Process for producing a surface gate of an integrated electro-chemical sensor, consisting of a field-effect transistor sensitive to alkaline-earth species and sensor obtained

A process for producing a surface gate comprising a selective membrane for an integrated chemical sensor comprising a field effect transistor, and the integrated chemical sensor thus produced, wherein the surface gate is particularly sensitive to alkaline-earth species, and more particularly, sensitive to the calcium ion. The process comprises forming grafts on the surface gate, and making the grafts operative utilizing phosphonate-based, iono-sensitive molecules.

FIELD OF THE INVENTION 
The present invention relates to an integrated chemical sensor, comprising 
a field-effect transistor in which the metal gate is replaced by a 
selective membrane of the ionic species to be dosed and placed in contact 
with the solution to be analyzed. 
Such sensors result from the evolution or development of the properties of 
the field-effect transistors comprising, in a semi-conducting substrate, 
two doped zones, called source and drain, and, in a layer of a dielectric 
material covering the semi-conductor and the doped zones, a central zone 
covered by a gate whose variable electrical supply makes it possible to 
modulate the passage of the current between the source and the drain. 
DESCRIPTION OF THE RELATED ART 
In publication IEEE TRANS, BIOMED. Eng. BME, 17 (1970) 70, P. BERGVELD 
proposed eliminating the gate and placing the surface of the dielectric 
material directly in contact with an electrolytic solution containing 
H.sup.+ and OH.sup.- ions and in which a polarization electrode is 
immersed. For a given polarization, a drain current is obtained, 
representative of the concentration of the electrolytic solution. Any 
modification of concentration modulates the drain current. The transistor 
then functions as an integrated chemical sensor. 
In order to broaden the domain of application limited to the medium 
containing H ions, it has been proposed to interpose an iono-sensitive 
surface gate between the electrolytic solution and the surface of the 
dielectric material. Such a surface gate may comprise an organic or 
inorganic membrane, connected by gluing, by supply or by chemical deposit 
in vacuo. Such a method makes it possible to produce a surface gate 
specific to a product and thus capable of analysis of the latter in a 
specific medium. 
Such a development has brought a certain advantage with respect to 
broadening of the range of application, but is not entirely satisfactory, 
due to ageing of the added organic or inorganic membrane, which lacks 
reliability over time. 
In order to overcome this problem, it has been proposed, particularly in 
Application FR-2 600 212 (86-08989), to make a directly integrated 
sensitive surface gate with the objective of eliminating the presence of 
the added membrane. According to this technique, the surface gate is 
subjected directly to a treatment of hydroxidation, then to a treatment of 
impregnation with a solution of silane, followed by condensation of the 
silane. 
This technique is satisfactory but may be considered as being of limited 
application, due to the necessity of having available, for the phase of 
impregnation, a silane in solution incorporating the group specific to the 
sensitivity to be given to the surface gate being treated. 
Furthermore, it has been determined that such a technique does not allow 
maximum performance to be obtained, particularly for certain specific 
sensitivities, such as those to alkaline-earth species and, more 
particularly, to the calcium ion. 
Now, such sensors are a subject of real need in numerous applications 
requiring electrochemical analysis, including big-medical analysis, water 
hardness testing, agronomy (analysis of nutritive solutions), and the 
environment (analysis of fresh- and sea-water). 
SUMMARY OF THE INVENTION 
The object of the present invention is to solve the above problem by 
proposing a novel process for making, on an electrochemical sensor, a 
selective surface gate particularly sensitive to alkaline-earth species, 
and more particularly, to the calcium ion, and wherein the operational 
characteristics of the sensors are reliable over time and allow a thorough 
miniaturization of sensors comprising a selective surface gate. 
The invention comprises a process for making a surface gate of an 
integrated electrochemical sensor comprising a field-effect transistor 
having two doped zones, wherein the doped zones are a source and a drain, 
and having a surface on which a surface gate is to be formed, wherein the 
process comprises the steps of: subjecting the surface to a treatment of 
hydroxylation in order to obtain 10.sup.14 to 10.sup.15 OH sites per 
cm.sup.2, washing and drying the surface, and further comprising the steps 
of: grafting the surface with a grafting product in order to produce 
grafts on some or all sites on the surface, preparing phosphonate-based 
iono-sensitive molecules, and making operative one or more of the grafts 
using the iono-sensitive molecules. 
