FR2623813A1 - Radiation-crosslinkable masking resin, process for obtaining it and mask employing this resin - Google Patents

Abstract

A new resin crosslinkable under the effect of an electromagnetic or particle radiation and usable as image-transfer material: poly(phenyl alpha -fluoroacrylate).

Description

CROSSLINKING MASKING RESIN
BY RADIATION, ITS PROCESS
PRODUCT AND MASK USING THIS RESIN
The present invention relates to a new resin which can be crosslinked under the action of electromagnetic radiation (rays & X-rays, ultraviolet radiation) or of particles (electrons, ions) and which can be used, for example, as an image transfer material in the production manufacturing masks in microelectronics or integrated optics. It is particularly suitable for use in dry etching processes, such as plasma etching techniques, because of its resistance properties a. this type of engraving.

 The masking resins used for image transfer in microelectronics can be classified into two categories according to their behavior under the effect of radiation: negative resins and positive resins.

 Negative resins will retire when subjected to appropriate radiation and then become insoluble in a number of solvents when they readily dissolve there before being irradiated. They are generally synthetic resins with a polyvinyl alcohol breeze or derivatives of polyvinyl vinylate or polyvinyl acrylate. They lead after development to a relief pattern.

Positive resins are degraded when subjected to appropriate radiation and then become soluble in solutions to which they fully resist before being irradiated. They give rise after development to a hollow pattern
Ideal masking resins must have a high sensitivity to radiation to allow high production rates, high contrast to lead to high resolution, good resistance to etching agents and in particular to dry etching (by plasma). plasma etching is a booming technique due to certain potential advantages over 8. low cost chemical etching, low physical and chemical pollution, and above all excellent resolution as a result of the anisotropy of the attack.

 It turns out that the same resin, whether negative or positive, generally does not meet these three requirements simultaneously. Negative resins have appreciable sensitivity to radiation but their contrast is insufficient.

In contrast, positive resins, which inherently have high contrast, are not sensitive enough to radiation. The resistance to etching, and in particular to dry etching, is better for crosslinkable negative resins, due to the formation of a three-dimensional network, than for positive degradable resins. Furthermore, whatever the type of resin (negative or positive), those which resist the best to dry etching are those which correlatively exhibit the lowest reactivity with respect to radiation during masking. This is for example the case of polystyrene, a negative resin which turns out to be an excellent mask for engraving but whose sensitivity to radiation is insufficient to be compatible with an industrial operation using high production rates. positive resins, poly (sulfone olefins) have a high sensitivity to electrons but a low resistance to dry etching. In contrast, poly (styrene sulfone) has very good plasma stability but lower sensitivity to electrons.

 Research has been carried out in order to find a compromise between the two parameters of radiation sensitivity and resistance to dry etching.

 On the one hand, attempts have been made to improve the resistance to etching of intrinsically sensitive resins. The article published in the "Journal of the Electrochemical Society, No. 128, p. 1304, 1981" describes the association of novolak chain resins with poly (sulfone olefins), which leads to improved resistance to dry etching compared to poly (sulfone olefins) used alone. The article published in the "Journal of the Electrochemical Society, No. 127 (2), p.

491, 1980 "shows that the addition of antioxidants to positive resins of the polymethacrylate type reduces their rate of erosion by a factor of around 2 without altering their sensitivity
A variant consists in incorporating, between the masking and etching operations, a resistant group (aromatic nucleus) in a sensitive resin which initially exhibits no resistance to etching capacity (see Vacuum 35, 10-11, p.

479, 1985).

On the other hand, attempts have been made to improve the sensitivity to radiation of resins intrinsically having a high resistance to dry etching. The incorporation of haloalkyl radicals or halogens on the aromatic nucleus of polystyrene increases the sensitivity to radiation while preserving its properties of stability to plasmas (Journal of
Vacuum Science Technology No. 19, p. 1121, 1981; Journal of the Electrochemical Society, No. 126, p. 1628, 1979; Review of
Applied Physics, vol. 20, no 3, p. 143, 1985). Copolymerization of monomers corresponding to resins resistant to dry etching (styrene for example) with monomers containing entities sensitive to radiation (modified styrene or monomer containing an unsaturated radical or a ring as a pendant group) has proven to be another way to achieve the desired objective (see for example
EP-A-5551), The thermal pre-crosslinking of positive resins by means of comonomer units has enabled a gain in sensitivity compared to the corresponding homopolymers due to the non-prohibitive use of a more powerful developer (IEEE Transactions on Electron Devices ED-29, 4 p.

