WO2000029637A1

WO2000029637A1 - Diffusion barrier materials with improved step coverage

Info

Publication number: WO2000029637A1
Application number: PCT/US1999/026408
Authority: WO
Inventors: Roy C. Gordon; Xinye Liu
Original assignee: President And Fellows Of Harvard College
Priority date: 1998-11-12
Filing date: 1999-11-08
Publication date: 2000-05-25
Also published as: KR20010080412A; JP2003522827A; WO2000029637A9

Abstract

A chemical vapor deposition process is provided for the formation of conformal layers containing metal nitrides. A film consisting mainly of titanium nitride is deposited from a vapor mixture of tetrakis(diethylamido)titanium, ammonia and trimethylamine on a surface heated to about 350 °C. The process can be used to form diffusion barrier layers between metals and silicon in computer microcircuits.

Description

DIFFUSION RA RTER MATERIALS WITH IMPROVED STEP

COVERAGE

Background of the Invention

1. Field of the Invention

This invention relates to a chemical vapor deposition (CVD) process for

metal nitride films with improved step coverage or conformality. A primary

application of the process is to form semiconductor microcircuits in which the

metal nitride acts as a barrier preventing diffusion of metals such as aluminum

or copper into silicon transistors.

2. Description of the Related Art

In computer processors and memory chips, metal circuits connect the

transistors and capacitors formed near the surface of the silicon. Aluminum,

copper and tungsten are the metals commonly used for these circuits. In order

to provide a functional and durable computer, the metals must be separated

from the silicon by a barrier layer. In the absence of such a barrier layer,

aluminum would alloy with the silicon, producing etch pits that can short out

the electrical circuits; copper would diffuse into the silicon and provide

deleterious recombination centers for the electrons and holes; or tungsten

would peel off of the silicon dioxide insulating layers.

Titanium nitride is the material that usually is used as the barrier layer.

The titanium nitride ordinarily is formed by the process of reactive sputtering

of a titanium target in a low pressure of nitrogen gas. The sputtered material has been satisfactory for the production of computer chips with feature sizes

down to about one-quarter of a micron. As the industry tries to make the

circuits operate faster and store more information, the feature sizes are being

reduced. For feature sizes less than about one-quarter of a micron, sputtering

does not cover adequately the sides and bottoms of the narrow holes and

trenches that are etched a micron deep into the substrates.

It is desirable that the coverage of the side walls and bottom of the

etched features have a film thickness on the same order as the outer surface.

This relationship is described as step coverage. "Step coverage" is defined as

the ratio of the thickness of the deposited film at the bottom of the holes to the

thickness of the film on the top of the silicon dioxide layer. It is desirable that

an the electronic feature have a step coverage close to one. Thus a need is

perceived for barrier layers deposited by a process that has better step coverage

than sputtering.

Chemical vapor deposition (CND) is a widely-used process for forming

solid materials, such as coatings, from reactants in the vapor phase. Often,

CND processes produce better step coverage than sputtering because they are

limited to line-of-sight coverage. Comprehensive reviews of CND processes

have been given recently in "CND of Nonmetals," W. S. Rees, Jr., Editor, NCH

Publishers, Weinheim, Germany, 1996; "CND of Compound Semiconductors,"

A. C. Jones and P. O'Brien, NCH, 1996; and "The Chemistry of Metal CND,"

T. Kodas and M. Hampden-Smith, Editors, NCH, 1994. It is known that thermal CND of tetrakis(dimethylamido)titanium or

tetrakis(diethylamido)titanium alone, i.e., without ammonia, leads to good step

coverage (Eizenberg et al, J. Vac. Sci. Technol. A, 13:590 ( 1995)). It is

believed that the good step coverage is due to the low sticking coefficient of the

titanium amido compound and of any reaction intermediates. Molecules with

low sticking coefficients can survive numerous collisions with the side walls of

feature without being deposited thereon. Hence, the molecule is more likely to

be finally deposited further within the trench of the feature and to give good

step coverage. Unfortunately, high carbon contamination of the films, high

electrical resistivity and porosity (low density) is observed along with the

observed good step coverage.