Another object of the invention is an integrated electrochemical sensor 
having a surface gate obtained by carrying out the process described 
herein. 
In order to attain the above objectives, the process according to the 
invention comprises the following steps: 
effecting grafting of the surface with a grafting product, for the purpose 
of producing grafts on at least certain sites on the surface, 
preparing phosphonate-based iono-sensitive molecules, 
making operative some or all of the grafts using the iono-sensitive 
molecules. 
Various other characteristics will appear from the description made 
hereinbelow with reference to the accompanying drawings which show, by way 
of non-limiting examples, embodiments of the object of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an integrated chemical sensor comprising a substrate 1 made of 
a semi-conducting material having undergone, from the surface 2, a doping 
operation, for the purpose of defining two zones 3 and 4 intended to 
comprise, respectively, a source and a drain. The surface 2 is coated with 
a layer 5 of silica of electronic quality. The layer 5, before or after 
the establishment of contacts 6 and 7 with the source and the drain, is 
coated with a mask 8 over the whole of its surface, apart from that part 
having to correspond to the establishment of a surface gate located, in 
superposition of planes, in the central part between the source 3 and the 
drain 4. 
The uncovered part of the layer 5 may be used as such or be treated, so as 
to obtain a layer 9 of a dielectric gate material, such as oxidized 
silicon nitride or oxynitride of silicon, of thickness in the range of 300 
to 2500 .ANG. (30 to 250 nanometers). 
The process according to the invention employs further operational phases 
or steps, of which the principal ones are: 
(I) a phase of grafting of grafts on some or all sites on the surface of 
the layer 5 or 9 of dielectric gate material, 
(II) a phase of preparation of iono-sensitive molecules adapted 
subsequently to ensure a sensitivity to the alkaline earth species and, 
more particularly, to the calcium ion, 
(III) a phase of making operative such grafts by the iono-sensitive 
molecules. 
Phase (I) 
Phase (I) for grafting the grafts comprises, according to a first variant, 
a preparatory step comprising hydroxylating the surface 10 in order to 
obtain from 10.sup.14 to 10.sup.15 OH sites per cm.sup.2 of surface. This 
step of hydroxylation may be conducted by pickling from a basic aqueous 
solution containing sodium hydroxide, at a concentration of 0.1 to 1M. 
This step 1 may be conducted by immersion for a duration of time in the 
range of 5 to 30 minutes, the pickling solution being maintained at a 
temperature in the range of 15.degree. to 40.degree. C. 
This step of hydroxylation may also be conducted, in the case of the layer 
5 comprising silica alone, by a hydration consisting in placing the 
surface 10 in contact with bidistilled water maintained at a temperature 
of 95.degree. C. Such hydration is continued for a duration of time in the 
range of 1 to 48 hours. A preferred embodiment continues the hydration for 
12 hours. 
Hydroxylation is then followed by a washing of the surface 10 by immersion 
or flow of a flux of distilled water delivered at ambient temperature. 
Another variant embodiment of phase (I) of grafting a graft comprises, for 
the purpose of obtaining the same number of OH sites, in ensuring 
hydroxylation from a sulfochromic mixture (attack solution) composed of 1 
ml of a saturated solution of potassium dichromate and 20 ml of 
concentrated (95-97%) sulfuric acid. This step may be conducted by 
immersion in the attack solution for a duration of time in the range of 1 
to 5 minutes, the attack solution being maintained at a temperature in the 
range of 15.degree. to 35.degree. C. 
This step of hydroxylation may be conducted in several steps, as described 
below: 
Immersion in the attack solution previously described (sulfochromic 
mixture) under the same conditions, 
Immersion in ultra-pure water of resistance 18 megohms for a duration of 
time in the range of 10 to 15 hours, at a temperature in the range of 
15.degree. to 35.degree. C. 
Immersion in the attack solution previously described (sulfochromic 
mixture) under the same conditions. 