518, 1982; Polymer Engineering Science No. 23, p. 968, 1983).

Another alternative, based on the use of multilayer systems, has also been widely exploited. In this case, there is total decoupling of the two parameters: sensitivity to radiation and resistance to etching which then fall under one or the other layer (see BJ Lin, ACS Symposium
Serles n ° 219, p. 288, 1983).

 However, all of these solutions, whether relating to single layers or using multilayer systems have major drawbacks.

They require the development of more sophisticated materials (for example copolymers or blends) and cause a complication of the etching process due to the existence of additional operations such as deposits, chemical treatments and heat treatments. This results in an increase in the cost which can be prohibitive for the industrial exploitation of such techniques.

 In order to overcome these drawbacks, the present invention provides a fluorinated resin which makes it possible to best meet the two requirements of sensitivity to radiation and resistance to dry etching.

n indiquant le degré de polymérisation.The subject of the invention is therefore a crosslinkable masking resin under the action of electromagnetic radiation or of particles, characterized in that it corresponds to the formula

n indicating the degree of polymerization.

 The invention also relates to a process for obtaining the masking resin defined above, characterized in that it consists in polymerizing the monomer d - phenyl fluoroacrylate in emulsion, by radical route, under vacuum and in the presence of 'a chain transfer agent.

 Another subject of the invention is a mask using the resin described above.

 In the field of radiation chemistry, a large number of studies have concerned fluorocarbon polymers.

In particular, several fluoroalkyl polymethacrylates have been studied for their behavior as a positive resin in electronic masking or in X-ray masking (see
Journal of the Electrochemicai Society No. 124, p. 1648, 1977).

It has also been shown that the incorporation of fluorine in the d position of an acrylate reversed the behavior under radiation compared to that observed with other substituents such as CN, CH3 and Cl, leading to a negative resin. poly (0 (- - methyl fluoroacrylate) or PMFA described in the "Journal of the Electrochemical Society, n0 128 (8), p. 1758, 1981" whose sensitivity to electrons is relatively low (a much higher dose is required at 10-5C / cm2 at 10 kV, which corresponds to an even higher dose at 20 kV) and whose resistance to dry etching, although better than that of certain positive resins such as PMMA (polymethyl methacrylate), is nevertheless lower than that of novolac resins and polystyrene (see data published in the "Journal of the
Electrochemical Society, vol. 132, no 6, p. 1390, 1985 ").

 French patent applications FR 2 444 022 and FR 2 444 052 describe the synthesis of an acrylic monomer constituted by phenyl fluoroacrylate and its bulk polymerization. The polymer obtained is intended to be used in the aeronautical field for its optical transparency properties. It can be used to make portholes for example. For such applications, the molecular weight of the polymer is not a critical parameter as it turns out to be in - microlithography. In fact, the higher the sensitivity of a masking resin, the higher its initial molecular weight. However, since the resin deposits are usually produced by centrifugation of a polymer solution in an organic medium, the mass of the polymer must be such that the resin remains soluble.

 One of the objects of the present invention relates to the optimization of the synthesis of poly (# - phenyl fluoroacrylate) in order to obtain an appropriate molecular mass and to allow its use as a masking resin of negative type.

n indiquant le degré de polymérisation. Le degré de polymérisation n sera de préférence compris entre 1500 et 2000.The masking resin according to the invention has the following structure

n indicating the degree of polymerization. The degree of polymerization n will preferably be between 1500 and 2000.

This resin is crosslinkable under the action of radiation (#, X rays, ultraviolet, electrons, ions). The radiation doses required to crosslink it during the masking operations are lower than those required for the PMFA already mentioned. On the other hand, the presence of an aromatic nucleus in its structure gives it a glass transition temperature (around 2250 C) much higher than that of PMFA (around 13O0C) which ensures increased thermal stability, therefore better resistance with dry etching. This resin also has good coating and adhesion properties.