Better quality titanium nitride films have been obtained by a CND

process from vapors of tetrakis(diethylamido)titanium and ammonia,

discovered by Gordon et al, US Patent 5,139,825 (1990). This process

operates at substrate temperatures below 400 °C, which are low enough for use

in semiconductor microcircuits. The step coverage obtained by this process is

adequate for forming barrier layers on microcircuits currently produced with

feature sizes around one-quarter micron and aspect ratios up to 4: 1. However,

the step coverage obtained by this process may not be high enough for the even

smaller feature sizes contemplated for future production of microcircuits.

Thus, it was believed that in order to produce low-carbon, low-resistivity

TiΝ films, reaction with ammonia was required. Reaction of the dialkylamido titanium complex with ammonia results in transamination and the formation of

the corresponding free dialkylamine. Weiller attempted to control the rate of

reaction of the metal-amido compounds, and possibly the quality of the step

coverage, by addition of the transamination product, dialkylamine, to the CND

reaction gases, tetrakis(dimethylamido)titanium vapor and ammonia gas, as

reported in the Journal of the Electrochemical Society, 144:L40-L43 (1997)

and in U.S. Patent No. 5,763,007. While addition of dialkylamine succeeding

in increasing reaction time, there has been no definitive evidence that the

addition of dimethylamine actually increases the step coverage of titanium

nitride films.

Thus, until now, the prior art was unable to provide a method for

depositing a metal nitride film having both good film quality and good step

coverage.

What is really needed is a method for increasing the step coverage

without adversely affecting the carbon content, electrical resistivity or density

of the titanium nitride films, and Weiller's U. S. Patent 5,763,007 does not

supply such a method.

Summary of the Invention A principal object of the present invention is to provide a process for the

deposition of metal nitride layers with improved step coverage.

Another object of the present invention is to provide a process for making metal nitride films, including transition metal nitrides, having high

purity, high electrical conductivity and effective diffusion barrier properties.

An additional object of the invention is to provide a process for chemical

vapor deposition of conformal metal nitride films from the vapors of stable and

homogeneous liquid solutions.

One particular object of the present invention is to provide a process for

depositing titanium nitride coatings having a good step coverage, low electrical

resistance and low carbon content.

Another particular object of the present invention is to provide a process

for depositing conformal titanium nitride films having good resistance to

diffusion of materials through the films.

A related object is to deposit conformal layers containing several metal

nitrides in a chemical vapor deposition process.

Another particular object of the present invention is to provide a process

for depositing conformal titanium nitride films having strong adhesion to

silicon and silicon dioxide.

A further particular object of the invention is to provide conformal

titanium nitride films onto which tungsten films adhere strongly.

Another particular object of the present invention is to provide a process

for depositing conformal, electrically conductive niobium nitride films having

good resistance to diffusion of materials through the films.

Other objects of the invention will be obvious to those skilled in the art on reading the instant invention.

The above objects have been substantially achieved by use of a chemical

vapor deposition process in which a gaseous mixture, comprising the vapors of

a metal alkylamide, ammonia and a tertiary amine, is brought in contact with a

hot surface on which a conformal film of a metal nitride is deposited. For

example, tetrakis(diethylamido)titanium vapor, ammonia gas and

trimethylamine gas are flowed onto a patterned substrate held at 350 °C, to

deposit a film of titanium nitride which uniformly covers holes and trenches in

the substrate.

The metal dialkylamides used in the process of the invention may have

the general formula M(NR* R )_n where R¹ and R² may be an alkyl group, or a

substituted alkyl group containing heteroatoms, such as nitrogen, and n is an

integer representing the oxidation state of the metal. Most preferred

compositions of the metal dialkylamides include ligands derived from

diethylamine, such as tetrakis(diethylamido)titanium or

tetrakis(diethylamido)niobium.

The tertiary amine is preferably is a trialkylamine, in which the alkyl

groups are the same or different. The alkyl group preferably contains less than

or equal to six carbons, and more preferably less than or equal to three carbons.

In the process of the invention, the trimethylamine may be replaced by other

tertiary alkylamines, such as the vapors of liquid triethylamine or pyridine.

