the silicon interface. The reaction(3) is likely to become (5):

(5)

Considering the reactions (1)–(3) we concluded thatthe improvement of the silicon anisotropic etching pro-cess could be realised by adding new compounds into theetching system. To choose theses compounds we mustconsider: (a) the acid-basic character; (b) the electrontransfer capacity (oxido–redox character); (c) the com-plexant character. The new compound must respect thenecessity of hydroxide ions generation into the system.OH− must be generated in important quantities and tobe accessible (free).

In our work a few aspects are followed: the process-pro-duced electrons capture, the increase of the availablequantity of hydroxide ions on the surface by complexingof M+ ions and solvability of Si(OH)4.

During the silicon etching reaction electrons andSi(OH)4 are produced and alkaline ions OH− are con-sumed. The electrons are captured in an aqueous mediumthe most probable by protons (H3O+) and hydrogenatoms are generated. The hydrogen atoms themselves canattack the silicon, or combine between them and molec-ular hydrogen results. We propose a schema of the processwith one input and two outputs to better explain our pointof view (I or II):

Hydrogen atoms generation involves the etching accel-eration because [H] atoms attack the silicon and formsilan bonds at the surface that initiate afterwards anotherchain of reactions, causing the solvability of silicon. Tobetter explain the mechanism, the reaction (*) is detailedas follows (schema III):

(2)

C. Moldo6an et al. / Sensors and Actuators B 58 (1999) 438–449440

If the steps a–f are accepted we can understand theeffect of the oxidants (e− acceptors) on the etch rate. Inour case the redox buffer (**) fix the electrons e− andthe [H] atoms quantity is reduced resulting in thestabilisation of the silicon etch rate and the improve-ment of the roughness.

(**) [Fe(CN)6]3−/[Fe(CN)6]4−

[Fe(CN)6]3− +1e−� [Fe(CN)6]4−

In this way the rate of H2O/HO− from the solutionis consumed by water hydrated to complex anion[Fe(CN)6]4− · 3H20 which is very stable resulting in avery quick decrease of H2O/HO− and stopping thereaction. The addition of KNO, allows continuing thereaction with a greater rate compared to the KOH 4.5M solution. It is an autocatalysis reaction, so, the etchproducts initiate a self reaction that continues in thepresence of the nitrous acid (HNO2) as impuritiestraces.

HNO2+HNO3�N2O4+H2O

N2O4�2NO2

2NO2+2e−�2NO2−

2NO2− +2H+�2HNO2

The nitrous acid (HNO2) generated in the reactionenters again in the reaction with the nitric acid from thesolution and the process is autocatalytic. As long as thenitric acid is regenerated in this reaction, the oxidantpower is due to the quantity of undissociated nitric acid(HNO3).

The pH value was found to be the most significantfactor affecting the etching characteristics. The schemaIII also shows, the needs of increasing the OH− ions atthe silicon surface.

We obtained the increase of OH− ions quantity onthe surface, involved in the silicon etching and in[Si(OH)4] solvability by complexing of K+ with: (A)ether-crown; (B) calix[4]arenes.

Any of the complexants (A, B) ‘immobilize’ the ionK sufficiently to create an increase of the OH ionquantity available at the silicon surface.

K+OH− +A (or B)�OH−free+K+A (or K+B).

The effect of very small quantities of A or B complex-ants is absolutely remarkable on the etch rate and onthe surface roughness and can be explained:

The complexant A (ether crown) has a surfactantcharacter, resulting in the complexing of K+ ions in theneighborhood of the silicon surface where OH− ionsbecome more accessible.

The complexant B (calix[4]arene) has a tensioactive,surfactant character and has phenol groups (similar topyrocatechol) that participate in removing Si(OH)4from the surface by complexing in conformity with thegeneral mechanism. The extremely favorable effect ofcalix[4]arene on the silicon etch rate is done to its tripleeffect: K+ ions complexant, tensioactive character andSi(OH)4 complexant.

3. Experiments

3.1. The experiment equipment

We use an equipment as shown in Fig. 1. It consistsof: (1) a double wall three-necked vessel; (2) a ther-mostated waterball and (3) a Biuret. The vessel allowsthe immersion of a 4¦ teflon wafer holder.