 The use of this resin in masking machines makes it possible to benefit from both high production rates and good resolution due to the possibility of using the dry etching techniques with which this resin is compatible as a result of its resistance properties.

 To obtain the resin which is the subject of the present invention, the phenyl o (- fluoroacrylate) monomer is polymerized by the radical route, in emulsion, under vacuum and in the presence of a chain transfer agent.

 The emulsion is obtained by dispersing the monomer in a supercritical aqueous solution of a surfactant, that is to say at a concentration greater than the critical micellar concentration for which the molecules of surfactant associate in the form of micelles.

The polymerization reaction is initiated by a free radical initiator soluble in the aqueous phase. The radicals created in the aqueous phase are trapped by the micelles into which the monomer has penetrated since it is not soluble in water, hence an intramicellar polymerization.
The chain transfer agent is added to limit the molecular weight so that the resin remains soluble.

 By way of nonlimiting example, we will describe the production of a resin according to the invention, that is to say one whose molecular weight is suitable for use as a negative type masking resin.

The polymerization is carried out in a vacuum-sealed tube at 50 ° C., with magnetic stirring for a determined period. Prior to sealing the tube, the reaction mixture is degassed in order to remove the dissolved oxygen which is a polymerization inhibitor
The surfactant used is sodium dodecyl sulfate (SDS), the radical initiator is potassium persulfate K2S208 and the chain transfer agent is lev potassium K2S2 O8 bromotrichloromethane CCl3Br.

 The polymer is recovered after opening the tube by extraction in an organic medium. It is then isolated by precipitation and then collected by filtration. After which, it is dried under vacuum.

 Its structure is checked by infrared spectrometry and by nuclear magnetic resonance. It is characterized by gel permeation chromatography, viscosimetry, differential enthalpy analysis and thermogravimetry.

 The method of using the resin according to the invention as an image transfer material is particularly simple and conforms to the standards of those skilled in the art. It is dissolved in a suitable solvent, for example methyl isobutyl ketone and then deposited in a thin layer (typically from 0.5 to 1, u) on a substrate (for example oxidized silicon or gallium arsenide) by centrifugation of this solution.

 The substrate is then irradiated in order to cause the formation of bridging bonds in the resin (crosslinking), making it insoluble in a suitable developer. Different types of radiation can be used. One can use for example an electron radiation (typically of energy between 5 and 80 keV) in Gaussian beam or in formed beam. One can also use ions (for example H or Ga of energy between 50 and 400 keV) either in parallel beam through a mask, or in focused beam. Photons can also be used ~ either the g radiation emitted 13t for example by conventional sources such as 55Cs or 60 2 # o, or near ultraviolet radiation # (wavelength between 300 and 400 nm) or distant (wavelength between 200 and 300 nm) coming for example from a mercury vapor lamp or a deuterium lamp respectively, or still soft X-ray (wavelength between 4 and 50 Angstroms), continuous (for example in the form of synchrotron radiation in a collision ring) or originating from the emission of characteristic lines.

 After revelation, the sample is heat treated before the etching step of the substrate to perfect the hardening of the resin. Dry etching techniques (for example reactive ion etching) which are capable of providing increased resolution compared to chemical etching can be used with this resin which exhibits effective resistance properties due to its aromatic structure. After etching the substrate, the resin can be removed by subjecting it to an oxygen plasma.

We will now describe an example of synthesis of the polymer according to the invention. In a tubo to be sealed provided with a magnetic bar, one pours a mixture made up of
- 25 g of distilled water, - 0.144 g of sodium dodecyl sulfate (5.10 -4 mole),
- 0.0135 g of potassium persulfate (5.

mole), - 10 g of o & - phenyl fluoroacrylate
(6.10-2mole),
- 0.5 g of bromotrichloromethane (2.5.10 3mole).

 This mixture is subjected to three degassing cycles (freezing / vacuum pumping / decon gelation) before sealing the tube under vacuum. The degassing pressure of the tube is controlled and kept constant (6 pascals). Then, the sealed tube is placed in an oil bath thermostatically controlled at 500C with magnetic stirring, and the polymerization is allowed to proceed for 5 hours.