Another preferred embodiment of the invention provides a process for the chemical vapor deposition of metal nitrides, using reactant vapors produced

by the flash vaporization of a liquid mixture of one or more metal

dialkylamides and a liquid tertiary amine. These mixed vapors are then mixed

in the gas phase with ammonia gas and, optionally, an inert carrier gas such as

nitrogen, and then brought in contact with a heated substrate. This process may

be used to form films, including, but not limited to, the nitrides of titanium and

niobium.

In another embodiment of the invention, mixed metal nitrides are formed

by vaporizing two or more metal dialkylamides and their vapors are mixed with

ammonia gas, the vapor of a tertiary amine and, optionally an inert carrier gas.

This vapor mixture is brought into contact with a substrate heated to a

temperature sufficient to deposit a material comprising two or more metal

nitrides. The process may be used to form multimetal nitride films, including,

but not limited to, titanium, zirconium, hafnium, vanadium, niobium, tantalum,

chromium, molybdenum, tungsten, aluminum, gallium, indium and tin.

Brief Description of the Drawing

The invention is described with reference to the Figures, which are

presented for the purpose of illustration only and are in no way intended to be

limiting of the invention, and in which:

Figure 1 are photomicrographs of (A) the top portion and (B) the bottom

portion and (C) full view of a feature coated as described in Example 1 , at the same magnification; and

Figure 2 are photomicrographs of (A) the top portion and (B) the bottom

portion (C) full view of a feature coated as described in Comparative Example

2, at the same magnification.

Detailed Description of the Invention

The present invention provides a metal nitride film of superior step

coverage, low carbon content and low resistivity. The method of the invention

includes a chemical vapor deposition process in which a gaseous mixture,

comprising the vapors of a metal alkylamide, ammonia and a tertiary amine, is

brought in contact with a hot surface on which a conformal film of a metal

nitride is deposited. For example, tetrakis(diethylamido)titanium vapor,

ammonia gas and trimethylamine gas are flowed onto a patterned substrate held

at 350 °C, on which a film of titanium nitride is deposited which uniformly

covers holes and trenches in the substrate. This process may be used to

provide good step coverage to features with dimensions below 0.25 microns in

diameter and with aspect ratios (depth: diameter) over 4: 1. Step coverages for

these geometries of greater than 70%, and approaching 95-100%, may be

obtained by the method of the invention.

Specific embodiments of this invention require the use of one or more

metal dialkylamides and tertiary amines.

The metal dialkylamides used in the process of the invention may have the general formula M(NR' R²)_n where R¹ and R² may be an alkyl group, or a

substituted alkyl group containing heteroatoms, such as nitrogen, and n is an

integer representing the oxidation state of the metal. Other ligands such as

alkyl groups or alkylimido groups may also be attached to the metal atom. In

preferred embodiments, the alkyl groups contain less than six carbons and more

preferably less than three carbons. Most preferred compositions of the metal

dialkylamides include ligands derived from diethylamine, such as

tetrakis(diethylamido)titanium or tetrakis(diethylamido)niobium.

The tertiary amine is preferably is a trialkyl amine, in which the alkyl

groups are the same or different. The alkyl group preferably contains less than

In the process of the invention, the trimethylamine may be replaced by other

tertiary alkylamines, such as the vapors of the liquids triethylamine or pyridine.

In preferred embodiments, the tertiary amine is trimethylamine or

triethylamine. In other preferred embodiments, the tertiary amine may be

pyridine.

A variety of tertiary amines may be used in the practice of this

invention. The preferred tertiary amines are all commercially available from

many suppliers. Trimethylamine is a gas at normal temperatures and

atmospheric pressure, and is normally supplied as a liquefied gas in a pressure

cylinder. Triethylamine and pyridine are liquids at normal temperatures and

pressures. These tertiary amines do not have hydrogen atoms directly attached to their nitrogen atoms, and thus they do not enter into the transamination

reactions characteristic of the secondary amines used in U.S. Patent 5,763,007.

The metal dialkylamides of this invention may be formed by reacting a

suitable dialkylamide salt of an alkali metal with a metal halide. For example,

lithium diethylamide may be reacted with titanium tetrachloride to form

tetrakis(diethylamido)titanium. Commercial suppliers of

tetrakis(diethylamido)titanium include Schumacher (Carlsbad, CA) and

Advanced Technology Materials (Danbury, CT). Commercial suppliers of

tetrakis(diethylamido)niobium include Chemat (Northridge, CA) and Advanced

Technology Materials.