The vessel is fitted with a water-cooled reflux con-denser to keep the concentration of etching solutionconstant.

3.2. Etching conditions

We used a solution of KOH and water as the basicetchant. The KOH produced by Merck was supplied in85 wt.% pellets. The KOH concentration was variedwithin the range 2–4.5 M. The selected temperatureswere in the range 60–100°C. The stability of the tem-perature during the etching was 0.1°C. To the KOHsolution we added a redox system and/or a complexantorganic compound. We studied the silicon etch rate,roughness, hillocks formation.

3.3. Materials

The organic compounds used are: ether dibenzo-18-crown-6, phenol, nonyl-phenol, p-tert-butyl phenol,calix[4]arene, p-tert-butyl-calix[4]arene, azo-calix[4]-arene [5].

Fig. 1. Experimental equipment: (1) double wall three-necked vessel;(2) thermostated waterbath; (3) biuret.


New macrocyclic complexants (p-tert-butylcalix[4]arene, azo-calix[4]arene) are analysed.

The azo-calix[4]arene is a K+ complexant, indicatorof pH and it contains three types of functional grouphaving a well-defined role:1. the hydroxyl groups complexants for K+

2. the azo-groups acceptors for the new formed hydro-gen atoms

3. the SO3H groups which increase the solubility of thecalix[4]arene and its derivatives

The tert-butyl calix[4]arene is a complexant but it isinsoluble in alkaline medium. A soluble calix-arenebearing soulphonate groups is obtained through diazo-tation reaction.

The results concerning the acceleration of the etchrate and the improvement of the quality surface at themoment of addition of calix[4]arenes in the KOH solu-tion encourage us to use other organic compound (phe-nol, nonyl-phenol, p-tert-butyl-phenol) because theyare substances easy to obtain and cheaper. We studiedthe influence of these compounds on the etch rate,roughness, hillocks [4].

Phenol has a weak tensioactive and surfactant char-acter. The nature of the substitute from ‘para’ position

was varied and the influence on hillocks formation isstudied. P-tert-butylphenol has a medium tensioactiveand surfactant character, and nonyl-phenol is anantifoamy.

If the organic compound has a great tensioactivecharacter, the etch rate increases proportionally and thesurface quality is improved.

3.4. Preparation of samples

3‘‘, n�100� 5–7 Vcm and p�100� 14–18 Vcm blankand patterned wafers were used. The blank wafers werecleaned as follows: ultrasound H2SO4:H2O2; BHF 10:1,D.I. water 18 MVcm. The patterned wafers wereprepared:1. 2 mm BPSG deposition or 3000 A SiON deposition;2. Photolithography, BPSG etching (BHF 10:1) or

SiON etching (planar plasma, CF4+O2 tc=10min);

3. cleaning: ultrasound H2SO4:H2O2: BHF 10:1, D.I.water 18 MVcm;

4. Both blank and patterned wafers were anisotropi-cally etched in various etchants and conditions.

Using the equipment from Fig. 1, the wafers pre-pared as shown before are introduced one by one in theetching solution in vertical position. using one waferteflon holder

After the wafer immersion in the etchant, the hydro-gen elimination is measured in volume and time. Afterthe etching time needed, the wafer is taken out of thesolution, it is cleaned in D.I. water and characterized(optical, SEM, AFM, thickness measurement).

4. Results and discussions

4.1. Redox system

We define the redox system: SIRED=K3[Fe(CN)6]0.1 M, K4[Fe(CN)6] · 3H2O 0.1 M and KNO3 1 M. Thechemical etch rate of the KOH 4.5 M+redox system(SIRED) is 25% faster compared to KOH 4.5 M solu-tion at equal temperatures. The proposed solution isnot toxic but it is corrosive. Both, the etch rate ofSi�100�n etched in 4.5 M KOH and the etch rate ofSi�100�n etched in 4.5 M KOH+redox system, func-tion of the temperature [°C] are presented in Fig. 2. Theredox system allows the utilization of BPSG as maskmaterial.

Remark: we presented in this paper only the relevantresults for our proposed topic. We studied the be-haviour of all the organic compounds (A–G) and theredox system in point of view of etch rate, hydrogenelimination, hillocks but only the better or relevantresults are presented.


