Electrochemistry and Structures of Silicon Surface Electrochemistry and Structures of Silicon Surface

Ab s tra c t: Accurate control and fabrication of silicon surface structures from atom ic scale to m icrometer scale, which may be random ly associated with surface roughness or have well defined patterns, is critical for the per


D iversity of Electrode Phenom ena
The functionality of most modern electronic de2 vices and m icro electromechanical system s (M EM S) are realized by the m icro structures on silicon surface.
Electrochem ical reactions of silicon are commonly in2 volved in wet cleaning and etching of silicon wafer for control and m anufacturing structures on silicon sur2 face [ 123 ] .The phenomena that can be generated by the electrochem ical reactions on single crystalline sili2 con electrodes are diverse, including oxide formation and passivation, current oscillation, anisotrop ic etch2 ing, form ation of porous silicon etc.Each of these phenomena has extremely rich details w ith comp lex relationship s bet ween structures and p roperties of sili2 con electrodes and bet ween p roperties and experi men2 tal conditions.
The physical and chem ical nature of the silicon / electrolyte interface, in term s of carrier type and den2 sity, charge distribution and transfer, surface reac2 tions, evolution of surface geometry, etc. are deter2 m ined by numerous variables as illustrated in Fig.
1 [ 3 ] .Each of these variables is a continuum of a w ide di mensional range and its effect on the electrode p rop2 erties involves ti m e and m icro surface geometry.
Thus, the possible conditions determ ined by the com2 binations of these variables are li m itless, responsible for the diverse phenomena and comp lex details ob2 served on silicon electrodes.These possibilities also form the basis for the cleaning and etching p rocesses that are w idely utilized in the fabrication of electronic devices.Reactions and surface structures are mutual2 ly related: reactions generally result in form ing and changing of surface structures while surface structures determ ine the nature and distribution reactions.

D issolution Reaction s
In the absence of redox coup les other than those Fig. 3 show s that the current increases exponen2 tially w ith increasing potential from OCP [ 6 ] .It breaks off from the exponential behaviour at larger overpoten2 tials, exhibits a peak, J 1 , and then attains a relative2  In KOH at OCP and at anodic potentials lower than V P , there is no participation of charge carriers and the reactions, hydrogen evolution and dissolution of silicon, are al most 100% chem ical such that the etching of silicon is characterized w ith the dissolution of one silicon atom and the evolution of t wo hydrogen molecules.A t potentials higher than V P , the surface is passivated and both silicon dissolution and hydro2 gen evolution cease [ 9 ] .A t cathodic potentials, hydro2 gen evolution on p 2Si is also chem ical due to the lack of electrons.However, for n2Si at cathodic potentials hydrogen evolution is mainly electrochem ical due to the abundance of electrons from the sem iconductor .which the contributions by kinetic and diffusion p rocesses are equal [ 8 ] .
In KOH solutions the rate li m iting p rocess at OCP is of chem ical nature, i .e. only r 6 is involved.
The electrochem ical p rocesses, that is r 4 and r 5 , are increasingly involved as potential is increased from OCP to V P .A t potential large than V P , the anodic re2 action is li m ited by the dissolution rate of the oxide,  erating conditions.This is often responsible for the someti m es large difference in etch rates that can be found in identical system s from different investiga2 tions.  2 ) The etch rates differ by several or2 ders of m agnitudes for different types of oxides w ith quartz being the slowest and anodic oxide being the fastest, reflecting the large difference in the structure of these oxides; 3) silicon as a solid is extremely stable in HF solutions compared to its oxide, as its dissolu2 tion rate is several orders of magnitude sm aller than even that of quartz [ 3 ] .

1 In HF Solution s
The etch rate of silicon in the absence of an oxi2 dant at room temperature at OCP is very low, on the order of 10 23 Δ / s in concentrated HF solutions ( > 25% HF ) [ 10 ] .It tends to decrease w ith i mm ersion ti me.In HF2 NH 4 F solutions, the etch rate decreases w ith increasing HF at constant NH 4 F concentration and it increases w ith increasing NH 4 F at constant HF concentration [ 11 ] .The etch rate of silicon in HF does not have a clear dependence on dopant concentration.
M echanistically, silicon dissolves mainly electrochem2 ically in HF solutions in which participation of charge carriers is require [ 3 ] .A t OCP the rate of dissolution