The vapors of the liquid precursors may be formed in a thin-film

evaporator, or by nebulization into a carrier gas preheated to about 150 °C. The

nebulization may be carried out pneumatically or ultrasonically. The liquid

metal dialkylamides are generally completely miscible with organic solvents,

including hydrocarbons, such as dodecane, tetradecane, xylene and mesitylene.

These solutions generally have lower viscosities than the pure liquids, so that in

some cases it may be easier to nebulize and evaporate the solutions rather than

the pure liquids. Thin-film evaporators are made by Artisan Industries

(Waltham, Massachusetts). Commercial equipment for direct vaporization of

liquids (DLI) is made by MKS Instruments (Andover, Massachusetts), ATMI

(Danbury, Connecticut), Novellus (San Jose, California) and CONA

Technologies (Tiburton, California). Ultrasonic nebulizers are made by Sonotek Corporation (Milton, New York) and Cetac Technologies (Omaha,

Nebraska).

Gaseous reactants, such as ammonia or trimethylamine, may be

introduced into the vapor through a mass flow controller, with or without an

inert carrier gas, to provide the desired partial pressure of the gas in the system.

The process of the invention can be carried out in standard equipment

well known in the art of chemical vapor deposition (CND). The CND

apparatus brings the vapors of the reactants into contact with a heated substrate

on which the material deposits. A CND process can operate at a variety of

pressures, including in particular normal atmospheric pressure, and also lower

pressures. Commercial atmospheric pressure CND furnaces are made in the

USA by the Watkins-Johnson Company (Scotts Valley, California), BTU

International (North Billerica, Massachusetts) and SierraTherm (Watsonville,

California). Low-pressure CND equipment is made by Applied Materials

(Santa Clara, California), Spire Corporation (Bedford, Massachusetts),

Materials Research Corporation (Gilbert, Arizona), Νovellus (San Jose,

California), Emcore Corporation (Somerset, ΝJ) and ΝZ Applied Technologies

(Woburn, Massachusetts).

Typical deposition temperatures lie in the range of about 200 to 400 °C.

The deposition reaction may also be accelerated by light, or by the electrical

energy of a plasma discharge, as well as by heat. Typical deposition pressures

range from normal atmospheric pressure down to a few milli-Torr. EXAMPLE 1

Titanium nitride films were made by atmospheric pressure chemical

vapor deposition in a Watkins-Johnson Model 965 belt furnace.

Tetrakis(diethylamido)titanium (99.995% purity) from Schumacher was

vaporized at 140 °C by an MKS Model LDS 100 liquid delivery and

vaporization system into nitrogen carrier gas purified of water and oxygen by

an Oxiclear purifier. Ammonia (NH₃, 99.995 % purity) and trimethylamine gas

(NMe₃, 99.5%o purity) from Matheson Gas Company were further purified of

water and oxygen contamination by passing them through Nanochem purifiers

designed for ammonia and then preheating to 160 °C. The

tetrakis(diethylamido)titanium vapor was mixed in the deposition zone with the

ammonia and trimethylamine gases. The gas-phase molar concentrations in the

deposition zone were 0.01% tetrakis(diethylamido)titanium, 1.0% ammonia

and 0.68%o trimethylamine, balance nitrogen carrier gas.

The substrates were silicon wafers previously coated with a layer of

silicon dioxide 2.4 microns thick, into which holes 0.7 microns in diameter and

2.4 microns in depth (an aspect ratio of about 3.5: 1) had been etched. The

substrates were preheated to 370 °C and then moved through the deposition

zone at a belt speed of 2 cm/minute. Control experiments with wafers having

thin thermocouple wires cemented to their surface showed that the substrate

temperature dropped to about 320 °C during its passage through the deposition

zone, the upper surface of which was held at a temperature of about 160 °C. After the deposition was complete, the wafer was cleaved and the

cleaved edge was examined with a scanning electron microscope. The step

coverage is shown in Figure 1 , and is determined by measuring the film

thickness at the surface of the film 100 of the film as shown in Figure 1A and at

the base of the trench 102 as shown in Figure I B. Taking into account the

experimental accuracy of the thickness measurements, the step coverage was

found to be between 95% and 100%.

