IE911058A1

IE911058A1 - Process and apparatus for producing a titanium nitride layer¹for circuits integrated on the very largest scale by¹means of chemical vapour deposition

Info

Publication number: IE911058A1
Application number: IE105891A
Authority: IE
Original assignee: Siemens Ag
Priority date: 1990-03-30
Filing date: 1991-03-28
Publication date: 1991-10-09
Also published as: EP0450106A1; JPH04225225A; KR100200910B1

Abstract

To form titanium nitride layers, which are used as adhesive, contact and barrier layers in VLSI circuits, a chemical vapour-phase deposition process in a multichamber high-vacuum system is employed. The choice of a nitrogen-containing organic titanium compound as starting substance and the use of thermal, optical and/or plasma excitation makes it possible to deposit layers even at low temperatures in the range 200-550 DEG C. The layers obtained are notable for a high edge coverage even with narrow vias and for good electrical and chemical properties. An apparatus in accordance with the invention for carrying out the process comprises a multichamber high-vacuum system whose chambers (1 - 6) are linked by high-vacuum locks (7), with the result that a particularly advantageous combination of several different processes is possible without interrupting the high-vacuum conditions.

Description

When semiconductor substrates containing silicon 15 or polysilicon are contacted with the aid of aluminium printed conductors, so-called spiking occurs due to the solubility of silicon in aluminium, i.e. silicon diffuses into the aluminium printed conductor and later precipitates again. These precipitations in aluminium, and the material loss in the silicon substrate associated therewith can lead to a weakening of the electrical contact, preferably if the spiking effect occurs in the region of the smallest structures, e.g. in the so-called contact holes. It is the prior art to prevent the interdiffusion of silicon and aluminium by introducing a diffusion barrier, e.g. made of titanium nitride or titanium/tungsten, between the substrate and the printed conductor, at least in the contact hole. Since these layers conduct substantially worse than the aluminium printed conductor, the aim is to keep their thickness as low as possible, but at the same time they must ensure an adequate barrier effect. A uniform and sufficient coverage of the side walls of the contact hole by the barrier layer is also desirable because in case of need it takes over the current conduction. 2. to improve the electrical characteristics of printed conductors It is possible with the aid of a titanium nitride layer to promote the preferred formation of a Ill-texture - 2 in the subsequently applied aluminium layers, as a result of which their stability and electromigration resistance are increased. Thin titanium nitride layers, which are applied, in addition, in or on aluminium printed conduc5 tors, serve the same purpose; these measures further suppress the formation of undesired material weak spots in the aluminium (for example hillocks). 3. as an adhesive and barrier layer in the case of contact holes filled with tungsten In this application, the adhesion of the tungsten to the underlying layer is improved by means of the intermediate titanium nitride layer. Moreover, it protects the free silicon or aluminium surfaces against the chemical attack of reactive fluorine compounds, which are released in conventional tungsten deposition processes.

The reaction with the free surfaces would lead to the formation of fluorine compounds which influence the electrical transition resistance.

In order to meet the requirements in the case of said applications, a production process for titanium nitride layers must lead to a homogeneous, dense, perfect layer, and have a good edge coverage even in the case of contact holes having an aspect ratio A (A = depth of the contact hole/diameter) > 1.

The following production processes are known for titanium nitride layers: 1. Physical sputtering Such a process is described in detail, for example, in EP-A-0,280,089, and can be applied when the surface to be coated has structures having aspect ratios A < 1. As is normal in the case of physical sputtering techniques, there are the disadvantages of a poor edge coverage, preferably for aspect ratios A > 1, and of a lack of conformity of the deposited layers. In order to achieve an adequately thick titanium nitride layer on the floor of a narrow contact hole, it would therefore be necessary for a very much thicker layer (over 100 nm) to be deposited on horizontal surfaces, and this ought to be avoided for the reasons set forth above. In the case of structures having aspect ratios A > 1, it can no longer be applied. Further disadvantages associated with this process are: a) The high number of loose particles knocked out during the process from the sputtering target or from other parts can lead to short circuits in the semiconductor circuit to be produced. b) An adequate barrier effect of these layers can be developed only once the layers have been brought into contact with air at raised temperatures. Due to the partial oxidation occurring in this process, which takes place preferentially at the grain boundaries of the sputtered titanium nitride, the interdiffusion of silicon and aluminium is suppressed. However, this necessary oxidation step increases the danger of further contamination, increases the production processing time, and in particular prevents integration with other processes, for example an immediately subsequent deposition of the aluminium layer in the same highvacuum facility. c) It is not generally possible with a sputtering process to deposit a titanium nitride layer having low mechanical stresses and at the same time a high barrier quality, as would be necessary for the said applications. This is explained in detail, for example, in the article by S. Kanamori in Thin Solid Films, 136 (1986), pages 195 to 214. d) In order to guarantee a reproducible and low-resistance contact with the silicon substrate, this process requires the additional deposition before the deposition of the titanium nitride of a metallic layer, for example of titanium, approximately 20 nm thick. 2. Chemical vapour deposition (CVD) This method, which is described in more detail in the article by N. Yokohama et al. in Journal of Electrochemical Society 136, No. 3 (1989), pages 882 to 883, uses titanium chloride and ammonia as the starting - 4 substances, and has been investigated predominantly for application in hard material coating. By contrast with the sputtering process, given suitable choice of the deposition conditions it is characterised by a substan5 tially better edge coverage even in the case of the smallest structures; however, it can only be used in a very conditional fashion in microelectronics because of the following disadvantages: a) The required relatively high deposition temperatures of 500 to 700°C can, on the one hand, lead to a strengthened oxidation of the silicon surface and thus to strongly increased electrical transition resistances. On the other hand, they render the application of the process impossible in the case of substrates which already have an aluminium layer, since here material changes such as, e.g. hillock formation, occur at process temperatures above 450°C. b) Chlorine and hydrogen are incorporated into the deposited layer, as a result of which the electrical characteristics, the material thickness and thus the barrier quality of the layer are altered in a negative fashion. c) Due to the chlorine-containing starting substances that are used, pumps, deposition chambers and layers are attacked and impaired, and increased demands are placed upon the process safety.

