JP5054066B2 - Film-type photodegradable transfer material - Google Patents

Description

  The present invention relates to a film-type transfer material used for pattern formation by photolithography.

  In the manufacture of semiconductors, etc., in order to obtain a high degree of integration, the resist image has been miniaturized, and in order to satisfy such a demand, a photolithography technique using a film type transfer material has been performed. Yes.

  Photolithography technology using a film-type transfer material is designed such that a photoresist layer of a film-type transfer material is laminated on a substrate for pattern formation, and then exposed, developed and etched. Get.

  At the time of exposure, exposure is performed through an exposure mask on which a predetermined pattern is formed. However, unintentional partial pattern distortion is often generated that is not as the exposure mask pattern.

  As a cause of pattern distortion, there is a so-called reflective notching caused by light reflected obliquely at the time of exposure. This is disclosed in Non-Patent Document 1.

  As one method for solving such a problem, a method of adding a dye to a photoresist disclosed in Patent Documents 1 and 2 and the like can be mentioned. However, if the above problem is solved by adding a dye to the photoresist to form a resist film having a high absorption with respect to the exposure wavelength, the sensitivity of the resist is reduced, or the problem during the curing process, alkali Other problems appear, such as resist thinning in the developer and dye sublimation during baking. In addition to the addition of dyes to the resist, surface imaging (TSI) methods are known.

  In addition to such a method, a method of applying a separate antireflection film before the formation of the photoresist layer is disclosed, but this also requires an additional step and is complicated.

US Pat. No. 4,575,480 US Patent No. 4,882,260 M.M. Bolsen, G.M. Buhr, H .; Merrem and K.M. VanWerden, Solid State Technology, Feb. 1986, p. 83

  Therefore, an aspect of the present invention is to provide a film-type photodegradable transfer material that can prevent reflection by a substrate during exposure.

  In one embodiment of the present invention, a film-type photodegradable transfer material that can be easily implemented in a target pattern shape and can improve resolution and fine line adhesion is provided.

  In order to solve the above problems, in one embodiment of the present invention, a film-type photodegradable transfer comprising: a support film; a photodegradable photoresist layer; and an antireflection layer formed on a surface laminated to the substrate. Provide material.

  According to a preferred aspect of the present invention, the film-type photodegradable transfer material may further include a protective layer.

  According to one embodiment of the present invention, the antireflection layer can have an absorptance of 90% or more for light having a wavelength in the range of 200 nm to 700 nm.

  According to one aspect of the present invention, the antireflection layer can include a black dye, a black pigment, or a pigment mixture that substantially exhibits a black hue in addition to the same composition as the photodegradable photoresist layer.

  According to one aspect of the present invention, the pigment mixture that substantially exhibits a black hue may include a mixture of a red pigment, a yellow pigment, and a blue pigment.

  According to one aspect of the present invention, the antireflection layer can have a thickness of 0.1 to 1.0 μm.

  According to one aspect of the present invention, the photodegradable photoresist layer can include an alkali-soluble resin and a photosensitive compound.

  According to one aspect of the present invention, the alkali-soluble resin can be a novolac resin.

  According to one aspect of the present invention, the alkali-soluble resin can be a cresol novolac resin.

  According to one aspect of the present invention, the photosensitive compound comprises 2,3,4,4′-tetrahydroxybenzophenone-1,2-naphthoquinonediazide-5-sulfonate, 2,3,4-trihydroxybenzophenone-1, 2-Naphthoquinonediazide-5-sulfonate and (1- [1- (4-hydroxyphenyl) isopropyl] -4- [1,1-bis (4-hydroxyphenyl) ethyl] benzene) -1,2-naphthoquinonediazide- It may be one or more selected from 5-sulfonates.

  According to one aspect of the present invention, 2,3,4-trihydroxybenzophenone, 2,3,4,4′-tetrahydroxybenzophenone and 1- [1- (4-hydroxyphenyl) isopropyl] -4- [1 , 1-bis (4-hydroxyphenyl) ethyl] benzene may include one or more sensitivity enhancers.

  As described above, since the film-type photodegradable transfer material according to one embodiment of the present invention can prevent distortion of a pattern at the time of exposure, a pattern having a desired shape can be realized. Fine wire adhesion can be improved.