Drying is then effected in the two embodiments, for example by means of a 
flux of inert gas at ambient temperature. 
The preparatory step is then terminated and the surface 10 is subjected to 
degassing in vacuo between 1 and 5 Pa, preferably. 1 Pa, by heating the 
surface 10 to a temperature in the range of 120.degree. to 160.degree. C. 
and, preferably at 140.degree. C. Such a degassing in vacuo is established 
for a duration of 1 to 5 hours, preferably limited to 2 hours, to obtain a 
completely dehydrated and degassad surface 10. 
The grafting phase is then carried out by causing to react, by immersion, 
the surface 10 in a grafting product such as a pure, mono or 
multifunctional silane, an aminosilane, an alkoxysilane or a halosilane, 
intended to constitute on at least certain of the sites of the grafts, of 
general formula: 
##STR1## 
in which: 
--R.sub.1 is either 
--(CH.sub.3).sub.2 --N--, 
--CH.sub.3 --O--, 
--CH.sub.3 --CH.sub.2 --O--, 
--Cl--, 
--R'.sub.1 is either: 
--CH.sub.3 --, 
--CH.sub.3 --O--, 
--CH.sub.3 --CH.sub.2 --O--, 
--R.sub.2 is in all cases: 
--Cl. 
so as to make the grafts operative. The silane is grafted by subjecting the 
surface 10 to a heating step in the temperature range of 
60.degree.-80.degree. C. for a duration of 15 to 48 hours, preferably 
70.degree. C. for 48 hours, in an enclosure at ambient pressure, in the 
presence of an inert (neutral) gas, such as nitrogen or argon. The grafted 
surface 10 is then washed with a solvent of the silane in excess, such as 
ether, tetrahydrofuran, by immersion or flow at ambient temperature and 
preferably by renewing the solvent one to four times. A rate of grafting 
of the OH sites in the range of 1 to 50% of the sites present is obtained. 
The grafted surface 10 is then subjected to a drying step in a flux of 
inert gas at ambient temperature. 
An example of carrying out this phase of grafting of a chloropropyldimethyl 
chlorosilane is illustrated hereinafter by formulae (i) and (ii). 
##STR2## 
Phase II 
Phase (II), which involves the preparation of iono-sensitive molecules, 
comprises providing the molecules with the following characteristics: 
a) presence of a group (CH.sub.3).sub.2 --N-- serving to ensure the 
chemical reaction of making operative the graft on the surface gate 10 
treated, 
b) presence of a spacer group --(CH.sub.2).sub.n --, 
c) presence of a group containing phosphorus-oxygen bonds and selected 
from: 
______________________________________ 
phosphonate --PO.sub.3 .sup.2- 
ethylphosphonate --PO.sub.3 Et.sup.- 
phosphate --O--PO.sub.3 .sup.2- 
______________________________________ 
Examples of the preparation of iono-sensitive molecules are given 
hereinafter. 
EXAMPLE 1 
3-Dimethylamine propylphosphonate of sodium: 
A mixture of 668 g (3.3 moles) of 1-3-dibromopropane and of 55 g (0.33 
mole) of triethylphosphite is heated to 150.degree. C. for 30 minutes. The 
excess of 1-3-dibromopropane is distilled in vacuo and makes it possible 
to obtain an intermediate product (A) of 85 g (0.33 mole) of formula: 
EQU Br--(CH.sub.2).sub.3 --PO.sub.3 Et.sub.2 
A mixture of 85 g of (A) and of 75 g (0.66 moles) of dimethylamine in 
aqueous (40%) solution and of 48 g (0.33 mole) of potassium carbonate is 
heated to 60.degree. C. for 12 hours. The insoluble parts are filtered and 
the solvent evaporated in vacuo. A water-ether extraction makes it 
possible to isolate in the ether phase 49 g (0.22 mole) of a product (B) 
of formula 
EQU (CH.sub.3).sub.2 --N--(CH.sub.2).sub.3 --PO.sub.3 Et.sub.2 
An acid hydrolysis of product (B) is then carried out by refluxing in a 48% 
hydrobromic acid solution for 20 hours. The solvent is evaporated, then 
the residue is neutralized, with a sodium hydroxide solution, up to 
complete neutralization (pH =12), to obtain the iono-sensitive molecules 
of formula: 
##STR3## 
EXAMPLE 2 
Dimethylamino methylphosphonate of sodium: 
A solution of 21.6 g (0.48 mole) of dimethylamine in 70 g anhydrous ethanol 
is made by bubbling gaseous dimethylamine in ethanol. Next, 14.4 g (0.48 
mole) of paraformaldehyde and 66.2 g (0.48 mole) of diethylphosphite are 
added to the solution, and the mixture is heated to 70.degree. C. for 2 
hours. 
