Negative resists with high thermal stability comprising end capped polybenzoxazole and bisazide

Cost-effective, negative resists having high thermal stability based on oligomeric and/or polymeric polybenzoxazole precursors are disclosed. Also disclosed are resist solutions having a high level of storage stability when they contain a photoactive component in the form of a bisazide and when the polybenzoxazole precursors are hydroxypolyamides having the following structure: ##STR1## where R, R*, R.sub.1, R.sub.1 * and R.sub.2 are aromatic groups, R.sub.3 is an aromatic group or a norbornene residue, and wherein n.sub.1, n.sub.2 and n.sub.3, are defined as follows: PA1 n.sub.1 =1 to 100, n.sub.2 and n.sub.3 =0 or PA1 n.sub.1 and n.sub.2 =1 to 100, n.sub.3 =0 or PA1 n.sub.2 =1 to 100, n.sub.1 and n.sub.3 =0 or PA1 n.sub.1, n.sub.2 and n.sub.3 =1 to 100 (with R.noteq.R* or R.sub.1 .noteq.R.sub.1 * or both) or PA1 n.sub.1 and n.sub.3 =1 to 100, n.sub.2 =0 (with R.noteq.R* or R.sub.1 .noteq.R.sub.1 * or both), PA1 on the condition that: n.sub.1 +n.sub.2 +n.sub.3 .gtoreq.3.

BACKGROUND OF THE INVENTION 
The present invention relates to negative resists having high thermal 
stability based on oligomeric and/or polymeric polybenzoxazole precursors, 
as well as to a method for manufacturing highly heat-resistant relief 
structures from these negative resists. 
Hydroxypolyamides are used as soluble polybenzoxazole precursors in 
high-temperature-resistant photoresists. Such photoresists are needed to 
directly structure organic insulating layers in a cost-effective manner. 
The polybenzoxazoles obtained from the hydroxpolyamides are distinguished 
by high temperature resistance, low water absorption, and excellent 
electrical properties. The polybenzoxazoles can be used as etching masks, 
particularly in alkaline etching processes, but they can also be used as 
organic dielectric materials in semiconductor manufacturing. 
Photostructurable polybenzoxazole precursors can find applications in both 
positive and negative resists. The positive resists contain a photoactive 
component in the form of a diazoquinone in addition to the polymer 
precursor (c.f., European Patent 0 023 662, European Published Patent 
Application 0 291 779 and German Published Patent Application 37 16 629). 
The negative resists have polymer precursors with cross-linkable, 
unsaturated groups (c.f., European Patent 0 041 677). 
However, storage stability problems can occur for both positive and 
negative resists. In the case of concentrated positive resist solutions 
containing hydroxypolyamides, a polycondensation reaction can occur. 
Negative resists tend to polymerize due to the presence of unsaturated 
groups, such as acrylic and metacrylic groups. In both resist types, this 
leads to an increase in viscosity or even to the formation of a gel. 
Moreover, in the case of positive resists, the photoactive component can 
be destroyed by terminal alkaline amino groups or by amine monomers and 
amide oligomers. 
SUMMARY OF THE INVENTION 
The present invention provides cost-effective, high-thermal-stability 
negative resists based on oligomeric and/or polymeric polybenzoxazole 
precursors and overcomes the storage stability difficulties previously 
encountered with these types of resists. 
These and other objectives are achieved by the present invention in that 
the resists contain a photoactive component in the form of a bisazide and 
in that the polybenzoxazole precursors are hydroxypolyamides having the 
following structure: 
##STR2## 
wherein R, R*, R.sub.1, R.sub.1 * and R.sub.2 are aromatic groups, R.sub.3 
is an aromatic group or a norbornene residue, and with respect to n.sub.1, 
n.sub.2 and n.sub.3, the following applies: 
n.sub.1 =1 to 100, n.sub.2 and n.sub.3 =0 or 
n.sub.1 and n.sub.2 =1 to 100, n.sub.3 =0 or 
n.sub.2 =1 to 100, n.sub.1 and n.sub.3 0 or n.sub.1, n.sub.2 and n.sub.3 =1 
to 100 (with R.noteq.R* or R.sub.1 .noteq.R.sub.1 * or both) or 
n.sub.1 and n.sub.3 =1 to 100, n.sub.2 =0 (with R.noteq.R* or R.sub.1 
.noteq.R.sub.1 * or both), 
on the condition that: n.sub.1 +n.sub.2 +n.sub.3 .gtoreq.3. 
From the hydroxypolyamides of the type mentioned above, relief structures 
can be unexpectedly produced in the presence of a bisazide using 
photolithographic means, which relief structures exhibit a high 
dimensional stability under heat both during and after tempering. 
The polybenzoxazole precursors themselves exhibit excellent storage 
stability in solution, both with respect to the viscosity of the solution 
and with respect to the stability of the photoactive component. Moreover, 
the photoresists of the present invention have a very low dark field loss 
during development. A resulting 35% loss in layer thickness during 
tempering corresponds to that of the pure polybenzoxazole precursors and 
represents the optimal value attainable for this system. 
Contrary to the known negative resists based on polybenzoxazoles, the 
resists of the present invention do not contain any hydroxypolyamides with 
unsaturated groups. Instead, these resists contain a photoactive component 
in the form of a bisazide. While the application of 
bisazide-group-containing photoactive components in negative resists is 
already known, they have previously been used only with natural and 
synthetic rubbers (c.f., U.S. Pat. No. 2,852,379 and U.S. Pat. No. 
2,940,853). The application of sulfone-group-containing bisazides is also 
known for rubbers as well as for other polymers, such as polystyrene, 
polyamide, novolak resins, poly (vinylphenol) and poly (vinylbutyral) 
(c.f., German Patent No. 29 48 324 and IEEE Trans. Electron Devices, Vol. 
ED-28 (1981), pp. 1306-1310). 
In contrast, the resists of the present invention use polybenzoxazole 
precursors in the form of specific hydroxypolyamides. In these 
hydroxypolyamides, the normally present amino terminal groups are modified 
in quite specific manner. Generally, R and R* are defined as follows for 
the hydroxypolyamides of the present invention: 
##STR3## 
R.sub.1 and R.sub.1 * are defined as follows, wherein H-atoms can also be 
substituted by Cl or Br: 
##STR4## 
R.sub.2 can be defined as follows: 
##STR5## 
wherein the aromatic residues can have alkyl substituents. R.sub.3 can be 
defined as follows: 
##STR6## 
wherein the aromatic residues can also have halogen substituents or alkyl 
substituents. Furthermore, a COOH group can be arranged on the aromatic 
residues in the o-position to the bond to the acid amide grouping 
(--CO--NH--). A corresponding group can be arranged in the 6-position of 
the norbornene residue. 
Finally, n=0 or 1, and X is defined as follows: 
##STR7## 
wherein Z=alkyl with 1 to 10 carbon atoms or aryl, and r=2 to 18. 
Serving as base materials for the hydroxypolyamides are polycondensation 
products, specifically: 
co-polycondensation products of aromatic diaminodihydroxy compounds and 
aromatic dicarboxylic acids or dicarboxylic acid chlorides; 
homo-polycondensation products of aromatic aminohydroxy carboxylic acids; 
co-polycondensation products of aromatic diaminodihydroxy compounds, 
aromatic dicarboxylic acids or dicarboxylic acid chlorides and aromatic 
aminohydroxy carboxylic acids. 
Preferred hydroxypolyamides are as follows: Polycondensation products of 
3,3'-dihydroxybenzidine and isophthalic acid dichloride; 
Polycondensation products of 3,3'-dihydroxybenzidine, 
2,2-bis(3-amino-4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane and 
isophthalic acid dichloride; 
polycondensation products of 3-amino-4-hydroxy benzoic acid. 
Besides 3,3'-dihydroxybenzidine (i.e., 
3,3'-dihydroxy-4,4'-diaminobiphenyl), isomers of this compound can also be 
applied as the diaminodihydroxy compounds. Other hydroxyl-group-containing 
aromatic diamines, such as 3,3'-dihydroxy-4,4'-diaminodiphenyl ether, can 
also be used. In addition to isophthalic acid dichloride, isophthalic acid 
can be used as a dicarboxylic acid. Terephthalic acid can also serve as a 
dicarboxylic acid, while terephthalic acid dichloride can serve as a 
dicarboxylic acid derivative. 
The base materials contain amino terminal groups, which become modified. 
More specifically, the amino groups are converted to acid amide groups (of 
aromatic carboxylic acids or norbornene carboxylic acid). The modification 
takes place in such a way that the amino terminal groups are reacted with 
suitable carboxylic acid derivatives and, in particular, with carboxylic 
acid chlorides or dicarboxylic acid anhydrides. 
Aromatic bisazides, preferably 2,6-bis(4'-azidobenzal)-cyclohexanone or 
2,6-bis(4'-azidobenzal)- 4-methylcyclohexanone, especially find 
application as a photoactive component in the negative resists of the 
present invention. Other compounds can also be applied, such as 
4,4'-diazidobenzophenone and 4,4'-diazidodiphenylmethane. The mass ratio 
of hydroxypolyamide to bisazide, advantageously is from 1:20 to 20:1, 
preferably from 1:10 to 10:1. 
To manufacture highly heat-resistant relief structures according to the 
present invention, a negative resist in the form of a layer or foil is 
applied to a substrate and exposed to actinic light through an overlay 
mask, or it is irradiated by directing a beam of light, electrons or ions. 
The unexposed or non-irradiated portions of the layer or foil parts are 
subsequently dissolved or otherwise removed and the resulting relief 
structures tempered. 
The photoresist of the present invention is advantageously applied to the 
substrate dissolved in an organic solvent. Preferably, one uses 
N-methylpyrrolidone as a solvent. However, other organic solvents with 
similar properties can find application, such as dimethylformamide, 
N,N-dimethylacetamide, and mixtures of the above-mentioned solvents. 
An adhesive agent and/or a wetting agent can be used in the manufacture of 
the relief structures. Adhesive agents or wetting agents can be added to 
the polymer solution, but they can also be applied to the substrate before 
the coating process. The polymer solution is preferably applied to the 
substrate by means of a centrifugal technique, by means of dipping or 
spraying processes, or by other coating methods, such as brushing and 
rolling. The substrate consists preferably of glass, metal (in particular, 
aluminum), plastic or a semiconducting material. 
The conversion of the structured polymer layers into polybenzoxazoles 
having high thermal stability is accomplished by means of a tempering 
process. Generally, temperatures of between 200.degree. and 500.degree. C. 
are selected. Preferably, the tempering process takes place at a 
temperature of between 300.degree. and 450.degree. C. 
The invention shall be clarified in greater detail based on the following 
preferred embodiments.

DETAILED DESCRIPTION 
1. Preparing a Hydroxypolyamide 
105 parts by weight of 
2,2-bis(3-amino-4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane and 62 
parts by weight of 3,3'-dihydroxybenzidine are dissolved under an argon 
atmosphere in 1380 parts by weight of N,N-dimethylacetamide and 250 parts 
by weight pyridine. 104 parts by weight of isophthalic acid dichloride, 
dissolved in 460 parts by weight of cyclohexanone, are then added 
dropwise, with vigorous stirring, to the above solution, after cooling to 
0.degree. C. After 21/2 hours of stirring, the reaction mixture is warmed 
to room temperature and stirred for another 31/2 h. After standing for 12 
hours, 32.2 parts by weight of benzoylchoride are slowly added to the 
reaction mixture in a dropwise manner while stirring, and the mixture 
allowed to stand for 12 h at room temperature. The reaction solution is 
subsequently added dropwise into 18 liters of water. The polymer that 
separates out is then washed and dried in a vacuum-drying oven over NaOH. 
2. Purifying the Hydroxypolyamide 
The hydroxypolyamide prepared according to Example 1 is purified by means 
of ion exchange materials. For this purpose, 150 parts by weight of a 
commercial anionic exchange material are suspended in distilled water, 
packed into a chromatographic column and washed until neutral; the water 
is subsequently replaced by N-methylpyrrolidone. Similarly, 100 parts by 
weight of a commercial cationic exchange material are suspended in 
distilled water, packed into a chromatographic column, and washed with 10% 
HCl until the eluate is acidic. The material is subsequently washed with 
water until it becomes neutral and chloride free. The water is 
subsequently replaced by N-methylpyrrolidone. 
To carry out the purification process, 200 parts by weight of dried 
hydroxypolyamide are dissolved in 2000 parts by weight of 
N-methylpyrrolidone. The solution is then added to the anionic and the 
cationic exchange materials. The polymer solution purified in this manner 
is added dropwise to approximately 17 liters water to precipitate the 
polymer. The polymer is then isolated, washed, and dried in a 
vacuum-drying oven over NaOH. 
Solutions of polybenzoxazole precursors in the form of 
terminal-group-modified hydroxypolyamides prepared in this manner are 
distinguished from solutions of corresponding, non-terminal-group-modified 
hydroxypolyamides by their particularly high viscosity stability. 
Specifically, 33% solutions of the first-mentioned type are stable at a 
storage temperature of 40.degree. C. for nearly 150 th, while solutions of 
the second type experience an increase in viscosity after only about 15 
th. 
3. Preparing a Resist Solution 
15 parts by weight of a polybenzoxazole precursor prepared and purified in 
the above-described manner are added to a solution containing 3 parts by 
weight of 2,6-bis(4'-azidobenzal)-4-methylcyclohexanone in 51 parts by 
weight of N-methylpyrrolidone. The resulting solution is then 
pressure-filtered through a 0.8 .mu.m filter. 
This type of solution is storage stable with respect to both the viscosity 
and the technological properties of the solution. Thus at a storage 
temperature of 40.degree. C., there is no increase in viscosity within 160 
th, and at a storage temperature of -18.degree. C., the technological 
properties are still unchanged after 20 days. 
4. Manufacturing a Relief Structure 
An adhesive agent is first applied to a silicon disk (5000 
revolutions/min-30 s; 80.degree. C.-10 min.). The silicon disk is then 
coated with a photoreactive solution prepared according to Example 3 
(layer thickness, 2.7 .mu.m) by means of a centrifugal process (1200 
revolutions/min-20 s). The coated disk is subsequently exposed to light 
through an overlay mask for 30 s (MJP 55: 42 mW/cm.sup.2, measured with 
OAI 400 nm), developed with an MIF-developer NMD-3 (0.32%) and then 
tempered in a diffusion oven under nitrogen (tempering program: room 
temperature to 170.degree. C.: 1 h; 170.degree. to 300.degree. C.: 1 h; 
300.degree. to 400.degree. C.: 1 h; 400.degree. C.: 1 h; 400.degree. C. to 
room temperature: 4 h). 
Fine, heat-deformation-resistant relief structures are thereby formed. The 
loss in layer thickness (dark field loss) is very small (i.e., the total 
loss in layer thickness amounts to only 35%).