Fig. 2. Etch rate of silicon n�100� and p�100� in 4.5 M KOH+re-dox system.

Fig. 4. SEM picture of a p�100� silicon etching profile, etched in 4.5M KOH+redox system+complexant, at 80°C; (a) ether crowncomplexant; (b) calix[4]arene complexant; (c) simple 4.5 M KOH.

4.2. Complexants

4.2.1. Calix[4]arenes and ether dibenzo-18-crown-6The chemical etch rate of the KOH 4.5 M+ether

crown complexant is 25% faster compared with KOH4.5 M solution at equal temperatures and 50% fasterusing calix[4]arene complexant. Both, the etch rate ofSi�100�n and the etch rate of Si�100�p, function of thetemperature [°C] are presented in Fig. 3.

Using the ‘neutralization method’ from the potentio-metric analysis, the ‘titre’ T [6] of the used solutions iscalculated. The solutions of KOH and KOH+calix[4]arene are compared before and after 1-, 2-, 3-,4-, 5- and 6-h silicon etching on blank wafers. The usedsolution quantities are equal and the silicon sampleshave equal areas.

From [6] concentration of KOH and the concentra-tion of OH− ions in solutions before and after siliconetching were calculated. The results show that the

Fig. 3. Etch rate of silicon n�100� and p�100� in 4.5 M KOH+com-plexants.

quantity of KOH consumed in the etching process isincreased 50% for the KOH+calix[4] arene solutioncompared to simple KOH after every hour of etching.That means an increase of the OH− free ions in thesolution, resulting an increase of the etch rate. It is aqualitative method which indicates us the evolution ofthe OHfree

− ions during the etching. It is not a sensitivemethod.

For the proposed solution the chemical silicon etchrate is increased, the walls profile for the patterned


wafers is better (Fig. 4a, b), the lateral underetching isnegligible (Fig. 4b) and the roughness (Fig. 5a, b) isreduced (one order of magnitude smaller) compared tothe usual 4.5 M KOH (Fig. 4c, Fig. 5c) at the sametemperature. For the Fig. 4, Fig. 5, the two systems(redox and complexant) are mixed. The goal is toobtain an improvement of the etching process without aclear separation between the effects of the twotechniques.

The complexing of the silicates formed at the siliconsurface due to the phenol groups (accepted in [2]) leads

Fig. 6. The elimination rate of H2 for KOH and KOH+azo-calix[4]arene at 80°C.

Fig. 5. Roughness of a silicon wafer 200 mm thickness, etched in 4.5M KOH+redox system+complexant, at 80°C (3D-AFM investiga-tion): (a) calix[4]arene complexant; (b) ether crown complexant; (c)simple 4.5 M KOH.

to the solvability and the rapid diffusion of the silicatesfrom the silicon surface. Therefore, this process favoursnew attacks of the OH− ions on the silicon surface,which is free from the reaction products.

The catalyses character of the calix[4]arene is due tothe small stability of the complexant-silicon pair in thestrong alkaline medium, determining the reformation ofthe free calix[4]arene and its returning to the interfacesilicon alkaline solution.

For the calix[4]arenes the increase of the tensioactiveorganic compound quantity in the etching solutiongives the inhibition of the silicon etching probablybecause the compounds cover the silicon surface andthan diminish the access of the hydroxide ions (OH−)at the wafer surface.

Calix[4]arene is resumed to the silicon surface afterthe interaction with the silicon acid and the K+ ions.

The results concerning the addition of organic com-pounds in the KOH solution (phenol, nonyl-phenol,p-tert-butyl-phenol, calix[4]arene, p-tert-butyl-calix[4]arene, azo-calix[4]arene) have as main objectives:to follow the influence on the etch rate, the roughnessand the understanding of the mode of molecular hydro-gen formation.

Remarkable results were obtained in point of view ofetch rate increase and hillocks elimination using azo-calix[4]arene. The results concerning the acceleration ofthe etch rate at the moment of azo-calix[4]arene addi-tion in the etching system are reflected by the volume ofeliminated H2 (Fig. 6). The H2 elimination rate is aqualitative measurement. We have not defined an accu-rately relation between the etch rate and H2 eliminationrate but we remarked an increase of the etch rate withthe increase of the H2 eliminated volume.