  Hereinafter, the present invention will be described in detail.

The film-type photodegradable transfer material according to one embodiment of the present invention includes a support film and a photodegradable photoresist layer, and a reflection suppressing layer is formed on a surface in contact with the substrate.
The film-type photodegradable transfer material according to one embodiment of the present invention is used in contact with a substrate on which a pattern is to be formed.
Hereinafter, the term “film-type photodegradable transfer material” means “positive type photoresist film”, “photoresist resin film”, “positive type dry film photoresist”, or Although it may be referred to as “positive photoresist resin film” or the like, it will be easily understood by those skilled in the art.

  The reflection suppression layer is for reducing the reflection of light from the substrate. In this respect, the reflection suppression layer absorbs at least 90% of light having a wavelength in the range of 200 nm to 700 nm. it can. More preferably, at least 90% or more of light having a wavelength in the range of 200 nm to 410 nm can be absorbed.

  When the antireflection layer is included, the light that has passed through the photoresist layer is absorbed by the antireflection layer, so that there is no problem of reflection from the substrate. As the reflection suppressing layer, an inorganic or organic layer can be used. As an inorganic reflection suppression layer, for example, literature [see Proc. SPIE. vol. 1086 (1989), p. 242 described in 242. Nolscher et al .; K., described in Thin Solid Films, 200, 93 (1991). Bather, H.M. Schreiber's paper; Microelectronic Engineering, 21 (1993), p. 51 described in G.51. It can be a film of TiN, TiNO, TiW, an inorganic polymer having a thickness of 300 mm disclosed in a paper by Czech et al. In addition, an inorganic film such as a titanium film, a chromium oxide film, a carbon film, or an α-silicon film can be used. These inorganic antireflection layers can be usually formed by a method such as vacuum deposition, CVD, or sputtering.

  On the other hand, as the organic antireflection layer, a polymer film to which a dye that absorbs light having an irradiation wavelength is added [see Proc. SPIE, vol. 539 (1985), p. 342].

  As another method, a method of forming a reflection suppressing layer by chemically bonding a dye to the film-forming polymer can be mentioned. Examples thereof include Fahey et al. [Ref: Proc. SPIE, vol. 2195, p. 422] reported by reacting an acid anhydride group of poly (vinyl methyl ether-co-maleic anhydride) with an amino group-containing dye can be used as a material for the antireflection layer of one embodiment of the present invention. .

In addition to the above, any material that satisfies the following characteristics can be used as the antireflection layer forming material:
(A) Good film formability (b) High light absorptance at the intended exposure wavelength (c) Non-mixability with photoresist In this respect, the film-type photodegradable transfer material according to one embodiment of the present invention The antireflection layer may be a layer containing a black dye, a black pigment, or a pigment mixture that substantially exhibits a black hue in the same composition as the photoresist layer. Such a dye, pigment, or pigment mixture is preferably thoroughly mixed with components common to the photoresist layer.

  In the film-type photodegradable transfer material according to one aspect of the present invention, the antireflection layer includes a mixture of a red pigment, a yellow pigment, and a blue pigment as a pigment mixture that exhibits a black hue in addition to the same composition as the photoresist layer. Can be.

  Examples of the material for forming an antireflection layer that can be easily purchased commercially include AR40 manufactured by Rohm & Hass, but are not limited thereto.

  When the antireflection layer is formed, the thickness is not particularly limited, but a thickness of about 0.1 to 1.0 μm is considered appropriate. When the thickness is less than 0.1 μm, the light absorptivity can drop to 90% or less, and when it is thicker than 1.0 μm, the exposure amount increases and the workability can be deteriorated.

  On the other hand, in the one-film photodegradable transfer material of one embodiment of the present invention, the composition constituting the photodegradable photoresist layer is not particularly limited, but the photoresist layer is composed of an alkali-soluble resin and a diazide-based photosensitive material. In particular, a novolak resin can be used as the alkali-soluble resin, and a cresol novolak resin can be more preferably included.

  The novolac resin can be obtained by a polycondensation reaction of phenols alone or in combination of phenols, aldehydes and an acidic catalyst.

