The present invention relates to a dielectric (per)fluorinated material for the insulation of high frequency integrated circuits, under the form of films having a very good adhesion to the substratum and a thickness lower than 200 nm.
Specifically the invention relates to a dielectric perfluorinated material, for the insulation of high frequency integrated circuits, under the form of a homogeneous film, substantially without defects, obtained from a PTFE nanoemulsion having a particle diameter in the range 5-100 nm; the films of the invention are characterized by a very good adhesion to the substratum, a thickness lower than 200 nm, a dielectric strength higher than 4 MV/cm, and a weight loss at 425xc2x0 C. comprised between about 0.0008%/minute and 0.02%/minute.
The modern high frequency integrated circuits contain ten millions of transistors placed on few square centimeters of silicon crystal and they work with frequencies of the order of 1,000 MHz. All the transistors of said integrated circuits must be connected each other by electricity conductors. The modern integrated circuits contain up to six layers of conductor elements. Owing to the sizes and the density of the transistor positioning, it is preferable that both the sizes of the conductor elements and the separation space among the conductors are reduced as much as possible. At present, integrated circuits are produced with conductors having a thickness of 180 nm and separation is lower than 200 nm. In next future the reduction of the conductor thickness is planned up to 100 nm.
The reduction of the conductor sizes and of the separation among conductors causes some problems. The main problem is due to an increase of the resistance-capacity delay (RC-delay), connected to the resistance increase due to a decrease of the conductor section and to the increase of the capacity due to the conductor approach. Besides, the capacity increase implies the decrease of the signal intensity due to the interference among conductors and the heat developed from the integrated circuit increases with consequent increase of the circuit energetic consumption. This makes it necessary a more intense cooling of the circuit.
These problems can be solved by reducing the capacity among the conductor elements by using an insulating material having a lower dielectric constant. At present as insulator among the conductor elements of the integrated circuits, silicon oxide is used, which however shows a high dielectric constant (∈=4.2). A lower dielectric constant is that of air (∈=1.01), which however does not guarantee the insulation of the conductors, since it shows unacceptable values of dielectric strength, lower than 0.01 MV/cm. The voltage used by modern integrated circuits is of 3.3 V, the distance among the conductors is of the order of 200 nm, wherefore 3.3/200 gives a value of 0.165 MV/cm or 16.5 V/xcexcm. Therefore, the dielectric strength of the used dielectric material must be higher of at least one order of magnitude than this value. Besides, it is preferable to use an insulating material having a dielectric strength as high as possible, since in the case of porous dielectric material, the dielectric strength remains at acceptable values.
The film thickness of the dielectric material must be very low to guarantee high performances of the integrated circuits. Besides, in modern circuits thicknesses of about 500 nm are used and it is expected that the constant trend to miniaturization requires dielectric materials having a thickness lower than 200 nm.
The integrated circuits during the production process are subjected to various thermal treatments, and therefore, it is important that the dielectric material has a suitable thermal resistance so that it is not damaged during said treatments. In particular, the dielectric material must withstand for a short time temperatures higher than 350xc2x0 C.
It is known that polytetrafluoroethylene (PTFE) has one among the lowest dielectric constants (∈=2,05) of the solid materials and absolutely the lowest one with respect to non porous solid materials which withstand temperatures higher than 350xc2x0 C. Therefore it is the ideal material for the use as dielectric insulator for high frequency integrated circuits. The problem is to obtain a thin PTFE film without defects having a high dielectric strength.
In U.S. Pat. Nos. 5,889,104 and 6,071,600 it is described how to obtain a dielectric material for integrated circuits from PTFE aqueous dispersions by spin coating. In these patents there is described the obtainment of the dielectric material from PTFE dispersions with particles having a diameter lower than 100 nm. Tests carried out by the Applicant (see comparative Examples) show that said PTFE dispersions give films which show defects and unhomogeneity. These films therefore are not able to guarantee good electric properties. Besides in said patents no value of dielectric strength of the obtained films is reported.
In U.S. Pat. No. 5,928,791 a method for improving the dielectric strength of thin PTFE films used in integrated circuits is described. The method includes a quick cooling after the sintering film of PTFE. In the Examples PTFE dispersions with particles having an average diameter of the order of 200 nm are used and films are obtained having a dielectric strength lower than 4 MV/cm, of the order 3.25-3.5 MV/cm, but having a high thickness in the range 500 nm-1,000 nm. Said thickness of the film of the dielectric material results too high and therefore unsatisfactory to guarantee high performances of the integrated circuits.
The need was therefore felt to have available a dielectric material for integrated circuits, under the form of homogeneous film, substantially without defects, having the following combination of properties:
a very good adhesion to the substratum;
a high dielectric strength, higher than 4 MV/cm;
a thickness lower than 200 nm;
a weight loss at 425xc2x0 C. comprised between about 0.0008%/minute and 0.02%/minute.
An object of the present invention is therefore a formulation based on polytetrafluoroethylene (PTFE), homopolymer or modified, comprising:
1) latex of said PTFE having a particle diameter between 5 and 100 nm, comprising an anionic fluorinated surfactant in an amount in the range 2-25% by weight based on the PTFE, preferably 3-20% by weight;
2) a non ionic fluorinated surfactant added to the PTFE latex in an amount in the range 18-60% by weight based on the PTFE, preferably 25-45% by weight.
