SiO2 heterostructures. Application to the fabrication of silicon microsensors

Colloids and Surfaces, Elsevier

Science

53 (1991)

Publishers

169-182

169

B.V., Amsterdam

Analytical control of the preparation grafting of Si/SiO, heterostructures. fabrication of silicon microsensors

and the chemical Application to the

Yolanda Duvault-Herrera”, Nicole Jaffrezic-Renault”, Dominique Morelb, Joseph Serpinetb, Jean-Louis Duvault” and Guy Hollinger” “LPCI URA CNRS 404, Ecole Centrale de Lyon, B.P. 163,69131 Ecully Ceden (France) bLSA URA CNRS 435, Universitd Lyon I, 69622 Villeurbanne Cedex (France) “LEAME

URA CNRS 848, Ecole Centrale de Lyon, B.P. 163,69131

(Received

5 April 1990; accepted

13 June

Ecully Cedex (France)

1990)

Abstract Three Si/SiOz:

analytical methods neutron activation,

were applied for the analysis of grafted planar heterostructures of GC and XPS analysis. Neutron activation analysis of bromoalkyl

grafts allows the graft density to be quantified. Several parameters were studied: time and temperature of the hydration treatment, as well as temperature of the grafting process. GC allows the grafted alkyl and perfluorinated chains (C 2 8) to be identified and quantified. Optimal conditions have been defined to obtain dense alkyl layers. The XPS method allows a qualitative control of the surface to be performed at each physical and chemical process but a quantitative determination of the graft density cannot be done because of the carbon contamination and of the instability of bromoalkyl and perfluorinated grafts under the photon beam. From these results, optimal perimental conditions have been found for the fabrication of grafted selective membranes chemical

and biochemical

exfor

silicon sensors.

INTRODUCTION

Trends in electronic technology lead to more and more sophisticated equipment applied in different fields. The fabrication of electronic microsensors has greatly developed and most of these electronic devices are based on the silicon technology of MOS-type structures (metal-oxide semiconductor). One of the main purposes of these microsensors is the translation of a chemical or a biochemical signal into an electronic one. Immunosensors detect the antibody-antigen interaction through capacitance measurements on silicon/ silicon dioxide/immobilized antibody/biological buffer heterostructures [ 11. Specific ions are detected through the variations of the surface potential of a functionalized insulator of an ISFET (ion sensitive field effect transistor) in contact with a solution of this ion.

0166-6622/91/$03.50

0 1991-

Elsevier

Science

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B.V.

170

The selectivity of the ISFET sensor is conferred by different ion sensitive membranes. In particular, sensitivity for silver and uranyl ions was obtained by chemical grafting of the silica insulator surface with silanes which have a specific functional group at the end of the alkyl chain [ 2,3]. The principal parameters influencing their sensitivity are the number of grafted sites and the complexation constant with the detected ion. The higher the density of grafted sites is, the lower the detection limit. Thus, it is necessary to develop analytical methods for determining the number of grafted sites. The fabrication of this selective membrane implies a series of processes of preparation and chemical transformation of the surface which have to be controlled. In a first stage test, samples with large surface area, i.e., oxidized porous silicon samples, were prepared and analyzed by traditional analytical methods [ 41. The hydroxylation treatment and the grafting conditions were optimized with this material. These results are good first data for optimizing the experimental conditions of chemical grafting of the thermally grown silica surface, but the absolute value of the grafting density has to be obtained on planar Si/SiOZ structures to eliminate the influence of the porous structure on the graft coverage. The surface analytical methods used have to be sensitive because of the extremely small dimensions of the silicon substrates and the low masses of the functional groups. With this aim in mind, we have adapted physical and chemical methods to control the preparation and the chemical grafting of Si/SiO:! heterostructures. The physical method of XPS (X-ray photoelectron spectroscopy) is a powerful technique for surface elemental analysis, largely applied for the characterization of materials used in electronic technology. It allows one to determine the elemental composition and sometimes the chemical state of the surface compounds. Nevertheless, a quantitative analysis is very difficult and requires a well-known standard sample. Neutron activation analysis is one of the most sensitive analytical methods; it requires that the titrated elements can be activated by thermal neutron flux, with (n, y) reactions. Grafted wafers ( Si/SiOZ) with bromosilanes were analyzed with this technique, we can, thus, obtain a quantitative analysis of the density of grafts (,umol m-‘). For the analysis of other kinds of grafted silanes (alkyl or perfluorinated molecules), determinations can be obtained by a chemical method with GC (gas chromatography). The great advantage of this method is that we can identify the grafted molecule very precisely. It is a well-known method in the analysis of grafted silica gel with a large specific surface area. In our case, the samples have very low surface areas, and this method was adapted.

