Abstract:
The invention relates to solvent systems which are based on water-soluble amino derivatives and propylene glycol components, for removing photoresists. These solvent systems are comprised of 
     (a) from about 10 to 100% by weight of at least one water-soluble amine of the general formula ##STR1##  in which R 1 , R 2 , R 3 , R 4  denote H or alkyl groups 
     n,m denote 0 to 2, and 
     o denotes 1 to 3; and 
     (b) from about 0 to 90% by weight of at least one water-soluble propylene glycol derivative of the general formula ##STR2##  in which R 3 , R 4  denote H, alkyl or ##STR3##  and P denotes 1 to 3.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to novel solvent systems, based on water-soluble amino derivatives and propylene glycol compounds, that are useful for removing photoresists. 
     Photoresists are used as masks in the production of, via etching or implantation processes, integrated component parts, circuit boards, and the like. After having served their purposes, the masks are generally removed, frequently by means of agents which, depending on their mode of action, are known as &#34;strippers&#34; or &#34;removers&#34;. 
     Systems of this kind are described, for example, by W. S. de Forest in &#34;Photoresist: Materials and Processes&#34; pages 203 et seq., McGraw-Hill Book Co., New York, 1975. They contain, e.g., chlorinated hydrocarbons, such as methylenechloride; polar solvents, such as dimethylformamide, N-methyl-2-pyrrolidone and monoethanolamine; glycol ethers, such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether and the acetates thereof; strong bases, such as tetramethyl ammonium hydroxide; and strong acids, such as sulfuric acid/H 2  O 2  mixtures. 
     U.S. Pat. No. 3,673,099 describes a mixture of N-methyl-2-pyrrolidone and a strong base, such as tetramethyl ammonium hydroxide. It is known that mixtures of this type attack sensitive substrates, such as aluminum layers, when water is added. It is, however, desirable for a product to be suitable for removing photoresist coatings both from sensitive and less sensitive substrate layers, since quite frequently these two types of substrate layers are used simultaneously. Moreover, tetramethyl ammonium hydroxide easily decomposes when heated, and thus the stripping solution has a very short service life at elevated temperatures. 
     U.S. Pat. No. 3,796,602 describes a system comprising an organic solvent which is partially miscible with water. However, the compounds mentioned in this publication, such as ethylene glycol monobutyl ether and monoaminoethanol, present toxicological problems. Furthermore, these compounds evaporate easily due to their low boiling points and, hence, cannot be employed at elevated temperatures. 
     European Patent Application No. 0 075 329 describes a mixture of a derivative of 2-pyrrolidone and a diethylene glycol monoalkyl ether. Compounds of this kind are, however, hardly effective in their action. In the examples of the document, for instance, a stripper bath temperature of 75° C. is required for removal of a photoresist subjected to a post-bake treatment at about 150° C. 
     European Patent Application No. 0 145 973 describes a system of a piperazine derivative and a N-alkyl-2-pyrrolidone. The preferably employed N-aminoethyl piperazine is classified as being toxic, whereas n-hydroxyethyl piperazine is scarcely effective. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an agent for photoresist removal that can be used at low temperatures and at elevated temperatures, that does not cause toxic problems, that can be rinsed off with water and is completely, or at least largely, non-corrosive of sensitive substrates. 
     In accomplishing the foregoing object, there has been provided, in accordance with one aspect of the present invention, an agent for photoresist removal that is comprised of: 
     (a) 10 to 100% by weight of at least one water-soluble anine represented by the formula ##STR4## in which R 1 , R 2 , R 3 , R 4  denote H or alkyl groups 
     n, m denote 0 to 2, and 
     o denotes 1 to 3; 
     and 
     (b) 0 to 90% by weight of at least one water-soluble propylene glycol derivative represented by the formula ##STR5## in which R 3 , R 4  denote H, alkyl or ##STR6##  groups, and P denotes 1 to 3. 
     Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The solvent systems of the present invention are suitable for removing both positive-working resists and photopolymerizable resists. Resists can be developed with aqueous-alkaline solutions are preferably removed. 
     It has been found that mixtures made according to the instant invention, comprising water-soluble propylene glycol ethers and amino compounds, can be used as agents for photoresist removal, and do not exhibit the disadvantages of the prior art cited above. Preference is given to mixtures which, due to the high boiling temperatures (at least 180° C.) of the individual components, result in only slight weight losses when the agent is used at elevated temperatures over a prolonged period of time. A mixture of N-(2-amino-ethyl)ethanolamine and tripropylene glycol methyl ether is particularly preferred. 
     It was not foreseeable that the photoresist remover of the present invention, comprising high-boiling propylene glycol ether derivatives such as tripropylene glycol methyl ether, would exhibit satisfactory properties even at room temperature. Thus, it is possible to use the remover of the present invention to particular advantage for the purpose of the invention, since it can be employed effectively at room temperature and, due to its favorable evaporation characteristics, at elevated bath temperatures as well. 
     It has also been found that the particularly preferred mixture of tripropylene glycol methyl ether and N-(2-aminoethyl)-ethanolamine has a synergetic effect. 
     Since, in practice, photoresist layers which were not exposed to high temperatures have to be removed quite frequently, mixtures whose individual components have boiling points of 160° C. or higher are also preferable. Of these, mixtures of dipropylene glycol methyl ether and isopropanolamine are particularly preferred. These systems, too, exhibit the advantages described above, i.e., they can be used at room temperature and at elevated temperatures. Due to their higher evaporation rates, however, these removers are preferably employed at relatively low bath temperatures, and they have a synergetic effect. 
     It is true that the propylene glycol ethers, such as dipropylene glycol methyl ether or tripropylene glycol methyl ether which are preferably employed within this invention, are distinctly more hydrophobic than comparable ethylene glycol ether products, and hence they possess good solubilizing properties with respect to organic polymers. Nevertheless, the compounds used in the present invention can be easily rinsed off with water without leaving any residues. 
     Despite their high efficiency, the mixtures according to the present invention do not corrode sensitive substrates, such as aluminum layers. 
     Mixtures containing 10 to 90% by weight, and in particular 20 to 70% by weight, of the amine and 10 to 90% by weight, and in particular 30 to 80% by weight, of the propylene glycol derivative, are preferred. It is particularly preferred that these ratios be employed with isopropanolamine and/or N-(2-amino-ethyl)ethanolamine and di- or tripropylene glycol methyl ether. 
     Apart from their low toxicity, the mentioned systems exhibit the advantages of having only a very faint odor, so that it is, for example, also possible to use them in uncovered baths. 
     It is self-evident that mixtures of one or several amines with one or several propylene glycol derivatives also come within the scope of the present invention. 
     Although it is customary to remove photoresists in an immersion process, whereby the substrates carrying photoresist coatings are usually moved in a bath or, alternatively, the bath is moved, for example, by means of a circulation pump, the remover solutions of the present invention are also suitable for use in other processes, such as spray or puddle processes. 
     The present invention will now be explained in greater detail by references to the following, non-limiting examples. 
    
