Chemically modified parylene gate field effect transistors

J. Electroanal. Chem., 106(1980) 413--418

413

© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Preliminary n o t e CHEMICALLY MODIFIED P A R Y L E N E GATE FIELD E F F E C T TRANSISTORS P R E P A R A T I O N OF pH INSENSITIVE P A R Y L E N E GATE F O R CHEMICAL MODIFICATION

MASAMICHI FUJIHIRA*, MOTOO FUKUI** and TETSUO OSA Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980 (Japan)

(Received 29th October 1979)

An ion-selective field effect transistor (ISFET), introduced independently b y Bergveld [ 1,2] and Matsuo' et al. [ 3,4], has been studied extensively from b o t h theoretical [5--7] and practical [8--14] viewpoints. The concept has been generalized to include sensors for uncharged chemical species [15--17], and the devices are called as chemically sensitive field effect transistors (CHEMFETs) [18] which are also included within a broader category of chemically sensitive semiconductor devices (CSSDs) [5]. Results obtained with these devices to date have recently been reviewed b y several authors [ 14,18--20]. Since the beginning of our studies of chemically modified electrodes [21--23], we have attempted to produce a new type of CHEMFET whose gate surfaces are chemically modified with c o m p o u n d s interacting specifically with some species in solution, e.g. the ionophores (crown ethers, cryptands, and antibiotics such as valinomycin) for alkali metal ion sensors [24] and the antigens or antibodies for immunosensors. It has been found, however, that these sensors are subject to interference from protons ~n solution because a considerable amount of surface silanol on the SiO2 or Si3N4 gate remains unreacted b y any means so far as these inorganic gate surfaces are chemically modified directly. In order to eliminate such interference from protons, we have tried to cover the inorganic gate surfaces with pH insensitive thin organic polymer films having little a m o u n t of surface hydroxyl. If we could succeed to prepare such films and fortunately we could introduce the specific ligands described above onto their surfaces without introduction of any appreciable amount of pH sensitive surface species, it would be possible to produce a better sensor with almost no sensitivity to pH. As promising candidates of such polymer films, polyvinylchloride (PVC) and poly-p-xylylene (Parylene N, Union Carbide Corp.) have been tried. The advantage of using PVC is that the film is easily formed b y a dip-coating from T H F solution. However, it has such disadvantage as difficulty in the chemical modification after the film coating. On the other hand, the Parylene film deposited in a vacuum by thermal decomposition of di-p-xylylene has several excellent characteristics as (a) precise control of coating thickness and uniformity, (b) tough, *To whom correspondence should be addressed. **On leave from Asahi Glass Co., Tokyo, Japan.

414

pinhole-free coatings as thin as 100 nm, (c) excellent dielectric characteristics, and (d) moderate surface reactivity capable of chemical modification. Preliminary results of the Parylene C (poly-monochloro-p-xylylene) gate TSFETs for the purpose of obtaining a pH insensitive reference ISFET have been reported by Matsuo and Esashi [25], but it has been found later on b o t h in Matsuo's [26] and in our laboratories that Parylene gate field effect transistors (PGFETs) prepared according to their method show poor reproducibility in their pH sensitivity (10--30 mV per pH unit, from pH 1 to 13) and occasionally show such high pH responses as the uncoated nitrige gate ISFET does. The latter, very high almost Nernstian responses might correspond to pinhole formation and the former, comparatively high pH responses of 10--30 mV per pH unit observed constantly on the pinhole-free PGFETs might be due to the introduction of surface functional groups containing oxygen during the annealing after film deposition. Pinholes were easily formed even in the films thicker than 100 nm when we intentionally skipped the careful cleaning procedure (described later in Table 1) before Parylene deposition. Also the PGFETs prepared by Matsuo's m e t h o d (method 3 in Table 1), when they were dipped i n a n aqueous sample solution, showed a large drift in their threshold voltage, Eth, of 1--3 V within an initial few hours and also showed unsaturation in their drain current, ID, vs. drainsource voltage, EDS, (ID--EDS) characteristic curves after several hours of immersion of the PGFETs. The poor adhesion of the Parylene film to substrate nitride may account for the large drift in E t h and unsaturation in I D - - E D s c u r v e s . Consequently surface pretreatment such as cleaning and alkylation to achieve better adhesion and pinhole-free coating and the conditions of annealing such as temperature, duration, and atmosphere to attain the low pH sensitivity have been investigated. Together with Matsuo's method, our procedures for surface cleaning and alkylation of the nitride gates before Parylene deposition are shown in Table 1. Alkylation of an oxide top layer [13] on the nitride gate with TABLE I Procedures for p r o d u c t i o n of pH insensitive P G F E T s Step M e t h o d I 1. 2. 3. 4. 5. 6.

7. 8. 9. 10.

Method 2

Method 3

Ultrasonic clean (USC)* in freshly distilled (f.d.) t r i c h l o r o e t h y l e n e (TCE) twice. USC in f.d. acetone twice. 2. USC in f.d. methanol. USC in doubly distilled water twice. USC in f.d. acetone twice. 4. USC in f.d. m e t h a n o l . USC in f.d. TCE twice. Boiling in n-butanol 6. Dipping in CH2N 2 6. Dipping in d i m e t h y l d i c h l o r o for 2 h. ethereal solution silane toluene solution for 1 h. for 1 day. Boiling in f.d. TCE for 10 rain. 7. USC in f.d. toluene. Cleaning in TCE vapor for 5 rain. -Parylene deposition in vacua ( 1 2 0 - - 1 3 0 ° C in sublimation r o o m . 6 5 0 - - 6 7 0 ° C in thermal d e c o m p o s i t i o n room, and 70°C in deposition room). Annealing in a v a c u u m of ca. 1 × 10 - 3 Torr at 150°C for 2 days.