The composition of the film was determined by helium ion scattering

experiments to be titanium nitride with some hydrogen and oxygen

contamination. There was no carbon detected in the films above the detection

level of 3 atomic percent.

COMPARATIVE EXAMPLE 2

Example 1 was repeated with the trimethylamine flow turned off. The

step coverage is shown in Figure 2, and is determined by measuring the film

thickness at the surface of the film 200 of the film as shown in Figure 2 A and at

the base of the trench 202 as shown in Figure 2B. The step coverage was found

to be 70%, which is significantly lower than obtained in Example 1. The

chemical composition, resistivity and density of the films were otherwise

identical to those of Example 1.

EXAMPLE 3

Example 1 was repeated with a preheat temperature of 390 °C. The step

coverage was determined to be 45%o. COMPARATIVE EXAMPLE 4

Example 3 was repeated with the trimethylamine flow turned off. The

step coverage was found to be 35%o, which is significantly lower than obtained

in Example 3. The chemical composition, resistivity and density of the films

were otherwise identical to those of Example 3.

EXAMPLE 5

Tetrakis(diethylamido)titanium was mixed with 20 times its volume of

liquid triethylamine, and the liquid solution was vaporized at 140 °C by an

MKS Model LDS 100 liquid delivery system into purified nitrogen carrier gas.

This vapor mixture was reacted with ammonia in an experiment similar to that

of Example 1 , except that trimethylamine was not used. Similar results were

obtained.

EXAMPLE 6

Example 1 was repeated with tetrakis(diethylamido)niobium in place of

the tetrakis(diethylamido)titanium. A niobium nitride film with excellent step

coverage was obtained.

The liquids and solutions disclosed in these examples all appeared to be

non-pyrophoric by the methods published by the United States Department of

Transportation. One test calls for placing about 5 milliliters of the liquid or

solution on an non-flammable porous solid, and observing that no spontaneous

combustion occurs. Another test involves dropping 0.5 milliliters of the liquid or solution on a Whatman No. 3 filter paper, and observing that no flame or

charring of the paper occurs.

The precursors generally react with the moisture or oxygen in ambient

air, and should be stored under a dry atmosphere such as nitrogen.

Those skilled in the art will recognize or be able to ascertain using no

more than routine experimentation, many equivalents to the specific

embodiments of the invention described specifically herein. Such equivalents

are intended to be encompassed in the scope of the following claims.

What is claimed is:

Claims

1. A process for forming a material containing one or more metal

nitrides, comprising:

providing a vapor comprising one or more metal alkylamides,

ammonia, and a tertiary amine; and

contacting the vapor mixture with a heated surface in a deposition

process to deposit a material containing one or more metal nitrides.

2. The process of claim 1 in which the metal or metals are selected

from the group consisting of titanium, vanadium, niobium and molybdenum.

3. The process of claim 2 in which the metal is titanium.

4. The process of claim 2 in which the metal is niobium.

5. The process of claim 1 , wherein the tertiary amine is an

alkylamine.

6. The process of claim 5, wherein the alkyl group is the same or

different.

7. The process of claim 6 wherein the alkyl group is comprised of

less than or equal to six carbons.

8. The process of claim 6 wherein the alkyl group is comprised of

less than or equal to three carbons.

9. The process of claim 1 in which the tertiary amine is

trimethylamine.

10. The process of claim 1 in which the tertiary amine is triethylamine.

1 1. The process of claim 1 in which the tertiary amine is pyridine.

12. The process of claim 1 , wherein the metal alkylamide comprises

an alkyl group having less than or equal to six carbons.

13. The process of claim 1 , wherein the metal alkylamide comprises

an alkyl group having less than or equal to three carbons.

14. The process of claim 1, wherein the surface is heated to a

temperature in the range of about 200-400 °C.

15. The process of claim 1 , wherein step coverage is greater than

75%.

16. The process of claim 1 , wherein step coverage is on the order of

95-100%.