  At this time, the phenols are not particularly limited, and phenol, o-cresol, m-cresol, p-cresol, 2,3-xylenol, 2,5-xylenol, 3,4-xylenol, 3, Monohydric phenols such as 5-xylenol, 2,3,5-trimethylphenol-xylenol, 4-t-butylphenol, 2-t-butylphenol, 3-t-butylphenol, 4-methyl-2-t-butyl; and List polyhydric phenols such as 2-naphthol, 1,3-dihydroxynaphthalene, 1,7-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, resorcinol, pyrocatechol, hydroquinone, bisphenol A, phloroglucinol, pyrogallol Can choose from these alone There may be used in combination of two or more. In particular, a combination of m-cresol and p-cresol is preferable.

  The aldehydes are not particularly limited, but formaldehyde, trioxane, paraformaldehyde, benzaldehyde, acetaldehyde, propyl aldehyde, phenylacetaldehyde, alpha or beta-phenylpropyl aldehyde, o-, m- or p-hydroxybenzaldehyde, Glutaraldehyde, terephthalaldehyde, etc. can be mentioned, and it can be used alone or in combination of two or more.

  The cresol novolak resin preferably has a weight average molecular weight (when based on the GPC measurement method) of 2,000 to 30,000, and the cresol novolak resin has a photosensitivity and a remaining film ratio depending on the content ratio of meta / paracresol. Therefore, it is preferable that the content of meta / paracresol is mixed in a ratio of 4: 6 to 6: 4 on a weight basis.

  If the content of metacresol in the cresol novolak resin exceeds the above range, the photosensitivity increases and the residual film rate decreases rapidly. On the other hand, if the content of paracresol exceeds the above range, the photosensitivity decreases. There are disadvantages.

  As the cresol novolac resin, a cresol novolak resin having a meta / paracresol content of 4: 6 to 6: 4 on a weight basis can be used alone, but more preferably, different resins can be mixed and used. it can. In this case, the cresol novolak resin is mixed with a cresol novolak resin having a weight average molecular weight of 8,000 to 30,000 and a cresol novolak resin having a weight average molecular weight of less than 2,000 to 8,000 in a ratio of 7: 3 to 9: 1. It is preferable to use a mixture.

  As described above and hereinafter, “weight average molecular weight” is defined as a converted value of polystyrene equivalent determined by gel permeation chromatography (GPC).

  On the other hand, in the composition of the photoresist layer, the diazide-based photosensitive compound acts as a dissolution inhibitor that reduces the solubility of the alkali-soluble resin in alkali, and when irradiated with light, it changes to an alkali-soluble substance and becomes alkali-soluble. It serves to increase the alkali solubility of the resin. As described above, the exposed portion of the film-type photodegradable transfer material of the present invention is developed by the change in solubility due to light irradiation.

  The diazide-based photosensitive compound can be synthesized by an esterification reaction between a polyhydroxy compound and a quinonediazide sulfonic acid compound. The esterification reaction for obtaining a diazide-based photosensitive compound is carried out by converting a polyhydroxy compound and a quinonediazide sulfonic acid compound into dioxane, acetone, tetrahydrofuran, methyl ethyl ketone, N-methylpyrrolidone, chloroform, triethylamine, N-methylmorpholine, N-methylpiperazine or After a basic catalyst such as 4-dimethylaminopyridine is dropped and condensed, the resulting product can be obtained by washing, purifying and drying.

  Here, examples of the quinonediazide sulfonic acid compound include 1,2-benzoquinonediazido-4-sulfonic acid, 1,2-naphthoquinonediazide-4-sulfonic acid, 1,2-benzoquinonediazide-5-sulfonic acid, and Examples include o-quinonediazidesulfonic acid compounds such as 1,2-naphthoquinonediazide-5-sulfonic acid, and other quinonediazidesulfonic acid derivatives.

  The quinonediazide sulfonic acid compound itself has a function as a dissolution inhibiting system that lowers the solubility of the alkali-soluble resin in an alkali. However, it is decomposed during exposure to become alkali-soluble, thereby having the property of promoting the dissolution of the alkali-soluble resin in the alkali.