An important aspect of the experiment is the spec-trophotometric determination of the azo-calix[4]arenereduction (Fig. 7). The reduction reaction:

−N=N−orange

+2[H]�−NH−NH−no color


takes place only in the presence of atomic hydrogen.That shows that azo-calix[4]arene and atomic hydrogenare simultaneously found on the silicon surface.

The lose of color of the azo-calix[4]arene shows thatthe azo-calix derivative captures the atomic hydrogenand than the electrons appearing in step a, scheme IIIare taken by the protons in step b, scheme III and thehydrogen atoms formed are added at to the azo-groupwhich is changed to hydroazo-group.

The formation of the molecular hydrogen (step c,scheme III) cannot be stopped because of the hydrogenquantity, which is greater than the azo-calix[4]arenefrom the solution medium.

The experiments performed prove that the azo-calix[4]arene passes in hydro-azo-calix[4]arene by thecapture of the hydrogen atoms and not of the molecu-lar hydrogen.

Theses experiments consist in the hydrogenation ofthe molecular hydrogen in alkaline medium by bubble.The stability of the colorant more of 72 h in alkalinesilicon etching system was found.

The experiments and the observations ask for a newestimate of the classical mechanism [1,2] of reactionabout the electrons e− capture from the silicon surfaceby way of water molecules and justify step b, schemeIII.

The acceleration of the etching process using azo-calix[4] arene can be explained if two azo-calix[4]arenecharacteristics are accepted:1. the complexing capacity of the K+ and

SiO2(OH)22− on the silicon surface;

2. the tensioactive character.The tensioactive character of the azo-calix[4]arene

favours the etching process in two different ways:1. it determines the gathering of the complexant com-

pound at the silicon surface;2. it stabilizes the small dimension hydrogen bubbles

and it does not permit the gathering of H2 on thesilicon surface or the stirring of the system.

Fig. 8. Optical density in visible domain of azoic colorant.

The two effects lead to a uniform etching and arhythmically elimination of the hydrogen bubbles into aquasi laminar flow. The azo-calix[4]arene reduction inpresence of silicon in an alkaline medium was confi-rmed by the experiments of reducing an azoic bluecolorant under the same conditions.

The absorbtion spectrums in the visible domain ofthe azoic colorant are presented in Fig. 8. BPSG.Si3N4, SiON were used as mask materials.

4.2.2. Phenol compounds KOH+phenolThe hydrogen elimination rate function of the phenol

concentration is presented in Fig. 9. The etch rate ofsilicon in KOH+phenol is presented in Fig. 10. Wenoticed a decrease of the etch rate with the decrease ofthe concentration (Fig. 10) and a decrease of the H2

volume.If the organic compound has a great tensioactive

character, the etch rate increases proportionally.If for the calix[4]arenes the increase of the tensioac-

tive organic compound quantity in the etching solutiongives the inhibition of the silicon etching probably

Fig. 9. Hydrogen elimination rate for silicon n�100� etched in KOH4 M+phenol, at 80°C.

Fig. 7. Optical density in visible domain of azo-calix[4]arenic col-orant.


Fig. 10. Etch rate of silicon n�100� in KOH 4 M+phenol atdifferent concentration, at 80°C.

Table 1Hillocks density and size versus compounds added in KOH 4.5 M at85°C

Compound added in KOH 4.5 M at 85°C Hillocks

Medium density,Phenol 10−2 (M l−1)small sizeMedium density,Phenol 10−3 (M l−1)small sizeSmall density, bigPhenol 10−4 (M l−1)size

Phenol 10−5 (M l−1) Small density, bigsize

p-tert-Butylphenol 10−3 (M l−1) Medium density,medium size

p-tert-Butylphenol 10−4 (M l−1) Very smalldensity, small sizeSmall density,p-tert-Butylphenol 10−5 (M l−1)small sizeVery smallAzo-calix[4]arene 10−5 (M l−1)density, small size

Redox system (K3[Fe(CN)6] 0.1 M, Very smallK4[Fe(CN)6]3H2O 0.1 M, KNO3 0.1 M) density, small size

because the compounds cover the silicon surface andthen diminish the access of the hydroxide ions (OH−)at the wafer surface, the phenol has a different be-haviour because it has a weak tensioactive character. Itis not returned to the silicon surface after the interac-tion with the silicon acid like the calix[4]arene but itreacts with silicon acid forming stable reactionproducts.