The anionic fluorinated surfactants used during the polymerization for obtaining the PTFE-based dispersion of the invention, are selected from the following compounds:
Txe2x80x94Oxe2x80x94Rfxe2x80x94CFXxe2x80x94COOMxe2x80x83xe2x80x83(IA)
wherein: Xxe2x95x90F, CF3; M=H, NH4, Na, Li, K;
T is a C1-C3 (per) fluoroalkyl group, optionally containing one Cl atom; preferably it is selected from xe2x80x94CF3, xe2x80x94C2F5, xe2x80x94C3F7, xe2x80x94CF2Cl, xe2x80x94C2F4Cl, xe2x80x94C3F6Cl; optionally one or two F atoms can be replaced by H;
Rf is a (per)fluoropolyoxyalkylene radical having a number average molecular weight Mn in the range 200-2,000, preferably 350-1,000; Rf is selected in particular from the following classes:
(a) xe2x80x94(CF2CF(CF3)O)m(CFXO)nxe2x80x94
wherein m and n are integers such that the n/m ratio is in the range 0.01-0.5 and the molecular weight is in the above range;
(b) xe2x80x94(CF2CF2O)p(CF2O)qxe2x80x94
wherein p and q are integers such that the q/p ratio is in the range 0.5-2 and the molecular weight is in the above range;
(c) xe2x80x94(CF2CF(CF3)O)rxe2x80x94(CF2CF2O)sxe2x80x94(CFXIIO)txe2x80x94
wherein r, s and t are integers such that r+s is in the range 1-50, the t/(r+s) ratio is in the range 0.01-0.05 and the molecular weight is in the above range;
(d) xe2x80x94(CF(CF3)CF2O)uxe2x80x94
wherein u is an integer such that the molecular weight is in the above range;
(e) xe2x80x94(CYZxe2x80x94CF2CF2O)vxe2x80x94
wherein Y and Z, equal to or different from each other, are F, Cl or H; v is a number such that the molecular weight is in the above range;
(f) xe2x80x94(CF2CF2O)wxe2x80x94
w is a number such that the molecular weight is in the above range.
Among the compounds of formula (IA) as anionic surfactants those of type (a) are preferred:
Txe2x80x94Oxe2x80x94(C3F6O)m(CF2O)nxe2x80x94CF2xe2x80x94COOM
Optionally the compounds of formula (IA) can be used in admixture with the following anionic surfactants:
CF3(CF2)nCOOMxe2x80x83xe2x80x83(IIA)
wherein n can range between 4 and 12,
Fxe2x80x94(CF2xe2x80x94CF2)nxe2x80x94CH2xe2x80x94CH2xe2x80x94SO3Mxe2x80x83xe2x80x83(IIIA)
wherein Mxe2x95x90H, NH4, Na, Li, K and n can range between 2 and 5. The amount of optional surfactant (IIA) and/or (IIIA) is lower than 50% by weight with respect to the surfactant (IA). The amount of anionic surfactant used in polymerization is such that the ratio by weight between the surfactant and the TFE converted into polymer is lower than 1, preferably in the range 0.02-0.25. The surfactant can also be fed partly at the starting of the polymerization and partly during the polymerization reaction.
The non ionic fluorinated surfactants which are added to the PTFE latex obtained from the polymerization have the following structure:
CF3(CF2)yxe2x80x94Lxe2x80x94Rhy=3-20xe2x80x83xe2x80x83(IB)
Txe2x80x94Oxe2x80x94Rfxe2x80x94Lxe2x80x94Rhxe2x80x83xe2x80x83(IIB)
wherein:
Rf is selected from the above structures (a), (b), (c), (d), (e), (f);
L is a divalent organic group, a linking group between Rf and Rh, selected from: xe2x80x94COxe2x80x94NR1xe2x80x94, xe2x80x94CH2(OCH2CHR2)axe2x80x94Oxe2x80x94, xe2x80x94CH2(OCH2CHR2)bxe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94(CH2)cxe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94; wherein R1 is xe2x80x94H or a C1-C4 alkyl; R2 is xe2x80x94H or a C1-C2 alkyl; a, b are numbers from 0 to 6, preferably from 0 to 2; c is a number from 1 to 3;
Rh is a radical having a polyoxyalkylene structure selected from:
(i) xe2x80x94(CH2CH2O)qCH2CH2Z, wherein: q is an integer from 5 to 70, preferably from 6 to 25; Z is selected from xe2x80x94OH, C1-C4 alkoxy;
(ii) xe2x80x94(CH2CH2O)r(CH2CH(CH3)O)s CH2CHR3Z, wherein r+s is an integer from 5 to 70, preferably from 10 to 50; the r/s ratio is in the range 0.1-10, preferably 0.5-5; R3 is selected between xe2x80x94H and xe2x80x94CH3; Z is selected between xe2x80x94OH, C1-C4 alkoxy;
Preferably as non ionic surfactants the following compounds are used:
the compounds of structure (IB) with y=5, L=xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94, and Rh=xe2x80x94(CH2CH2O)qCH2CH2OH wherein q=6, said compounds are commercialized with the name FORAFAC 1110D*;
the compounds of structure (IIB) having Rf of structure (a) with T=xe2x80x94C3F6Cl, m and n such to give a molecular weight in the range 450-650; L=xe2x80x94CONHxe2x80x94; Rh=xe2x80x94(CH2CH2O)qCH2CH2OCH3 wherein q=21, said compounds are commercialized with the name Fluorolink C455*.