    
     EXAMPLES 1 TO 5 
     To test the effectiveness of the remover, photoresist coatings conventionally employed in the production of integrated circuits were used. A commercially available photoresist on a diazoquinone/novolak basis (AZ 1350 J) was spin-coated onto silicon wafers at 4,000 rpm and dried at 90° C. for 30 minutes. A coating thickness of 1.8 μm was measured. Subsequently, the substrates were baked at 200° C. for 30 minutes. 
     The removal tests were performed in a moving bath having a temperature of 80° C. After a dwell time of 10 minutes in the bath, the wafers were rinsed with deionized water and the remaining thickness of the resist coating was measured. The results, which are compiled in Table 1, clearly show the superior effectiveness of N-(2-aminoethyl)-ethanolamine (AEEA) and isopropanolamine (IPA), which were employed in accordance with the invention. Triethylene-tetraamine also had a good removing action, but because of its toxicity this compound is not preferred. 
     The amines used for comparative tests were added in equimolar amounts. 
     EXAMPLES 6 TO 12 
     The following examples also served to test the effectiveness of various amino compounds. Here, too, the compositions were mixed such that equimolar amounts were present in each case. 
     The test conditions were the same as in Examples 1 to 5, except that the substrates had been baked at 160° C. for 30 minutes. The bath temperature was 22° C., and the dwell time in the remover bath was 10 minutes. Again, the thickness of the resist coating was measured. 
     The results shown in Table 2 indicate that even at 22° C. the removing action of N-(2-amino-ethyl)-ethanolamine and triethylenetetraamine is superior. Whereas in this case the combination of isopropanolamine and tripropylene glycol methyl ether (TPM) is considered less preferable, the mixture of isopropanolamine and dipropylene glycol methyl ether exhibited good properties at room temperature. 
     EXAMPLES 13 TO 18 
     As described in Examples 1 to 5, various mixtures of N-(2-aminoethyl)-ethanolamine and tripropylene glycol methyl ether were tested. The substrates had been baked at 200° C. for 30 minutes, and the baths had a temperature of 80° C. 
     The results achieved with a bath dwell time of 4 minutes are compiled in Table 3. Under these conditions, the photoresist coatings were not yet removed completely, but the synergetic effect between N-(2-aminoethyl)-ethanolamine and tripropylene glycol methyl ether was impressively demonstrated by the measurement of the resist thicknesses of incompletely removed photoresist portions. A mixture comprised of 51% by weight of N-(2-aminoethyl)ethanolamine and 49% by weight of tripropylene glycol methyl ether was found to be the optimum composition. 
     EXAMPLES 19 TO 21 
     As described in Examples 13 to 18, the synergetic effect between isopropanolamine and depropylene glycol methyl ether (DPM) was determined. In these examples, too, the dwell time in the remover bath had been chosen such that measurable resist coatings remained on the substrate. 
     The results compiled in Table 4 show that an optimum effectiveness is achieved when the mixture is comprised of 22.5% by weight of isopropanolamine and 77.5% by weight of dipropylene glycol methyl ether. 
     EXAMPLES 22 AND 23 
     In these examples, the behavior of the removers towards aluminum layers was tested by heating the compositions corresponding to Examples 13 and 19 to a temperature of 80° C. and allowing them to act on aluminum-coated wafers for 30 minutes. No attack of the aluminun layer was observed. 
     