KOH+p-tert-butyl-phenol

The etch rate of silicon n�100� in KOH with differ-ent concentrations [7] compared to KOH (differentconcentrations)+p-tert-butyl phenol (10−4 Ml−1) isshown in Fig. 11.

For p-tert-butyl phenol 10−4 Ml−1 added in KOH,the etch rate is not increased compared to KOH but thesurfaces are very smooth due to the modification of thesurface tension forces and to the effect of the siliconwettability. P-tert-butyl phenol has a medium tensioac-tive character but it is a good surfactant and this could

explain the low etch rate and the excellent quality of theetched surfaces.

The solution temperature changing shows the samebehaviour of the solutions of KOH+organic com-pound or KOH+redox system compared to simpleKOH. That proves that the adding of the organiccompound and/or redox system does not change thebehaviour of the etching solutions versus temperature.The increase of temperature leads to the increase of theetch rate (ex. Fig. 2, Fig. 11) for all the solutions. Si3N4,SiON were used as mask materials.

4.3. Hillocks determination

Considering the hillocks formation on the siliconsurface an intrinsic mechanism of the anisotropic etch-ing [8–10] we characterized every solution in point ofview of hillocks. The hillocks are evaluated by opticalmicroscopy and by SEM. They are evaluated in densityand size [11].

Because the phenol can be used in different concen-trations and then the different results can be compared,we have evaluated the hillocks size and density (Table1, Fig. 12).

The real dimensions of the area presented in thefigures (Figs. 12–16) are 150×120 mm2.

The SEM investigations (Fig. 17, Fig. 18) of thehillocks show the typical pyramids with octagonal basecharacterizing the silicon surface etched in alkalinesystems [8].

P-tert-butyl phenol is soluble at 10−4–10−5 M l−1

concentration. Smooth surfaces are obtained for p-tert-butylphenol 10−4–10−5 M 1−1 concentration (Table1, Fig. 13).

Fig. 11. Etch rate of silicon n�100� etched in KOH 4.5 M and KOH4.5 M+10 4 M l−1 p-tert-butyl phenol.


Fig. 12. Top view microscop picture of n type n�100�Si etched 1 h at 85°C in 4.5 M KOH+phenol at different concentration: (a) 10−2 M l−1

phenol; (b) 10−3 M l−1 phenol; (c) 10−4 M 1−1 phenol; (d) 10−5 M 1−1 phenol.

The azo-calix[4]arene is used at 10−5 M l−1 concen-tration and is soluble. We obtained small hillocks witha very small density, resulting a smooth surface (Fig.14). The aspect of the silicon surface presented infigures is relevant and characterizes the entire surface ofa 3¦ wafer. For p-tert-butylphenol and azo-calix[4]arene very good results are obtained: Themacroscopic aspect of the wafers etched l h at 80°C is‘mirror polished’ and the microscopic aspects are al-most ‘free of hillocks’ (Fig. 13, Fig. 14).

Smooth surface is also obtained, using the redoxsystem added at KOH 4.5 M (Fig. 16), working at85°C.

The etching starts on the hydrophilic surface [12].The hydrophobic surface leads to a rough surface.Silicon is a hydrophobic surface due to the watercondensation and deposition of small dust particles.The cleaning wafer procedures before the anisotropicattack are not perfect and the silicon surfaces can resthydrophobic. The surfactants adding in the alkalinemedium helps the surfaces become or resting hy-drophilic. The hydrophilic surfaces can prevent thehillocks formation. A hydrophilic surface is a smooth

surface, also we can explain the good results obtainedin point of roughness using compounds with a greatsurfactant character (ex:azo-calix[4]arene). Our surfac-tants increase the ability to supply HO-ions on thewafer surface leading to minimization of hillocks andincreasing the etch rate. Si−H bonds (schema III)

Fig. 13. Top view microscop picture of n type �100�Si etched 1 h at85°C in 4.5 M KOH+10−4 M l−1 p-tert-butylphenol.