  Examples of the polyhydroxy compound include trihydroxybenzophenones such as 2,3,4-trihydroxybenzophenone, 2,2 ′, 3-trihydroxybenzophenone, and 2,3,4′-trihydroxybenzophenone; Tetrahydroxybenzophenones such as 4′-tetrahydroxybenzophenone, 2,2 ′, 4,4′-tetrahydroxybenzophenone, 2,3,4,5-tetrahydroxybenzophenone; 2,2 ′, 3,4,4 ′ Pentahydroxybenzophenones such as pentahydroxybenzophenone and 2,2 ′, 3,4,5-pentahydroxybenzophenone; 2,3,3 ′, 4,4 ′, 5′-hexahydroxybenzophenone, 2,2 ′, 3,3 ′, 4,5′-hexahydroxybenzophenone and other hexes Hydroxybenzophenones; gallic acid alkyl esters; such as oxy flavones can be exemplified.

  Specific examples of the diazide-based photosensitive compounds obtained from these include 2,3,4,4′-tetrahydroxybenzophenone-1,2-naphthoquinonediazide-5-sulfonate, 2,3,4-trihydroxy Benzophenone-1,2-naphthoquinonediazide-5-sulfonate and (1- [1- (4-hydroxyphenyl) isopropyl] -4- [1,1-bis (4-hydroxyphenyl) ethyl] benzene) -1,2 -1 or more types chosen from naphthoquinone diazide-5-sulfonate can be mentioned.

  As for such a diazide-based photosensitive compound, it is advantageous in terms of developability and solubility that the composition of the photoresist layer is 30 to 80 parts by weight with respect to 100 parts by weight of the alkali-soluble resin. .

  Meanwhile, in the film-type photodegradable transfer material according to an aspect of the present invention, the photoresist layer may include a sensitivity enhancer. This is to improve sensitivity. Examples include 2,3,4-trihydroxybenzophenone, 2,3,4,4′-tetrahydroxybenzophenone and 1- [1- (4-hydroxyphenyl) isopropyl] -4- [1,1-bis. One or more selected from (4-hydroxyphenyl) ethyl] benzene.

  When the sensitivity enhancer is included, the content thereof is preferably 3 to 15 parts by weight based on 100 parts by weight of the alkali-soluble resin in terms of improving the photosensitive effect and window process margin.

  Furthermore, the photoresist layer can contain other components and additives such as leveling agents, fillers, and antioxidants.

  A composition containing such an alkali-soluble resin, a diazide-based photosensitive compound, etc. is dispersed in a certain amount of solvent to form a liquid, and after coating on a film support film and drying, a photoresist layer is formed. can do.

  Examples of the solvent include ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, propylene glycol monoethyl ether acetate, acetone, methyl ethyl ketone, ethyl alcohol, methyl alcohol, propyl alcohol, and isopropyl alcohol. , Benzene, toluene, cyclopentanone, cyclohexanone, ethylene glycol, xylene, ethylene glycol monoethyl ether, and diethylene glycol monoethyl ether.

  The mixture is applied to the support film at a thickness of 1 to 100 μm. Forming a photoresist layer on the support film is a composition mixed with the solvent by a commonly used coating method such as a roller, a roll coater, a Meyer rod, a gravure, or a spray. This can be carried out by coating the product on the above described film and drying it to volatilize the solvent in the composition. If necessary, the applied composition may be heat-cured.

  On the other hand, in the film-type transfer material, the support film is not particularly limited as long as it satisfies the transparency that does not impair the light transmittance. Examples are polycarbonate film, polyethylene (PE) film, polypropylene (PP) film, stretched polypropylene (OPP) film, polyethylene terephthalate (PET) film, polyethylene naphthalate (PEN) film, ethylene vinyl acetate (EVA) film, polyvinyl Including films, other suitable polyolefin films, epoxy films and the like. Particularly preferred polyolefin films are polypropylene (PP) film, polyethylene (PE) film, ethylene vinyl acetate (EVA) film and the like. Preferred polyvinyl films are polyvinyl chloride (PVC) films, polyvinyl acetate (PVA) films, polyvinyl alcohol (PVOH) films, and the like. Particularly preferred polystyrene films include polystyrene (PS) films, acrylonitrile / butadiene / styrene (ABS) films, and the like. The support film preferably has a thickness in the range of about 10-50 μm, preferably about 15-50 μm, more preferably about 15 in order to serve as a skeleton for shape support of the dry film resist. It can have a thickness in the range of ˜25 μm.