171 EXPERIMENTAL

Sample preparation Oxidation Silicon wafers (32 mm diameter) were provided by Siltronix (Geneva, Switzerland). Substrates were p-type, (100) single crystal and double face polished. Wafers were cleaned successively with three organic solvents (trichloroethylene, acetone and isopropanol) sonicated for 5 min in each solvent. Native silica was removed with a 5% HF solution. Oxidation was carried out in a furnace for 10 min at 900’ C under a dry oxygen flow (N45 Alphagaz, Paris, France). Thus, we obtained a 100 A silica layer [ 51. Hydration (silanol surface formation) The creation of silanol groups (Si-OH) at the silica surface is necessary since the grafting reaction of a silane molecule (R, R, R, R, Si) is a condensation reaction with these sites. Different treatments were used: wafers were hydrated in water at different temperatures and times of treatment; samples were treated with a sulfochromic solution (50 ml H2S04, 0.2 g K2Cr207, 2 ml H,O) for 3 min, rinsed and dipped for 2 h in water (18 ML?) and treated at once in a sulfochromic solution and rinsed. Preparation of the different silanes Four types of silanes were synthesized. The experimental process depends upon the initial product which is commercially available (Table 1). In general way the first step is a hydrosilylation of a 1-alkene (with an alkyl or a functionalized alkyl chain) by dimethylchlorosilane in the presence of chloroplatinic acid according to the reaction: R-C&ZH~~_~)-CH

= CH2 + H-%I dH3

M pt “6~) (H2

R-C,H2,-S;I-Cl

P3 CH3

where R is CH, or Br. This reaction was carried out under a dry nitrogen flow. The reagent vapours were condensed under isopropanol flowing at - 10°C. After 18 h, the total disappearance of 1-alkene was checked by infrared spectroscopy. Chlorosilane was distilled in a vacuum. The chlorosilane may be directly grafted, but the densities of graft obtained are lower and less reproducible than with the aminosilanes [ 6-8 J; nevertheless we had to use the chlorosilanes when the R group was bromine, since the ami-

172 TABLE I Experimental

conditions

of the organic synthesis

of grafting reagents Amination

product

product

Initial reagent (commercial origin)

Hydrosilylation

Octadecene Fluka, Switzerland

Octadecyldimethylchlorosilane

Octadecyldimethyl amino)silane

Docosene Riedel de Haan, F.R.G.

Docosyldimethylchlorosilane

Docosyldimethyl(dimethylamino)silane

6-Bromohexene Fluka, Switzerland

6-Bromohexyldimethylchlorosilane

Tridecafluorooctyldimethylchlorosilane Petrach, U.S.A.

(dimethyl

Tridecafluorooctyldimethyl (dimethylamino ) silane

nosilane was very unstable. The alkylchlorosilane silane were aminated by the following reaction: FH3

CH,3

Cl-j$i-C,H2,-CH3

+ ZCH3)2-NH

---w CH:

chloro-

,CH3

N-Sj-C&,-CH3

CH3

and perfluorinated

+ (CH3)2NH*HCI

CH3

Grafting process The grafting process was described before [7,8]. The hydroxylated wafers were heated at 140°C in a vacuum for 3 h to remove physisorbed water. Samples were then coated with silane in a 2% v/v solution of silane in isopentane at - 30’ C in a vacuum for about 2 h. The condensation reaction took place at different temperatures for each reagent: 140” C for the alkylsilanes ( C18, 0; 80’ C for bromohexylchlorosilane and a controlled increase of temperature from 40 to 80°C for the perfluorinated silane. The reactions were carried out in a flow of N, over 48 h. The grafting reactions were, respectively: ‘+

&-OH

+

73

N-

Si -

C/H,

CH3

(CH2),

-

CH3

FH3 Cl-_Si-_(CHzk--Br

4

--+

cp3 Y $I-O-,S~-(CH~)~-CH~+N(CH~~H 1 CH3

C,H3 -

ii-0-pi-((312)6-b

CH3

(73

FH3

N-_Si--(CH2)2-(CF2h--CF3 C;lj

CH3

+HCI

CH3

-W

p. SH3 ~l-O-si-(CH2)2-((CF2)S-CF3 CH3

+

N(CH~)*H

173

and the excess of silane was eliminated by washing with tetrahydrofuran. ensure optimal density, it was necessary to repeat the silanizing treatment.