Fig. 14. Top view microscop picture of n type n�100�Si etched 1 h at85°C in 4.5 M KOH+10−5 M 1−1 azo-calix[4]arene.

Fig. 16. Top view microscope picture of n type n�100�Si etched 1 hat 85°C in 4.5 M KOH+redox system.

1. it increases the OHfree− ions concentration at the

silicon surface by using the complexants of K+;2. the complexants used (A–G) have a complexant

and surfactant character. The calix[4]arenes have acleaning Si surface character too (solvability ofSi(OH)4) and azo-calix[4]arene is the best solutionof silicon alkaline etching which offer an importantincrease of the chemical etch rate and an excellentroughness;

3. it minimizes the roughness and prevents the hillocksformation;

4. it allows the utilization of usual mask materialsConsidering the two systems: redox and complexant,

we can conclude that the best results in point of view ofhillocks minimization were obtained using: the redoxsystem (K3[Fe(CN)6] O. 1M, K4[Fe(CN)6] 3H2O 0.1 M,KNO3 0.1 M), azo-calix[4]arene and p-tert-butyl phe-nol. In point of view of etch rate acceleration the bestresults were obtained using: the redox system(K3[Fe(CN)6] 0.1 M, K4[Fe(CN)6] · 3H2O 0.1 M, KNO3

0.1 M), ether-dibenzo-18-crown, calix[4]arene, azo-calix[4]arene.

determine a hydrophobic surface. The H2 generationand elimination lead to a hydrophilic surface. The H2

volume eliminated from the solution is a quantitativemeasurement of the hydrophobic–hydrophilic surfacecharacter. The H2 volume increase (Fig. 6) leads to asmooth surface (Fig. 14).

5. Conclusions

New anisotropic etchants of silicon are used. Theredox system has the following advantages:1. it increases the chemical etch rate by e− capture;2. it is not toxic;3. it minimizes the roughness;4. it allows the utilization of BPSG as mask material.

The macroscopic and the microscopic roughness ofthe silicon etched were very good. The etching samplekeeps the aspect of a mirror polished wafer independentof the silicon-etched thicknessThe complexant systemhas the following advantages:

Fig. 15. Top view microscop picture of n type n�100�Si etched 1 h at85°C in 4.5 M KOH.

Fig. 17. SEM picture of hillocks of silicon n�100� etched 1 h at 85°Cin 4.5 M KOH+10−4 M l−1 phenol.


Fig. 18. SEM picture of hillocks of silicon n�100� etched 1 h at 85°Cin 4.5 M KOH+redox system.

[11] H. Schroder, E. Obermeier, A. Steckenborn, Formation, preven-tion and removal of micropyramids on KOH etched {100}silicon, Proc. 9th Micromechanics Europe Workshop MME’98,Ulvik in Hardanger, Norway, June 3–5, 1991, pp. 28–31.

[12] T. Abe, A contamination-free microstructure in a humid envi-ronment by means of a combination of hydrophilic and hydro-phobic surfaces, J. Microelectromech. Syst. 7 (1998) 94–101.

Biographies

Carmen Moldo6an was born in Craiova, Romania.She received the M.S. degree in electrical engineeringfrom the ‘Politehnica’ University of Bucharest in 1983.She was a Ph.D. Student at the Politehnica Universityfrom 1995. In 1983, she joined Microelecronica S.A.,Bucharest. She has been engaged in semiconductortechnologies. In 1993 she joined the Institute of Micro-technology, Bucharest and she is involved in the R&Dof micromachining technologies. From 1995 she hasbeen Head of the R&D Laboratory of Microfabrica-tion. Carmen Moldovan is a member of the IEEE andof the Romanian Academy Commision of Science andTechnology of Microsystems.

Rodica Iosub was born in Bucharest, Romania. Shereceived the M.S. degree in chemistry from the Univer-sity of Bucharest in 1980. She was a teacher from 1980to 1983. In 1983, she joined Microelecronica S.A.,Bucharest. She has been engaged in semiconductortechnologies. In 1993, she joined the Institute of Micro-technology, Bucharest and she is involved in the R&Dof micromachining technologies.