  Further, the manufactured positive photoresist film of the present invention may further include a protective layer on the upper surface of the photoresist layer. Such a protective layer serves to protect the photoresist layer from air blocking and foreign matter, and is preferably formed of a polyethylene film, a polyethylene terephthalate film, a polypropylene film, etc., and has a thickness of 15 to More preferably, it is 30 μm.

  The formation of the antireflection layer is performed by a coating method using a roller, a roll coater, a Meyer rod, a gravure, a spray, etc. on the surface of the photoresist layer, as in the formation of the photoresist layer. A protective layer can be formed on the protective layer, or alternatively, a reflection suppressing layer can be formed on the protective layer by a method such as a roller, roll coater, Myer rod, gravure, or spray. A method of attaching a support film formed on the surface of the resist layer is also possible, but the method is not particularly limited thereto.

On the other hand, the method of forming a pattern using the photoresist resin film of the present invention is the same as the method of using a general photoresist resin film,
(I) A dry film photoresist is placed on a glass substrate so that the antireflection layer of the dry film photoresist produced according to the present invention is in contact, and the support film is peeled off from the photoresist resin film as necessary. The step of:
(II) a step of directly irradiating ultraviolet rays through a mask or without passing through the mask in order to obtain a desired pattern on the coating; and (III) when the support film is not peeled off from the photoresist resin film. And removing the photoresist layer in the irradiated portion by a development process, and then forming a resist pattern film.

  The step (I) is a step of forming a positive-type photoresist resin film on the anti-reflection layer by adhering the anti-reflection layer of the positive-type dry film photoresist on the substrate. At this time, the support film of the dry film photoresist may not be peeled off. Moreover, it is not necessary to dry the antireflection layer and the photoresist resin film formed on the substrate.

  If necessary, when developing in step (III), before or after step (II) as necessary, in order to strengthen the determination power with the base material so that the resist pattern film is not washed off. A heat treatment (baking) step is required. More specifically, after performing the step (II), after forming the positive photoresist resin film on the base material, after peeling the support film from the photoresist resin film, Therefore, after performing the step (II), the support film of the photoresist resin film is peeled off, and the heat treatment (Baking) is performed to enhance the fixing power with the substrate. It can be performed. Or after performing a process (II), a support body film can be peeled from the said photoresist resin, and this peeled film can also be heat-processed, and the adhesiveness with respect to a base material can also be strengthened. That is, the heat treatment process can be variously repeated depending on the requirements of the photoresist film, the complexity of the solvent system, and the difference in boiling point.

  In this way, a desired resist pattern film is formed by the steps (I), (II) and (III).

  The developer for developing the positive photoresist resin film of the present invention is preferably 2.38% tetramethylammonium hydroxide (TMAH).

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on the Example of this invention, the scope of the present invention is not limited to these Examples.

  In the following examples, the light absorptivity of the antireflection layer is specifically determined so that the antireflection layer is on an optical polyethylene terephthalate film (thickness, 100 μm) and the thickness after drying is 0.1 to 1.0 μm. It applied so that it might become. With respect to the film thus obtained, the light transmittance at a wavelength of 200 to 700 nm was measured by a magnetic spectrophotometer (UV-3101PC, SHIMADZU) method.

<Example 1>
34 parts by weight of 2,3,4-trihydroxybenzophenone-1,2-naphthoquinonediazide-5-sulfonate as a photosensitive compound with respect to 100 parts by weight of an alkali-soluble resin (phenol novolac resin); 3.6 as a sensitivity enhancer A solution containing parts by weight of 2,3,4-trihydroxybenzophenone; and 220 parts by weight of methyl ethyl ketone was prepared.

  The prepared photoresist layer forming solution was filtered through a 0.2 μm millipore Teflon filter to remove insoluble materials.

  The resulting solution was applied as a film substrate on a polyethylene terephthalate film (thickness, 19 μm) to a thickness of 3 μm to form a photoresist layer.

  The antireflection layer was produced by mixing the same composition as that of the photoresist, and further mixing 56 parts by weight of carbon black (Mikuni) with propylene glycol monoethyl ether acetate as a solvent.

  On the other hand, a reflection suppressing layer (thickness 0.1 μm, absorption rate 90% for 200 to 700 nm wavelength light) was formed on the photoresist layer by the gravure coating method using the thus prepared reflection suppressing layer.