To

Analytical methods XPS XPS analysis is the only method that can be applied to measure the surface composition at each process of preparation and chemical transformation of the samples. The spectra were recorded with a VSW spectrometer and monounder the experimental conditions the chromatic Al Kcyl,Z radiation; Au ( 7f7,2) line, measured at 84.0 eV, had a full width at half-maximum of 0.7 eV. Neutron activation Neutron activation was applied for the quantitative determination of graft densities (pm01 mF2) in samples grafted with bromoalkylsilane. The analysis was performed in the Pierre Sue laboratory (CEN Saclay, France). Fragments of grafted wafers with controlled area were irradiated with a standard sample of grafted silica gel and aliquots of NH,Br solutions in the Orphee reactor (thermal flux 1013 n crne2 s--l) for 30 min. The cooling time was 24 h. The activity was measured using a Ge-Li detector. Pulses were stored for 15-30 min. Determination of graft densities by GC Considering the small dimensions of the Si/SiO, wafers the determination of graft density was very difficult. The classical method for analyzing grafted silica gel was based on the acidic rupture of the siloxane bond with HF [9]; the fluorosilane obtained was analyzed by GC, following the reaction: p

CH3

h

GH

$Si-O-&H,

HF ),-R

3

-

CH3 F-&H2

,,-R

214,

where R= (CH2)6Br, C20, C,, or (CH,),-(CF,),-CF,. This reaction was performed with 2.5 ml of a 40% HF solution and 0.2 ml of concentrated H,SO, in a Teflon beaker; 3 ml of an organic phase was added for the extraction of the fluorosilane. This phase was a solution of an internal standard, the solvent being first distilled. Bearing in mind the low masses obtained for grafted planar silica surfaces ( z lo-’ mol), the solution must be evaporated in order to concentrate the fluorosilane and to detect it. Two ways were used: for long alkyl molecules, C,, or Co, the concentration was made with an evaporation injection system (Ross injector). More volatile reagents were evaporated in a reactor at 0” C with a N, flow, in order to concentrate the fluorosilane by a factor of 10. These techniques could not be applied for lighter molecules.

174 TABLE 2 Conditions

of the chromatographic

analysis of the grafted molecules

Reagent

Solvent

Evaporation

Column

Stationary

C 18 C B:C6

Heptane Heptane Hexane Hexane

Ross Ross Not possible Reactor

Capillary Capillary Macrobore

C.813

phase

Standard

Chromatograph

SE 30 SE 30

C 19 C 23

Girdel Girdel

CPSIL SB 15 pm

C,

Hewlett Packard

The experimental chromatographic conditions for the analysis of different grafted molecules and the chromatograph used are presented in Table 2. Detection was performed with a flame ionization detector, results were recorded with a Spectra Physics SP 4270 integrator. For quantitative determinations we needed a reference solution composed of the grafted reagent, where the exact concentration was known, and an internal standard which was an alkane with a molecular weight similar to that of the fluorosilane. The mass of fluorosilane was calculated according to the expression: S,M,S;M;,

n/r,=

S,S;M:

where MF is the fluorosilane mass, SF the area of the chromatographic peak of the fluorosilane, M, the internal standard mass and S, the area of the chromatographic peak of the internal standard. ML, Sk, ML and Sk are the corresponding values for the reference solution. The density of grafted alkyl chains (N) inpmol m -’ was defined as:

where S, was the wafer area in m2 and M, fluorosilane.

was the molecular weight of

RESULTS

Preparation

of the wafer surface before the grafting process

Oxidized wafers were analyzed with XPS; typical peaks of 0 (1s) and Si (2~) corresponding to oxidized silicon are present; a carbon peak was also observed corresponding to hydrocarbonated species, this was due to carbon impurities. Intensities of the different peaks were recorded, the variation of the ratio of the C (1s)and Si (2~) intensities were followed after different surface treatments (Table 3).

175

TABLE 3 Ratio C (1s) /Si (2p) of core level intensities, obtained ments of the SiO, surface before the grafting process

by the XPS method,

Treatment

C(ls)/Si(2p)

After thermal oxidation After organic solvent treatments (acetone, trichloroethylene, isopropyl alcohol) After hydration After sulfochromic treatment

0.14

for different

treat-

(SiO,) intensityratio

0.67 0.23 0.04

J

0

20

40

60

80 hr

Fig. 1. Kinetics of hydration, the samples being grafted with bromohexyldimethylchlorosilane. Graft density (pm01 m-‘) as a function of the time of hydration. (A) Hydration in boiling water and (B) hydration at ambient temperature.