2 In A lka li ne Solution s
The most commonly used alkaline solutions have been KOH and EDP ( or EPW ) which is a m ixture of ethylenediam ine ( ED or E) , pyrocatechol( P) and wa2 ter ( W ) [ 16217 ] .O ther solutions such as NHOH 4 , hadrazine, ethanolam ine and tetram ethyl ammonium hydroxide ( TMAH ) are also used.The developm ent and app lication of these etching solutions are dictated by a num ber of factors such as etch rate, anisotrop ic selectivity, corrosiveness to masking materials, sur2 face quality, p rocessing controllability, safety, and more recently environmental i mpact .
Silicon etching in KOH solutions have been ex2 tensively investigated, resulting in a body of informa2 tion that shapes the current understanding of the etch2 ing behavior of silicon in alkaline solutions.The ma2 jor characteristics and the p rincip le reaction p rocesses involved in all alkaline solutions appear to be si m ilar to that in the KOH system although the detail charac2 teristics vary from system to system.Most notably, an alkaline solutions show the sensitivity of etch rate to crystal orientation, which is the basis for anisotrop ic etching.A lso, all these etchants show an etch rate reduction for highly boron doped m aterials.The alka2 line etchants can be made of organic and inorganic so2 lutions, but all of them appear to require the p resence of water to etch silicon at significant rates.
The etch rate of silicon in KOH may vary from as low as 1 Δ / s in dilute KOH ( e. g. 0. 5 mol/L ) at room temperature to as high as 2 000 Δ / s in a con2 centrated solution ( e. g. 40% KOH ) at high tempera2 tures.For a given type and orientation the etch rate is essentially independent of dop ing concentration up to a concentration of about 10  Etch rate is the highest at OCP.A t potentials positive of the passivation potential etching stop s due to the Anisotrop ic dissolution of crystal surface results in the formation of surface contour whose geom etric features depend on the crystal orientation [ 27 ] .During steady state etching the etched surface p rofile exhibits a characteristic shape: convex or concave [ 30 ] .Etching

Fig. 1 1 )
Fig. 1 The major variables that affect the electrochem ical p roperties of silicon electrode 2008 China Academic Journal Electronic Publishing House.All rights reserved.http://www.cnki.netly constant value at J 2 .Exam ination of the surface anodized at different potentials, indicates that form a2 tion of porous silicon occurs in the exponential region but not at potentials more positive than the peak po2 tential .The potential corresponding to the maxi mum slope of the i～V curve is about the upper li m it for formation of uniform porous silicon layer .A t poten2 tials bet ween the m axi mum slope and the current peak, porous layer may still form but its surface cov2 erage is not uniform.V isible hydrogen evolution oc2 curs in HF solutions at anodic potentials in the expo2 nential region anodic of OCP.The rate of hydrogen evolution substantially decreases as potential app roa2 ches the current peak, J l .Hydrogen evolution ceases above the current peak.

Fig. 3
Fig. 3 Current2potential curve of the p + silicon samp le in 1% HF solution w ith potential sweep rate of 2 mV / s

M
any p rocesses in different phases in the sili2 con / electrolyte interface region are involved during electrochem ical reaction, Each of the possible p roces2 ses, as schematically illustrated in Fig.4, in the multi2layer silicon / electrolyte interface region can be the rate li m iting p rocess under certain conditions, as has been summarized in reference[ 3 ].For examp le, the anodic reaction p rocesses on n2Si in the dark is li m ited by the m inority hole transport in the bulk of silicon, that is r 2 .For p 2Si and illum inated n2Si in HF solutions at potentials negative or positive of the current peak, J l , the reaction rate is determ ined by the charge transfer p rocess across the electrode / elec2 trolyte interface, that is, r 4 and r 5 .A t potentials posi2 tive of J l , i .e. the electro polishing region, the rate determ ining step in the anodic reaction is the dissolu2 tion of the anodic oxide fil m , that is r 10 .The dissolu2 tion of the oxides form ed at low fluoride concentrations is mainly kinetically controlled, that is r 10 , while for high fluoride concentrations the p rocess is m ainly dif2 fusion controlled, r 11 .There is a critical concentra2 tion, depending on pH , rotation rate and potential, at © 1994-2008 China Academic Journal Electronic Publishing House.All rights reserved.http://www.cnki.netr 10 , which, unlike in fluoride solutions, is comp letely a surface controlled p rocesses in KOH solutions.