An acid hydrolysis is carried out by refluxing the mixture in a 48% 
solution of hydrobromic acid for 20 hours. After evaporation of the 
solvent, the residue is neutralized by a sodium hydroxide solution up to 
complete neutralization (pH=12), and comprises the iono-sensitive 
molecules of formula: 
##STR4## 
Phase (III) 
Phase (III) of making the graft operative consists in placing about 0.5 g 
of the iono-sensitive molecules according to Example 1 or 2 in 40 ml of 
methanol in order to obtain a reactive sensitization mixture. The surface 
10, treated as stated previously, is placed in contact with the mixture, 
which is taken to reflux of the methanol, i.e. at about 65.degree. C. for 
a duration of 1 to 5 days. 
Surface 10 is then rinsed with water and dried in a flux of an inert gas at 
ambient temperature, resulting in 1 to 100% of the grafted sites being 
made operative. 
According to another embodiment of the invention, 100 to 500 mg of sodium 
iodide are added to the mixture to accelerate the reaction or to increase 
the percentage of the grafted sites made operative. 
A surface 10, treated in accordance with the three phases described 
hereinabove with iono-sensitive molecules according to Example 1, has been 
the subject matter of a test of characterization by means of a mass 
spectrometer of secondary ions. The mass spectrometer features high 
lateral resolution, and is equipped for electronic imagery and for 
analysis of the insulants, and is capable of attaining a depth of analysis 
of the order of 10 to 20 .ANG. (1 to 2 nanometers). 
Such a spectrometer was of the SIMS-LAB type manufactured by VG INSTRUMENT 
equipped with a MIG 300 source. 
An extreme surface elementary analysis test was conducted on a square 
surface with sides measuring 20 micrometers. The density of the current 
applied was of the order of 10.sup.-7 A/cm.sup.2, for the purpose of 
creating a very low speed of abrasion, while presenting a sufficient 
sensitivity for a detection of the ionic species characteristic of 
chlorine and phosphorus. 
The smooth curve obtained, as illustrated in FIG. 2, makes it possible to 
identify the ionic species of the phosphorus present on the surface 10 by 
the process of treatment of the invention. 
A field-effect transistor, provided with a surface 10 treated with the 
iono-sensitive molecules of either Examples 1 or 2, was tested for 
sensitivity in a test medium comprising the following: for the acid pH 
values, the medium comprised a mixture of 0.01M acetic acid, 0.01M sodium 
acetate and 0.1M sodium nitrate in sufficient quantity of water; and, for 
the basic pH values, the medium comprised 2% tri hydroxymethylaminomethane 
with 0.5M KCl. 
FIGS. 3 and 4 show the characteristic response curves obtained, showing two 
zones of inflexion at about pH 3.5-4 and 8, which are characteristic of 
the sensitivity of the grafted and operative surface gate. 
FIG. 5 illustrates the response curves to the calcium ion in a 2% solution 
of tri hydroxymethylaminomethane with 0.5M KCl at pH 10, the sensor 
C.sub.1 and C.sub.2 prepared in accordance with Examples 1 and 2, in 
comparison with a sensor C.sub.0 whose surface 10 is basic dielectric not 
treated in accordance with the invention. This FIG. illustrates a high 
sensitivity which may be considered as close for the sensors C.sub.1 and 
C.sub.2. 
POSSIBILITY OF INDUSTRIAL APPLICATION 
A sensor obtained by the process of the invention finds a typical 
application in the determination of the degree of water hardness. 
The invention is not limited to the examples described and shown, as 
various modifications may be made thereto without departing from its 
scope.