When wafers were hydrated in water the ratio of C ( 1s) /Si (2~) increased, it reached high values when wafers were in contact with the organic solvent (acetone, trichlorethylene and isopropyl alcohol). This value was considerably reduced by a sulfochromic treatment. In a previous paper on oxidized porous silicon [ 41, it was shown that this treatment also created a considerable number of silanol sites. Effect of the time and temperature of the hydration treatment and of the temperature of condensation of bromohexylsilanes Samples grafted with bromohexyldimethylchlorosilane were analyzed by neutron activation analysis. Firstly two groups of samples were studied, in both groups the time of hydration varied; the first group of samples was hydrated in boiling water and for the second group, hydration was carried out in water at ambient temperature. All samples were then grafted at 80°C. Results show that wafers hydrated in

176 TABLE 4 Graft density (pm01 m-‘) of samples grafted with bromohexyldimethylchlorosilane conditions of hydration and grafting temperature Grafting temperature

( aC )

Hydration

time (h)

Graft density

24 12 19

80 80 120

in different

(urn01 m-*)

2.93 3.49 4.18

TABLE 5 Graft density (pm01 m-*) of samples grafted with different and XPS at different temperatures

long alkyl molecules, analyzed by GC

Silane graft

Grafting temperature (“C)

Graft density (pm01 m-‘)

XPS intensity [C(ls)lSi(2p)l

Amino Amino Amino Chloro Chloro Amino

140 140 100 140 120 80

4.8 4.6 4.5 4.3 3.3 2.6

1.58 1.01 1.07

C22 C22 Cl8 Cl8 Cl8 C,F,,

ratios

boiling water (Fig. 1) had low densities of graft, without coherent values; it is possible that the silica layer is not preserved in this case. Samples hydrated at ambient temperature show an increase in density of graft as a function of the time of hydration. This means that the formation of silanol sites at the surface increases with the time of hydration which has been observed on oxidized porous silicon through direct titration of the hydroxyl groups [ 41. A third group of samples was analyzed, where the time of hydration and temperature of grafting varied (Table 4). Here, again, the time of hydration had an important role. The role of the grafting temperature is well known, the condensation reaction takes place at relatively high temperatures [7]. This fact is confirmed by the optimal density obtained at 120°C. Study of the grafting of alkyd chains and peFfh.4oFinated chains Chromatographic

analysis

Samples grafted with long alkylsilane ( CZ2,C,,) and perfluorinated chains were analyzed with this method (Table 5). For bromosilanes we found difficulties in the concentration of the solution, since bromohexyldimethylfluorosilane is volatile. It is noticeable that optimal values were obtained for long chains, this means

177

that the grafting conditions are well adapted. The densities of graftedperfluorinated molecules are low (2.55 +-0.39)) this is due to the volatility of the reagent. When the grafting process is performed at 140’ C, which is the optimal temperature for the condensation [ 71, the reagent is lost via the N, flow, new experiments have to be done in a sealed reactor. The chromatographic method seems to be the best adapted for analyzing g-rafted wafers with low areas. It is very selective, it identifies exactly the grafted molecules, but it is not applicable for light hydrocarbon chains (carbon number <8). XPS analysis of the grafted wafers Our principal aim was to compare the XPS method to the quantitative methods. First, we analyzed samples grafted with long alkylsilanes, where the density of graft was determined by GC. The C (ls)/Si(2p) ratios obtained on grafted samples were also compared with the ratio of a blank sample, which followed the same chemical treatment without any silane reagent (Table 6). The C ( 1s) /Si (2~) ratio on the blank sample is higher than the ratio of samples treated with sulfochromic solution. This is due exclusively to the contact of the samples with organic solvents (isopentane and tetrahydrofuran). On the other hand, grafted wafers show a C (1s) /Si (2~) ratio greater than that of the blank sample which is due to the grafted molecules. For a quantitative determination with XPS, it is necessary to get a standard sample which has to be free of carbon contamination. This is difficult since the grafting process uses organic solvents, the principal source of carbon contamination. In order to compare XPS analyses with neutron activation results, samples grafted with bromohexylsilane, where the density of graft was determined by neutron activation, were analyzed. On these samples, the observed C (1s) / Si (2~) ratio is representative of a grafted sample (Table 6). The cross-section value [ 10 ] of Br(3d) is 2.85 times greater than the value for C (Is). Thus, for the bromohexyl graft, the expected C (1s) /Br (3d) ratio is TABLE