Fig. 4 3
Fig. 4 Schematic illustration of the p rocesses involving the transport of charge and species in the different pha2 ses in the Si/ electrolyte interface region r l ) & r 2 ) majority&m inority carrier transport, r 3 ) transport of holes to the surface, r 4 ) charge transfer across the Hel mhotz layer, r 5 ) electron in jection, r 6 ) chem ical dissolution, r 7 ) oxide formation, r 8 ) ionic transport in oxide, r 9 ) injection of oxidants, r 10 ) dissolution of oxide, r 11 ) mass transport in elec2 trolyte 3 Ra te of S ilicon D issolution The t wo p rincip le etching solution system s for sil2 icon are HF solutions and alkaline solutions [ 3 ] .This is because silicon is inert in aqueous solutions due to the formation an insoluble surface oxide, excep t for HF solutions or alkaline solutions in which the oxide is soluble.Various chem ical agents can be added into these t wo solutions to control etch rate, etch selectivi2 ty, solution stability, and quality of the etched sur2 face.One major difference bet ween these t wo system s is that the etch rate of silicon in HF solutions is si m i2 lar among the various crystalline orientations, i .e. isotrop ic, while in alkaline solutions it strongly de2 pends on the crystalline orientation, that is anisotrop2 ic.Another difference is that silicon oxide, which m ay be p resent on silicon surface p rior or during an etching p rocess, etches fast in HF solutions while it etches very slow ly in alkaline solutions relative to the etch rate of silicon.Several general points m ay be made regarding etch rate of silicon: i) silicon can etch at a w ide range of rates, as much as 9 orders of magnitude; ii)

Fig. 5
Fig. 5 Etch rate of different silicon oxides as a function of HF concentration The etching of silicon oxides is particularly i m2 portant in silicon technology.Deposited silicon oxides are used as an dielectric layer, passive layer or mask2 ing m aterials for device fabrication and as a native ox2 ide it is an essential part of the surface condition.In particular, the oxide formed on silicon during electro2 chem ical p rocesses p lays a critical role in the p roper2 ties of silicon electrode; most of the electrode phenom2 ena observed on silicon are associated w ith the forma2 tion and dissolution kinetics and the p roperties of the surface oxides.Fig. 7 show s the etch rate of different types of silicon oxides as a function of HF concentra2 2008 China Academic Journal Electronic Publishing House.All rights reserved.http://www.cnki.nettion.Several general rem arks can be m ade on the da2 ta: 1 ) Etch rate of all the oxides increases w ith HF concentration; is low because there are few carriers.The dissolution rate increases w ith potential to generate carriers at the surface as shown in.Fig.4.Addition of oxidants in HF solution can greatly increase etch rate.The most used oxidants for etching are HNO 3 , B r 2 , H 2 O 2 and CrO 3 .In particular, HF HNO 3 etching system is the most used isotrop ic etch2 ant for silicon[ 12213 ]  .The m ixtures of these t wo con2 centrated acids can be diluted w ith water or other di2 lutant to give a w ide range of etch rates.On the other hand, the HF2C rO 3 etching system is w idely used for defect etching and delineation of junctions bet ween silicon layers of different dop ing concentrations[ 14215 ]  .

3 .
rate of boron doped silicon drastically decreases w ith increasing dopant concentration, particularly for boron doped materials.Thus, reduction by as much as three orders of m agnitude can be obtained by varying the boron concentration This feature has been w idely used as an etch2 stop technique for the fabrication of silicon m icro2 structures.The etch rate of all silicon materials in KOH de2 pends, to a varying extent, on potential[ 20222 ]  .How2 ever, the contribution of electrochem ical reactions, relative to chem ical reactions, in etch rate is sm all .
Fig. 6 show s the condition for occurrence of PS forma2 tion and electropolishing [ 6 ] .The three regions in rela2 tion to current density and HF concentration are es2 sentially independent of the silicon substrate dop ing type and concentration, which means that the differ2 ences in sem i2conducting p roperties of the silicon samp les have little effect on the occurrence of these regions.The various parameters involved in PS forma2 tion such as potential, dop ing, and illum ination, af2 fect the occurrence of the different regions through their relation to the current density.Low current and high HF concentration favours PS formation while high

Fig. 6
Fig. 6 Occurrence of different regions as a function of HF concentration The rich details of PS morphology are determ ined by the numerous factors involved in the anodization.Generally, p 2Si and n2Si have distinct differences in the correlation bet ween the form ation conditions and PS morphology.A lso, among all formation conditions dop ing concentration appears to show the most clear functional effect on morphology.A s the most quantifi2 able param eter the diameter of pores appears, as a rough generalisation, to have certain qualitatively cor2 relation w ith the various morphological aspects.More detailed descrip tion of the morphology of porous sili2 con and discussion of the formation mechanism s are docum ented in a recently published book: " Electro2 chem istry of Silicon and Its Oxide" [ 3 ] .