The latex of PTFE homopolymer or modified, having a particle diameter in the range 5-100 nm is obtained by the radical polymerization process of tetrafluoroethylene in the presence of a microemulsion containing the above anionic surfactants (IA), said process being described in EP 969,027 in the name of the Applicant, herein incorporated by reference. The microemulsions used for the polymerization are described in U.S. Pat. Nos. 4,864,006 and 4,990,283.
PTFE nanoemulsions having an average diameter of the latex particles in the range 5-100 nm, preferably 10-50 nm, of the following classes, are the preferred ones for the present invention:
nanoemulsions of PTFE homopolymer: the use of the PTFE homopolymer nanoemulsions allows to obtain a dielectric material having a greater thermal stability with respect to the modified PTFE;
nanoemulsions of modified PTFE, i.e. TFE copolymers with one or more comonomers containing at least one unsaturation of ethylene type in an amount up to 6% by moles, preferably up to 1% by moles. Generally, the use of nanoemulsions of modified PTFE allows to obtain a dielectric material having improved electric properties with respect to the PTFE homopolymer.
The comonomers which can be used for preparing the modified PTFE, are both of hydrogenated and fluorinated type; among the hydrogenated comonomers we can mention:
ethylene, propylene, acrylic monomers, for example methylmethacrylate, (meth)acrylic acid, butylacrylate, hydroxyethylhexylacrylate, styrene monomers, such as for example styrene.
Among the fluorinated comonomers, we can mention:
C3-C8 perfluoroolefins, such as hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylene CH2xe2x95x90CHxe2x80x94Rf, wherein Rf is a C1-C6 perfluoroalkyl;
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
CF2xe2x95x90CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
CF2xe2x95x90CFOX (per)fluoro-oxyalkylvinylethers, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl; fluorodioxoles, preferably perfluorodioxoles.
Fluorinated comonomers are preferred, preferably those which do not compromise the PTFE thermal stability, in particular perfluoromethoxydioxole (MDO), perfluoropropylvinylether (PPVE), perfluoromethylvinylether (PMVE) and perfluoropropene (PFP).
The formulations of the present invention are used under the form of dielectric film for the insulation of integrated circuits. The thickness of the obtained films is lower than 200 nm, their dielectric strength higher than 4 MV/cm.
The dielectric films of the invention for the insulation of integrated circuits are obtained by the deposition of the formulation on a substratum (integrated circuit), preferably using the spin coating technique, subsequent sintering at a temperature higher than the PTFE melting T, subsequent air-cooling.
In a preferred embodiment of the invention, the deposition is carried out by spin coating, preferably using a constant spinning rate in the range 3,000-10,000 rpm for a time generally comprised between 30 seconds and 5 minutes to assure the uniformity of the thickness and the homogeneity of the deposited film. Then, the obtained film is sintered at a temperature higher than 320xc2x0 C., preferably in the range 390xc2x0 C.-410xc2x0 C.; subsequently the sintered film is air-cooled. A film of dielectric material having a good adhesion to the substratum is obtained, with a dielectric constant lower than 2.2, a thickness lower than 200 nm, the dielectric strength being higher than 4 MV/cm and the weight loss at 425xc2x0 C. in the range 0.0008-0.02%/min.
The total amount of non ionic and anionic surfactants used in the present invention must be such to guarantee a good wettability of the silicon wafer. An excessive amount of surfactants produces surface defects in the obtained films. By using an amount of fluorinated anionic surfactant higher than 25% by weight with respect to PTFE, a dispersion able to wet the wafer surface can be obtained, but the obtained films are unhomogeneous and one cannot obtain a continuous film having a thickness lower than 200 nm. It is presumed that this effect can be due to the fact that by increasing the amount of the anionic surfactant, the thickness of the double electric layer around the dispersion particles increases. This implies a significant increase of the dispersion viscosity and it does not allow the particles to get near each other to form a compact and homogeneous film.
The formulation of the present invention can optionally be added with water, organic solvents, such for example ethyl or isopropyl alcohol; adhesion promoters, etc., foaming agents and other additives, such for example silicon oxide to improve the mechanical properties.
As said, the formulation of the present invention allows to obtain insulating films having a dielectric constant lower than 2.2, a thickness lower than 200 nm, preferably in the range 50 nm-150 nm, and a dielectric strength higher than 4 MV/cm and having a weight loss at 425xc2x0 C. in the range 0.0008-0.02%/min. Such combination of properties is very good for the use as dielectric material for the insulation of conductors in integrated circuits.
The following Examples are mentioned for illustrative purposes, but not limitative of the scope of the invention.