Once this time has elapsed, the tube is opened and its contents are diluted with acetone. T, a solution obtained is poured dropwise on a volume of hexane at least ten times greater. The precipitating polymer is then collected by filtration and then dried in a vacuum oven at 300 ° C. for 48 hours, in the presence of phosphoric anhydride to remove the water. The monomer / polymer conversion rate is 48 8. The intrinsic viscosity of the latter, measured in the monomethyl ether of ethylene glycol acetate (EMMEGA) at 300C is 1.48 dl / g. thermogravimetric analysis reveals a remarkable thermal stability of this polymer (weight loss
Lower 1 i% up to 2250C and lower than 2% up to 3150 C). The differential enthalpy analysis thermogram reveals a glass transition temperature of 2280C
We will now describe the behavior under electronic irradiation of the polymer developed above. To use this polymer as a masking resin, it is dissolved in a mixture consisting of EMMEGA and acetone (weight ratio 26.3 / 73.7). The concentration by weight of polymer in the solvent is 5 90. The resin is then deposited by centrifugation of the solution in the form of a thin film (thickness 0.5 μl) on two types. of substrate. The first substrate is silicon oxidized on the surface including the oxide layer
SiO2 is approximately 1000 angstroms thick. The second substrate consists of gallium arsenide. No post-coating firing was carried out in order to avoid any thermal crosslinking.

 The samples thus prepared are irradiated with the EPG 102 electronic masker (electron acceleration voltage 20kV, Gaussian beam) by drawing a resolution target with a range of doses. The samples are then revealed by immersion in a mixture of acetone and isopropanol (proportion 90/10 by volume) for 30 seconds at 200 ° C., which leaves the irradiated areas on the substrate. As an indication, for the sample on oxidized silicon, the dose making it possible to obtain the insolubilization of 70% of the initial thickness of resin is 5.10 6 C / cm2. Lines of width less than 0.3, u have been drawn by superimposing single scans.

 The behavior of the resin under dry etching was analyzed on the sample on gallium arsenide. For this, a reactive ion etching frame with planar configuration was used, the interelectrode distance of which is 4 cm and operating at 13.56 MHz. The gas used for the etching of gallium arsenide is dichlorodifluoromethane. Between the revelation and the etching, the sample was annealed at 2000C for 30 min, after which it was introduced into the etching chamber and this was carried out under the following conditions: power 0.5 W / cm2, pressure 4 pascals, gas flow 2 cm3 / min, self-polarization voltage -50 V, etching time 3 min. It then appeared that the polymer remained insensitive to etching, its thickness remaining perfectly constant, which was not the case with another negative resin not containing an aromatic nucleus, the copolymer of thioglycidyl methacrylate. and methyl methacrylate (of proportions 1/3), the loss of thickness of which after engraving was around 10%.

Claims (10)

 1. Masking resin crosslinkable under the action of electromagnetic radiation or particles, characterized in that it corresponds to the following formula

 n indicating the degree of polymerization.  2. Masking resin according to claim 1, characterized in that the degree of polymerization n is between 1500 and 200.  3. Masking resin according to one of claims 1 or 2, characterized in that said electromagnetic radiation consists of rays, X-rays or ultraviolet radiation.  4. Masking resin according to one of claims 1 or 2, characterized in that said particles are electrons or ions.  5. Method for obtaining the masking resin according to any one of claims 1 to 4, characterized in that it consists in polymerizing the monomer # - phenyl fluoroacrylate in emulsion, by radical route, under vacuum and in the presence of a chain transfer agent.  6. Method according to claim 5, characterized in that the emulsion is obtained by dispersing said monomer in a supercritical aqueous solution of a surfactant.  7. Method according to claim G, characterized in that the tenso-aetif agent used is sodium dodecyl sulfate.  8, Process according to any one of claims 5 to 7, characterized in that potassium persulfate is used as a radical initiator.  9. Method according to any one of claims 5 to 8, characterized in that the chain transfer agent used is bromotrichioromethane.  10. Resin mask, characterized in that it uses a resin according to any one of claims 1 to 4.