6

Ratio C ( 1s) /Si (2~) of core level intensities,

after sulfochromic

pretreatment,

for grafted samples

and blank sample (solvent treatment) Treatment

Intensity

ratio [C (Is) /Si (2p) ]

Graft density (pmol m-‘)

Sulfochromic Blank sample Graft Cl8

0.04 0.44 1.58

4.3

Graft Cl8 Graft BrC6

1.01 1.12

3.3 4.2

L

0

1000

Auger

peak

600

Auger peak

Fig. 2. XPS general spectrum of the Si/Si02

0.1

0.2

0.3

units

INTENSITY

arbitrary

600

heterostructures,

Fls

1

200

Si2p

grafted with tridecafluorotetrahydrooctyldimethyl

400

SiZs

1

ENERGY

w

(eV)

(dimethyIamino)silane.

BINDING

0

1 2, t=o 2, t=30 3

Sample

min

C(b)293 0.11 0.01 0.01 0.03

C(b)285

1.86 0.03 0.04 0.05

F(ls)

4.07 0.37 0.30 0.15

Experimental

C(ls)293/F(ls) 0.2

C(ls)285/F(ls) 0.069

C(ls)293/F(ls) 0.027 0.027 0.033 0.020

0.457 0.08 0.13 0.033

Theoretical

1s) (CF,) of core level intensities of grafted samples with perfluor-

C(ls)285/F(ls)

Theoretical and experimental ratios C (ls)285/F (Is) (CH,) and C (ls)293/F( inated molecules

TABLE 7

180

J

181

2.807. In spite of this fact, no bromine signal was observed neither in the general scan, nor in the binding energy region of Br (3d). The absence of a bromine signal can be explained by the well-known instability of the C-Br bond [ 131; the photon beam may break it. It is, thus, not possible to detect bromoalkyl grafts with XPS through the Br (3d). In order to solve the problem of carbon contamination, samples grafted with perfluorinated molecules were analyzed through F( 1s). Three reasons were behind this choice: (1) fluorine [ F( 1s) ] has a cross section 4.4 times greater than the C( 1s) [lo]; (2) the C-F bond is very stable [ 131; and (3) the chemical shift in C (1s) induced by fluorine is well known [ 111 and we can differentiate CF, from CH,. A general scan, obtained on a wafer with perfluorinated grafts shows a conspicuous F (1s) peak (Fig. 2 ). The scanning region of C (1s) shows an important peak at 285 eV characteristic of CH,, a small peak at 293 eV characteristic of CF, which decreases in time (Table 7 and Fig. 3) and in some samples, an intermediate peak at 289.3 eV, attributed to the CF, forms. These observations indicate the presence of grafted molecules. Considering the cross section and the number of carbon and fluorine atoms, the theoretical ratios of the peak areas C (1s) 285/F (1s) and C (ls)293/F (1s) were calculated (Table 7) and compared with the experimental values. The experimental ratios [C (1s) 285/F ( 1s) ] show an excess of the CH, form and on the contrary, the experimental ratios C(ls)293/F(ls) (CF, forms) show an excess elemental fluorine. These observations indicate that the perfluorinated molecule degrades during the XPS analysis. A quantitative analysis of the perfluorinated graft cannot be done with this technique. CONCLUSIONS

Three analytical methods were applied for the analysis of grafted planar heterostructures of Si/Si02, characterized by low areas (16 cm2 per wafer). On wafers grafted with bromoalkylsilanes, analysis by neutron activation allows the graft density to be quantified. This study leads to the evidence of the kinetics of the creation of silanol sites as a function of the time of the hydration treatment at ambient temperature and the role of the temperature of the grafting process. Chromatographic analysis allows the grafted molecule to be identified very precisely, and to quantify the density of graft; this method is well adapted for the analysis of samples with low areas, grafted with long alkyl chains (carbon number > 7). By the XPS method we can make a qualitative analysis of the composition of the surface at each physical or chemical process. From the quantitative determination of long alkyl grafts by chromatographic analysis, it has been shown that the optimal conditions have been reached to obtain dense alkyl layers on Si/SiO, structures. These layers have

182

been characterized by capacitance and ellipsometric measurements, the thickness of the layer has been found to be close to the theoretical chain length

[121.

These results will be applied in order to optimize the fabrication of grafted selective membranes, for chemical or biochemical silicon sensors. ACKNOWLEDGMENTS

The authors are much indebted to Dr G. Revel and the staff of the Pierre Sue Laboratory (CEN Saclay, France) for their help in carrying out the activation analysis.

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