                       TABLE 1______________________________________Example                       Thickness ofNo.    Composition*           resist coating______________________________________1      64.4%    Di-(n-propylene)-triamine                             0.380 μm  35.6%    Tripropylene-glycol-methyl           ether (TPM)2      50.7%    Diethylene-triamine                             0.249 μm  49.3%    TPM3      71.8%    Triethylene-tetraamine                             0     μm  28.2%    TPM4      36.9%    Isopropanolamine (IPA)                             0     μm  63.1%    TPM5      51.0%    N--(2-aminoethyl)-ethanol                             0     μm           amine (AEEA)  49.0%    TPM______________________________________ *Percentages expressed by weight 
    
     
                       TABLE 2______________________________________Example                      Thickness ofNo.    Composition*          resist coating______________________________________6      63.9%    Hydroxyethyl piperazine                            1.517  μm  36.1%    TPM7      51.6%    Aminoethoxyethanol                            0.396  μm  48.4%    TPM8      50.7%    Diethylene triamine                            0.369  μm  49.3%    TPM9      36.9%    IPA              0.293  μm  63.1%    TPM10     71.8%    Triethylene tetramine                            0      μm  28.2%    TPM11     51.0%    AEEA             0      μm  49.0%    TPM12     36.9%    IPA  63.1%    Dipropylene glycol methyl                            0      μm           ether (DPM)______________________________________ *Percentages expressed by weight 
    
     
                       TABLE 3______________________________________Example                      Thickness ofNo.    Composition*          resist coating______________________________________13     75%     N--(2-aminoethyl)-ethanol                            0.652 μm          amine (AEEA)  25%     Tripropylene-glycol-methyl          ether (TPM)14     51%     AEEA              0.387 μm  49%     TPM15     35%     AEEA              0.758 μm  65%     TPM16     25%     AEEA              0.860 μm  75%     TPM17     15%     AEEA              0.896 μm  85%     TPM18     10%     AEEA              1.174 μm  90%     TPM______________________________________ *Percentages expressed by weight 
    
     
                       TABLE 4______________________________________Example                 Thickness ofNo.         Composition resist coating______________________________________19          10.0%   IPA     1.22     μm       90.0%   DPM20          22.5%   IPA     0.50     μm       77.5%   DPM21          36.9%   IPA     0.83     μm       63.1%   DPM______________________________________ *Percentages expressed by weight 
    
     EXAMPLES 24 TO 27 
     To test the effectiveness of removers of the present invention, photopolymerizable photoresists were used that are developable by means of aqueous-alkaline developers and are customarily employed in the production of circuit boards. A commercially available dry resist (Ozatec® T 138, made by Hoechst AG) was bonded to a copper-clad laminate board at 115° C., exposed for 8 seconds to a 5 kw metal halide lamp arranged at a distance of 60 cm, and developed in a 1% strength aqueous Na 2  CO 3  solution having a temperature of 30° C. for 70 seconds. 
     The removal tests were performed in a moving bath having a temperature of 50° C. After 10 minutes, the results compiled in Table 5 were achieved. The resists were removed both by undiluted removal agents and by agents that had been diluted with water. 
     
                       TABLE 5______________________________________Example No.  Composition Resist removed______________________________________24           51%     AEEA    yes        49%     TPM25           12.75%  AEEA    yes        12.25%  TPM        75%     water26           22.5%   IPA     yes        77.5%   DPM27           5.63%   IPA     yes        19.37%  DPM        75%     water______________________________________