  Thereafter, a polyethylene terephthalate film (thickness: 16 μm) was coated on the antireflection layer to produce a positive photoresist film.

<Example 2>
Although manufactured by the same method as in Example 1, it was manufactured so that the thickness of the photoresist layer was 2.0 μm and the thickness of the antireflection layer was 1.0 μm.

<Example 3>
The same composition and thickness as in Example 1 were prepared, but the antireflection layer was first coated on a protective film polyethylene terephthalate film (thickness 16 μm), and then the photoresist layer and antireflection layer were in contact with each other. Made by wearing.

<Comparative Example 1>
A positive photoresist film was produced in the same manner as in Example 1, but no antireflection layer was formed.

<Evaluation>
The produced film was laminated to a glass substrate at a lamination speed of 2 m / min, a temperature of 110 ° C. and a heating roll pressure of 70 psi, and then the support film was peeled off, and the positive photoresist layer was heat-treated on a 100 ° C. hot plate for 120 seconds. Then, it is irradiated (exposed) with ultraviolet rays through a photomask, developed with a 2.38% TMAH alkaline developer for an appropriate period of time, then washed and dried for 30 seconds to leave the unexposed part and form a circuit. The resolution at this time was evaluated with an electronic microscope.
When the resolution was 3 μm or less, it was evaluated as “very good”, and when it exceeded 3 μm, it was evaluated as “good”.
Also, the obtained pattern is observed using a scanning electron microscope (SEM), and if the pattern end face shape is similar to a square, the pattern stability is evaluated as “very good”, and similar to a trapezoid. If so, the pattern stability was evaluated as “good”.
These resolutions and pattern shape observation results are summarized. If the resolution is 3 μm or less and the pattern end face shape is similar to a square, the adhesion is evaluated as “very good”, the resolution exceeds 3 μm, In addition, if the pattern end face shape was similar to a trapezoidal shape, the adhesion was evaluated as “good”.

  The results are shown in Table 1 below.

  * Very good: ◎, good: ○

  The present invention is applicable to a film type transfer material used for pattern formation by photolithography.

Claims (10)

A support film;
A photodegradable photoresist layer; and an antireflection layer formed on the surface laminated to the substrate;
Including
The film-type photodegradable transfer material, wherein the antireflection layer includes a black dye, a black pigment, or a pigment mixture that substantially exhibits a black hue in addition to the same composition as the photodegradable photoresist layer. .   The film-type photodegradable transfer material according to claim 1, further comprising a protective layer.   The film-type photodegradable transfer material according to claim 1 or 2, wherein the antireflection layer has an absorptance of 90% or more for light having a wavelength in the range of 200 nm to 700 nm.   The film-type photodegradable transfer material according to claim 1, wherein the pigment mixture that substantially exhibits a black hue includes a mixture of a red pigment, a yellow pigment, and a blue pigment.   The film-type photodegradable transfer material according to claim 1, wherein the antireflection layer has a thickness of 0.1 to 1.0 µm.   The film-type photodegradable transfer material according to claim 1, wherein the photodegradable photoresist layer contains an alkali-soluble resin and a photosensitive compound.   The film-type photodegradable transfer material according to claim 6, wherein the alkali-soluble resin is a novolak resin. The film-type photodegradable transfer material according to claim 6 or 7 , wherein the alkali-soluble resin is a cresol novolac resin.   Photosensitive compounds are 2,3,4,4′-tetrahydroxybenzophenone-1,2-naphthoquinonediazide-5-sulfonate, 2,3,4-trihydroxybenzophenone-1,2-naphthoquinonediazide-5-sulfonate and One selected from (1- [1- (4-hydroxyphenyl) isopropyl] -4- [1,1-bis (4-hydroxyphenyl) ethyl] benzene) -1,2-naphthoquinonediazide-5-sulfonate The film-type photodegradable transfer material according to claim 6, which is as described above.   2,3,4-trihydroxybenzophenone, 2,3,4,4′-tetrahydroxybenzophenone and 1- [1- (4-hydroxyphenyl) isopropyl] -4- [1,1-bis (4-hydroxyphenyl) The film-type photodegradable transfer material according to claim 6, wherein the film-type photodegradable transfer material comprises one or more sensitivity enhancers selected from the group consisting of: