Method for manufacturing microcrystalline cubic boron-nitride-layers

The method of the present invention provides in a simple manner, the deposition of boron nitride layers with microcrystalline cubic structure which are suitable as insulating layers in VLSI-circuits, as mask membranes in x-ray lithography, as well as coating hard substances. Due to the use of excited starting substances that already contain boron and nitrogen in one molecule and are preferably liquid or solid, and the use of a plasma-CVD-method, the method can be performed using in temperatures of below 500.degree. C. The excitation of the starting substance proceeds preferably in inductive or capacitative fashion in a hollow cathode.

BACKGROUND OF THE INVENTION 
The present invention relates to methods for manufacturing microcrystalline 
cubic boron-nitride-layers through chemical vapor deposition in an 
electromagnetic alternating field (PECVD-method). 
For a number of technical applications, boron-nitride layers are of great 
interest. For example, boron-nitride layers have applications as 
insulating layers in integrated circuits. For intermetal insulating and 
final passivation layers for use in VLSI semiconductor circuits, 
dielectric layers having small dielectric constants .epsilon..sub.r, good 
insulating and blocking properties, and high breakthrough stability, are 
necessary. Additionally, it is necessary that the layers have an optimally 
conformal edge covering and are not hygroscopic. Boron-nitride layers are 
suitable for these requirements, particularly because they have a 
sufficiently low dielectric constant in either an amorphous or 
polycrystalline form. 
The article of W. Schmolla and H. Hartnagel, in Solid State Electronics 26, 
No. 10, page 931 to 939, discloses that for the manufacture of crystalline 
boron-nitride layers, relatively high temperatures are necessary. 
Additionally, the article discloses that because of the increased energy 
gap, improved insulating properties are to be expected with cubic 
crystalline boron nitride as compared with other crystalline forms. 
Known methods for the production of cubic boron nitride are based on a 
multistage process. First, boron nitride in a different crystalline form, 
principally in a hexagonal or rhombohedral phase, is generated. This is 
then converted into a cubic phase using a high temperature and high 
pressure (see, e.g., EPA 0 216 932). 
Even for the production of the rhombohedral or hexagonal phase, typically, 
temperatures above 750.degree. C. are necessary. Because of such 
temperatures, these processes are not viable for the production of 
intermetal insulating and final passivation layers for VLSI semiconductor 
circuits. 
The article of A. Chayahara et al. in Appl. Surface Science 33/34 (1988) p. 
561-566, suggests a method for manufacturing cubic boron nitride using 
B.sub.2 H.sub.6 and nitrogen in a PECVD-method. This method is not 
suitable for manufacturing applications since it has a deposition rate of 
less than 6 nm/min. To this end, typical layer thicknesses of a few 100 nm 
are required for use as an insulation layer. Another disadvantage of the 
method is that it uses a very dangerous gaseous starting substance B.sub.2 
H.sub.6. 
Another application of boron-nitride layers is for mask membranes in x-ray 
lithography. For manufacturing of so-called VLSI-components with structure 
sizes below 0.5 .mu.m, the application of x-ray lithography is necessary. 
An x-ray mask requires the use of a thin membrane, that is highly 
permeable to soft x-rays. Additionally, the membrane must be dimensionally 
stable during the manufacturing process and during its use for the 
component manufacturing. As revealed in the article W. Johnson et al. in 
Journal of Vacuum Science and Technology B5, January 1987, p. 257 to 261, 
amorphous boron nitride is basically well suited for use as a mask 
membrane. It can be produced, for example, using the method described in 
the article of Schmolla and Hartnagel set forth above. 
There are some disadvantages, however, in the use of boron nitride as a 
mask membrane. In air, the resultant boron nitride layers change their 
surface and structure--due, for example, to boric acid crystal growth. The 
reduced transmission of the layers connected therewith makes the optical 
adjustment of the mask difficult when it is used for component 
manufacturing. Local changes of mechanical tensions in the boron nitride 
layer can cause a displacement of the mask. This results in adjustment 
inaccuracies that are not tolerable. 
Another disadvantage is the complicated system structure that is required. 
Further, the starting substances used for the manufacturing method are 
dangerous. 
A still further, application of boron-nitride layers is for coating hard 
material. Cubic boron nitride is principally used for the production of 
grinding discs and turning tools, as well as for the processing of 
hardened steel, tool steel, super alloys, and chrome nickel steel. 
The hardness of cubic boron nitride layers is preserved up to approximately 
600.degree. C. In contrast, the hardness of, for example, tungsten carbide 
considerably decreases at approximately 300.degree. to 400.degree. C. 
(see, e.g., M. Rand, J. Roberts in Journal Electrochem. Soc. 115, 1968, 
page 423). Such layers are principally generated from the hexagonal phase 
of the boron nitride at temperatures of above 1500.degree. C. and a 
pressure of above 80 kbar. The disadvantages of this method include the 
necessity of at least two unit processes and the high temperatures that 
are necessary, which change the structure of the workpiece to be hardened. 
SUMMARY OF THE INVENTION 
The present invention provides a method that enables the manufacture of 
cubic crystalline boron nitride layers under distinctly lower temperatures 
particularly below 500.degree. C., in a simple manner. The process is 
performed in a single stage and does not require dangerous starting 
substances nor a complicated system. The cubic boron nitride layers 
generated can be created so that they have properties required for the 
desired application. Particularly, a dielectric constant .epsilon..sub.r 
&lt;4 for the use as an intermetal dielectricum, a high permeability to soft 
x-radiation for the use in x-ray lithography, and a high Vickers-hardness 
for use in the coating of hard materials. 
To this end, a method for manufacturing a microcrystalline cubic boron 
nitride layer is provided. The layer is produced via chemical vapor 
deposition in an electromagnetic alternating field given a temperature in 
the range from 200.degree. to 450.degree. C. using an excited starting 
substance which contains boron and nitrogen in one molecule. 
In an embodiment, a compound with a coordinative boron-nitrogen combination 
is used as the starting substance. 
In an embodiment, a compound with a covalent boron-nitrogen combination is 
used as the starting substance. 
In an embodiment, a compound having a ratio of boron to nitrogen of &lt;1 is 
used as the starting substance. 
In an embodiment, the starting substance contains at least one group chosen 
from the group consisting of alkyl-, aryl-, NH.sub.2 - and/or hydrogen or 
halogen. 
In an embodiment, B.sub.3 N.sub.3 H.sub.6 (borazol) is used as the starting 
substance. 
In an embodiment, the starting substance includes a mixture of compounds 
containing boron and nitrogen. 
In an embodiment, ammonia or nitrogen is added during the method. 
In an embodiment, the method includes an inductive or capacitative 
excitation of at least the starting substance in a hollow cathode. 
In an embodiment, the following process parameters are used: 
______________________________________ 
Deposition temperature 
200-450.degree. C. 
Process pressure 1-100 mbar 
RF-power 100-800 W 
Electrode distance 0.3-1.5 cm 
Carrier gas flow 0-600 sccm N.sub.2 or He 
N.sub.2 - or NH.sub.3 -flow 
0-500 sccm 
Evacuator temperature 
25-120.degree. C. 
______________________________________ 
The mechnical, electrical or chemical properties of the deposited boron 
nitride layer can be adjusted by the selection of the starting substance 
and/or process parameter. 
Additional features and advantages of the present invention are described 
in, and will be apparent from, the detailed description of the presently 
preferred embodiments and from the drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The present invention provides a method for manufacturing microcrystalline 
cubic boron nitride layers. 
Pursuant to the invention, starting substances are used that contain boron 
as well as nitrogen in one molecule. In an embodiment, the substances are 
excited, preferably with gaseous nitrogen or ammonia in a hollow cathode, 
and disintegrated in an electromagnetic alternating field with thermal 
support (PECVD-method). Thereby, the B:N-ratio of the layers to be 
deposited and their structure can be modified by a suitable selection of 
the starting substances, by modification of a residue R in the molecule of 
the starting substance, as well as by varying the parameters of the 
PECVD-process. 
The FIGURE illustrates a system, diagrammatically, for chemical vapor 
deposition in an electromagnetic alternating field (PECVD). In an 
embodiment, this system is preferably used for the implementation of the 
method of the present invention. 
Referring to the FIGURE, in a reactor chamber 18, a wafer 1 to be processed 
is located on a wafer lift 3 between the gas intake area 6 and the 
susceptor 2. The movement of the wafer lift 3 is indicated by a double 
arrow. Through the wafer lift 3, the electrode distance is adjusted. 
To allow for the evacuation of the reactor chamber 18, two vacuum pumps 9 
are provided that are connected via a butterfly valve 8. For the 
generation of an electromagnetic alternating field a RF-generator is used. 
Individual sections of the reactor chamber 18 are protected by insulators 
4. 
A bubbler 12 is provided for storing, prior to use, the preferred liquid 
starting substance. Helium or nitrogen can also be located in the bubbler 
as a carrier gas. For adjusting the nitrogen content, further gas feeder 
lines 15, 16, and 17 are provided for N.sub.2 and NH.sub.3. All gas feeder 
lines are provided with flow controllers 11 and pneumatic valves 10. 
The deposition temperature in the reactor chamber 18, can be adjusted by 
lamps or a reflector module 13 that are provided. Irradiation of the 
system located in the reactor chamber 18 is performed through a quartz 
window. The excitation of the starting substance, and the N.sub.2 - or 
NH.sub.3 - gas if added, proceeds in the area of the gas admission 6. This 
excitation is achieved either in an inductive or capacitative manner, for 
example, in a hollow cathode. 
The application of the boron nitride layer proceeds at a temperature of 
from approximately 200.degree. to about 450.degree. C. and at a pressure 
of approximately 1 to about 100 mbar in the illustrated PECVD-system. The 
starting substances can be applied either as a liquid or solid, and 
preferably using an evaporator temperature of from approximately 
25.degree. to about 120.degree. C. 
The starting substance enters the process chamber 18 of the system by means 
of a carrier gas, usually helium or nitrogen, or by suction. Excitation 
proceeds such that the starting substance does not disintegrate thereby. 
Particularly suitable herefor is a hollow cathode excitation in the gas 
admission region 6. 
Additionally, nitrogen-containing process gases such as, for example, 
nitrogen or ammonia can be admitted to the process chamber and excited 
together with the starting substance. By the selection of a suitable boron 
or nitrogen containing starting substance, or a mixture of such starting 
substances, and the process parameters, boron nitride layers having the 
desired properties can be designationally generated. For example, the 
dielectric constant .epsilon..sub.r is controlled via the ratio of boron 
and nitrogen in the deposited layer; which again depends on the cited 
quantities. 
By way of example, process parameters for the PECVD-process should be as 
follows: 
______________________________________ 
Deposition temperature 
200-450.degree. C. 
Process pressure 1-100 mbar 
RF-power 100-800 W 
Electrode distance 0.3-1.5 cm 
Carrier gas flow 0-600 sccm N.sub.2 or He 
N.sub.2 - or NH.sub.3 - flow 
0-500 sccm 
Evacuator temperature 
25-120.degree. C. 
______________________________________ 
By way of example, and not limitation, examples of starting substances that 
can be used in the present invention are the following: 
I. Compounds Having a Coordinative Boron-Nitrogen Combination 
1. Adducts of borane to ammonia: 
H.sub.3 B.rarw.NH.sub.3. 
2. Adducts of borane to amines: 
H.sub.3 B.rarw.NH.sub.n R.sub.3-n wherein R is Alkyl or Aryl; 
for example, H.sub.3 B.rarw.NH(CH.sub.3).sub.2. 
3. Adducts of alkyl boranes to ammonia: 
H.sub.n R.sub.3-n B.rarw.NH.sub.3 wherein R is Alkyl or Aryl; 
for example, H(CH.sub.3).sub.2 B.rarw.NH.sub.3. 
4. Adducts of organylboranes to amines: 
H.sub.n R.sub.3-n B.rarw.NH.sub.m R.sub.3-m wherein R is Alkyl or Aryl; 
for example, H(CH.sub.3).sub.2 B.rarw.NH(CH.sub.3).sub.2. 
5. Adducts of halogen borane to amines or ammonia: 
H.sub.n X.sub.3-n B.rarw.NH.sub.m R.sub.3-m wherein: 
R is H, Alkyl, or Aryl; 
X is F, Cl, Br, or I; 
for example, HCL.sub.2 B.rarw.NH(CH.sub.3).sub.2. 
6. Adducts of alkylhalogenboranes to amines or ammonia: 
X.sub.n R.sub.3-n B.rarw.NH.sub.m R.sub.3-m wherein: 
R is H, Alkyl, or Aryl; 
X is F, Cl, Br, or I; 
for example, Cl(CH.sub.3).sub.2 B.rarw.NH(CH.sub.3).sub.2. 
7. Adducts of boron halogenides organylboron halogenides and hydrogen boron 
halogenides to pyridine derivatives: 
X.sub.n R.sub.3-n B.rarw.NC.sub.5 H.sub.5-m R'.sub.m wherein: 
R, R' is Alkyl or Aryl; 
X is F, Cl, Br, or I; 
for example, Cl(CH.sub.3).sub.2 B.rarw.NC.sub.5 H.sub.6. 
8. Adducts of boron halogenides organylboron halogenides and hydrogen boron 
halogenides to nitriles: 
X.sub.n R.sub.3-n B.rarw.NCR' wherein: 
R, R' is H, Alkyl, or Aryl; 
X is F, Cl, Br, or I; 
for example, Br(CH.sub.3).sub.2 B.rarw.NC(C.sub.2 H.sub.5). 
II. Compounds Having a Covalent Boron-Nitrogen-Bond: 
1. (Organyl-) amino-(organyl-) boranes: 
H.sub.n R.sub.2-n BNH.sub.m R'.sub.2-m wherein: 
R, R' is Alkyl or Aryl; 
for example, HCH.sub.3 BNHC.sub.2 H.sub.5. 
2. (Organyl-) amino-(organyl-) halogenboranes: 
H.sub.n R.sub.2-n BNH.sub.m R'.sub.2-m wherein: 
R, R' is Alkyl or Aryl; 
X is F, Cl, Br, or I; 
for example, HCH.sub.3 NBC.sub.2 H.sub.5 Cl. 
3. Borozines and borozine derivatives: 
a) B.sub.3 N.sub.3 R.sub.6 wherein R is H or NH.sub.2 ; for example, 
##STR1## 
b) B-Halogenborazines: B.sub.3 N.sub.3 H.sub.3 X.sub.3 wherein X is F, Cl, 
Br, or I; for example, 
##STR2## 
c) B-Triorganylborazines: B.sub.3 N.sub.3 H.sub.3 R.sub.3 wherein R is 
Alkyl or Aryl; for example, 
##STR3## 
d) N-Triorganylborazines: B.sub.3 N.sub.3 H.sub.3 R.sub.3 wherein R is 
Alkyl or Aryl; for example, 
##STR4## 
e) B,N-Hexaorganyl derivates: B.sub.3 N.sub.3 R.sub.6 wherein R is Alkyl 
or Aryl; for example, 
##STR5## 
3. B.sub.2 N.sub.2 R.sub.6 wherein R is H, F, Cl, Alkyl, Aryl, or NH.sub.2 
(B); for example, 
##STR6## 
4. 1,2,3,4-Tetraorganyl-1,3-diaza-2,4-diboracyclobutane: B.sub.2 N.sub.2 
R.sub.4 wherein R is H, F, Cl, Alkyl, Aryl, or NH.sub.2 (B); for example, 
##STR7## 
5. 
1,2,3,4,5,6,7,8-Octaorganyl-1,3,5,7-tetraaza-2,3,6,8-tetraboracyclooctane: 
B.sub.4 N.sub.4 R.sub.8 wherein R is H, Alkyl, Aryl, or NH.sub.2 (B); for 
example, 
##STR8## 
6. 
1,3,5,7-Tetraorganyl-2,4,6,8-tetrahalogen-1,3,5,7-tetraaza-2,4,6,8-tetrabo 
racyclooctane: 
B.sub.4 N.sub.4 R.sub.4 X.sub.4 wherein: 
R is H, Alkyl, Aryl, or NH.sub.2 (B) 
X is F, Cl, Br, or I; 
for example, 
##STR9## 
7. 2,3,4,6,7,8-Hexaorganyl-1,3,5,7-tetraaza-2,4,6,8-tetraborobi-cyclo 
(3,3,0)octane: 
B.sub.4 N.sub.4 R.sub.6 wherein R is H, Alkyl, Aryl, or NH.sub.2 (B); for 
example, 
##STR10## 
8. 1,2,-Diorganyl-1,2-azaborolanes: BNC.sub.3 H.sub.6 R.sub.2 wherein R is 
H, Alkyl, Aryl, or NH.sub.2 (B) ; for example, 
##STR11## 
9. 1,2-Diorganyl-1,2,azaborinanes: BNC.sub.4 H.sub.8 R.sub.2 wherein R is 
H, Alkyl, Aryl, or NH.sub.2 (B); for example, 
##STR12## 
10. 1-Bora-5-azabicyclo(3,3,0)octane: BNC.sub.6 H.sub.12 
##STR13## 
11. 1-Bora-6-azabicyclo(4,4,0)decane: BN(CH.sub.2).sub.8 
##STR14## 
III. Compounds Having a Ratio of Boron:Nitrogen&lt;1: 
1. Diaminoboranes: 
RB(NR'.sub.2).sub.2 wherein R, R' is H, Alkyl, Aryl, or NH.sub.2 ; for 
example, CH.sub.3 B(N(C.sub.2 H.sub.5).sub.2).sub.2 
2. Triaminoboranes: 
B(NR.sub.2).sub.3 wherein R is H, Alkyl, Aryl, or NH.sub.2 ; for example, 
B(N(C.sub.2 H.sub.5).sub.2).sub.3 
3. B-Aminoborazines: 
B.sub.3 N.sub.3 (NR.sub.2).sub.3 R'.sub.3 wherein R, R' is H, Alkyl, Aryl, 
or NH.sub.2 (B); for example, 
##STR15## 
IV. Others: 
1. B.sub.3 N.sub.3 H.sub.6 R.sub.6 wherein R is H, Alkyl, Aryl, or NH.sub.2 
; for example, 
##STR16## 
It should be understood that various changes and modifications to the 
presently preferred embodiments described herein will be apparent to those 
skilled in the art. Such changes and modifications can be made without 
departing from the spirit and scope of the present invention and without 
diminishing its attendant advantages. It is therefore intended that such 
changes and modifications be covered by the appended claims.