Dan Dascalu received the M.Sc. and Ph.D. degrees inelectronics from the Technical University of Bucharestin 1965 and 1970, respectively. From 1965 he served theDepartment of Electronics and Telecommunicationsfrom Technical University of Bucharest, where he hasbeen a full professor from 1990. From 1968 to 1969, hewas a research fellow at the University of Birmingham,UK. The industrial and academic activity of ProfessorDascalu covers semiconductor electron devices: techno-logical research of metal-semiconductor contacts in mi-croelectronics, prototyping of microwave digital radios,microwave generating diodes, space-change limited,physics and technology of microsystems. He is theauthor of three scientific monographs and of over 150papers which have been published or communicated atinternational conferences. Since 1991, Professor Das-calu has served also as general manager of the Instituteof Microtechnology, coordinating the linkage betweenmicroelectronics and microtechnology. From 1993, hehas been a full member of Romanian Academy ofScience. Professor Dascalu is coordinating differentjoint European projects within the TEMPOS, PECO,

Our research allows us to obtain a stable and wellcontrolled process for the silicon etching. Smooth sur-faces and shorter duration for the silicon etching wereobtained, new compounds to be added in alkalinesystems were tested and the reduction of azo-calix[4]arene in alkaline medium and in presence ofsilicon is experimented.

References

[1] L. Ristic, Sensor Technology and Devices, Artech House,Boston, pp. 67–68.

[2] H. Seidel, L. Csepregi, A. Heuberger, H. Baumgartel, An-isotropic etching of crystalline silicon in alkaline solutions, J.Electrochem. Soc. 137 (1990) 3612–3632.

[3] C. Moldovan, R. Iosub, D. Dascalu, Gh. Nechifor, C. Radu, Aninvestigation of an alkaline syatem for silicon anisotropic etch-ing, Abstract Book, workshop of physical chemistry of wetchemical etching of silicon, Holten, Netherlands, 1998, pp. 21–22.

[4] C.D. Gutsche, B. Dhawan, K. Hyum No, R. Mathukrishnan,Calixarenes. 4. the synthesis, characterization, and properties ofthe calixarenes from p-tert-butylphenol, J. Am. Chem Soc. 103(1981) 3782–3792.

[5] S. Shinkai, K. Araki, J. Shibata, D. Tsungawa, O. Manabe,Autoaccelerative diazo coupling with calix[4]arene: substituenteffects on the unusual co-operativity of the OH groups, J. Chem.Soc. Perkin Trans. 1 (1990) 3333–3337.

[6] L. Pauling, General Chemistry, W.H. Freeman, San Francisco,1970 (translated Bucharest 1972), pp. 461–462.

[7] K. Sato, M. Shikida, Y. Matsushima, T. Yamashiro, K. Asaumi,Y. Iriye, M. Tamamoto, Characterization of orientation-depen-dent etching properties of single-crystal silicon: effects of KOHconcentration, Sensors and Actuators A 64 (1998) 87–93.

[8] S. Tan, M.L. Reed, H. Han, R. Boudreau, Mechanism of EtchHillock formation, J. Microelectromech. Syst. 5 (1996) 65–71.

[9] P.M.M.C. Bressers, J.J. Kelly, J.G. E. Gardeniers, M. Elwen-spoek, Surface morphology of p-type (100) silicon etched inaqueous alkaline solution, J. Electrochem. Soc. 143 (1996) 1744–1750.

[10] M. Elwenspoek, Stationary hillocks on etching silicon, proceed-ings, 9th Micromechanics Europe Workshop MME’98, Ulvik inHardanger, Norway, June 3–5, 1991, pp. 70–73.


NEXUSEAST, NEXUSPAN and COPERNICUS pro-grammes. He is a senior member of the IEEE.

Gheorghe Nechifor was born in 1960. He received theM.S. degree in chemistry from the ‘Politehnica’ Univer-sity of Bucharest in 1985. He is a Ph.D. at the Babes-Bolyoi University of Cluj Napoca from 1996. He has

been Associate Professor to the Department of Analyt-ical Chemistry of the Politehnica University Bucharestfrom 1986. Gheorghe Nechifor is member of the Roma-nian Association of Analytical Chemistry, the Roma-nian Association of Colloid and Surfaces Science andthe Romanian Society of Chemistry. He is involved inselective membranes research activity

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Documents

Anisotropic etching of silicon in a complexant redox alkaline system