~'Every ultrasonic clean has been done for ca. 30 s.

415

butanol or diazomethane [27,28], followed b y the thorough cleaning in trichloroethylene vapor in the present methods 1 and 2, then the Parylene deposition, and finally the appropriate annealing described later, gave initial low drifts from 10 to 100 mV in Eth and ideal saturated ID--EDs curves even after several days of immersion of the PGFETs in aqueous solution (Fig. 1), in contrast with the above described large drifts in Eth and unsaturation in ID--EDs curves on the PGFETs prepared b y method 3. In addition to these observations, superiority of our butanol alkylation in method 1 to the treatment with dichlorodimethylsilane in method 3 has also been confirmed very recently b y Nakajima, Esashi, and Matsuo [29] to compare the lateral conductance of the Parylene films on the slide-glass substrates alkylated by our m e t h o d 1 and their method 3 in contact with water. EGS=I.5V

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In Fig. 2, the effect of the annealing conditions on the pH responses of the PGFETs pretreated by method 2 is shown. Similar results have also been obtained on the PGFETs pretreated by method 1. The potential change at the

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Parylene gate on the pH responses of the PGFETs pretreated by method 2. (A) PGFET annealed in the oxygen atmosphere at 150°C for 2 days. (B) PGFET annealed in a vacuum (ca. 1 × 10 -3 Torr) at 150°C for 2 days. (C) PGFET obtained by the esterification with diazomethane of the same PGFET as B. Fig. 3. Circuit for operating PGFET in constant drain current mode with a saturated calomel electrode [30 ].

Parylene gate and the aqueous buffer solution interface was measured by using the same circuit as described by Abe et al. [30] for operating PGFET in constant drain current mode with a saturated calomel reference electrode (SCE) as shown in Fig. 3. As the drain current is held constant, the change in the potential at the interface in response to a change in the activity of the measured ions in solution is read out directly. The PGFET annealed in the oxygen atmosphere at 150°C for two days shows the potential change more than 200 mV from pH 1 to 13 with a maximum slope of ca. 30 mV pH -1 , while the PGFET annealed in a vacuum (ca. 10 -3 Torr) at 150°C for 2 days shows small potential change ca. 50 mV in the same range of pH with the m a x i m u m slope ca. 10 mV pH -1 . Both potential--pH curves become S-shaped with their m a x i m u m slopes at ca. pH 5 and the curves appear to level off at the high and low pH ends. By assuming the presence of carboxyl and other oxygen containing functional groups (pH sensitive) on the Parylene gate surfaces, we can interpret the potential--pH changes in the pH range from 2 to 9 theoretically and the details will be reported elsewhere. In Fig. 4, the change in the Auger spectra of the Parylene N films deposited on silicon waters and annealed in a vacuum before and after dipping in distilled water is shown. Before contact with water, only a carbon Auger peak at ca. 272 eV is observed and an oxygen peak at ca. 505 eV is n o t detectable in contrast with Matsuo's results on Parylene C [25], while on the Parylene N film dipped in water a new but small oxygen peak at ca. 505 eV appears in addition to the carbon peak. It has not been clear yet whether the oxygen peak observed on the dipped Parylene film corresponds to the formation of a new surface species containing oxygen or to the strong adsorption of water and/or contaminant from

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distilled water. However, the presence of oxygen only at the surface of the dipped sample was established by the compositional profile also obtained by Auger electron spectroscopy combined with argon sputter etching as shown in Fig. 5. On the other hand, oxygen is absent from the surface and through the bulk on the Parylene film not contacted with water. It seems difficult at present to eliminate the small pH responses due to the surface acids described above completely by the change in the annealing conditions, unless the higher vacuum system with oil diffusion or ion pump takes the place of the present vacuum system with rotary pump connected in series to a liquid nitrogen trap for Parylene deposition and annealing. Therefore we have at~ tempted in the next step to make the unfavorable surface acids insensitive to pH. If the surface carboxyl and other acids sites are esterified, the resulting esters are expected to be pH insensitive. Consequently we tried the esterification of the surface acids on the Parylene gate with diazomethene. The pH response of the PGFET thus esterified is also shown in Fig. 2 (curve C). As we would expect, the potential change is further lowered and the maximum pH slope becomes less than 4 mV pH -1 .

418

In conclusion, the availability of the PGFETs with small pH response of ca. 50 mV from pH 1 to 13 by the improvement of adhesion and the annealing in a vacuum allows us to produce a better chemically modified CHEMFET. The feasibility of the use of Parylene gate to chemically modified CHEMFETs is further increased b y the realization of the suppression of the pH sensitivity of the Parylene surfaces by the esterification of the unavoidable surface acid functional groups. ACKNOWLEDGMENT

The authors are indebted to Professor T. Matsuo and the members of his research laboratory, Department of Electronic Engineering, Tohoku University, for their helpful discussion and their assistance to fabricate the Parylene deposition system. The present w o r k was partially supported b y a Grant-in-Aid for Scientific Research from the Ministry of Education (No. 310502).

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