It is therefore the object of the invention to specify a production process for titanium nitride layers as contact layer, barrier layer or adhesion layer in circuits integrated on the very highest scale, which is characterised by simplicity and elimination of dangerous or corrosive starting substances, facilitates production at temperatures far below 450°C, and allows integration with prior or subsequent processes. The layers deposited by means of this process are to be uniform, dense and perfect, and to have low mechanical stresses, good adhesion to the underlying layers and a high edge coverage even in the case of structures having an aspect ratio A > 1.

This object is achieved by means of the characterising features of Patent Claim 1. Further developments of the invention, preferably an apparatus for carrying out the process, are the subject of subclaims.

FIG 1 shows an embodiment of the apparatus in a diagrammatic representation.

Although a production process for titanium nitride layers which makes use of a nitrogen-containing organic titanium compound is specified in the article by K. Sugiyama in Electrochem. Soc. 122, No. 11 (1975), pages 1545 to 1549, this CVD process likewise requires process temperatures of approximately 400*C in order to deliver satisfactory results. The process is not envisaged for application in the case of circuits integrated on the very largest scale, nor is it suitable for this, since on the one hand it does not deliver uniform layers, and on the other hand the deposited titanium nitride layers have a layer resistance which is larger by the factor 10* than in the case of titanium nitride produced by means of known processes. Furthermore, by contrast with the process according to the invention all that is provided is a thermal excitation of the starting substances without the use of a reducing agent.

The problem of producing titanium nitride layers having the characteristics such as conformity, low stress, good edge coverage, high barrier effect, good conductivity and others even in the case of low tempera30 tures is solved by the present invention through the use of starting substances which already contain both titanium and nitrogen in a molecule and are designated below as nitrogen-containing organic titanium compounds, and through the use of thermal or optical excitation or excitation in the electromagnetic alternating field (so-called plasma excitation) or an arbitrary combination of these types of excitation. Representatives of the following classes of substances come into consideration as starting substances, for example: - 6 1. Ti(NR2)4 where R = alkyl, aryl, CF3 2. Ti(NHR)A where R = alkyl, aryl, CF3 3. Ti(NR2)2R'2 where R = alkyl and R' = alkyl, aryl, CF3 According to the invention, the titanium-nitrogen 5 ratio and the structure of the deposited titanium nitride layer can be adjusted in the short-range order both by suitable choice of the starting compound, by modification of the radical R, R', and by variation of the parameters of the deposition process, as well as by a possible addition of gaseous nitrogen or ammonia.

The conformity, and thus a good edge coverage is achieved even in narrow structures by using a deposition process which proceeds as a surface-controlled reaction. This is achieved by the choice of an adequately high and stationary partial pressure of the nitrogen-containing organic titanium compound (i.e. the rate of delivery of this compound is very much higher than the rate at which it is consumed by the reaction) and by the choice of suitable reducing agents such as H2 and NH3. These two molecules decompose preferentially on metallic and metallike surfaces with the formation of particularly reactive, atomic hydrogen, which in turn enables the decomposition of the nitrogen-containing organic titanium compound directly on the surface. This reaction already occurs at process temperatures of approximately 200°C.

According to the invention, the starting substance can also be excited optically or by a plasma at room temperature. The combination of thermal excitation with a further type of excitation, for example by a plasma, is advantageous in order, for example, to achieve a higher deposition rate in the case of a preselected process temperature. The addition of a reducing agent can be eliminated.

The use of a high-vacuum facility, which in accordance with the embodiment represented in FIG. 1 preferably has a plurality of high-vacuum chambers 1-6 for possibly different processes and a high-vacuum lock 7 connecting these chambers, is particularly advantageous for carrying out the process according to the invention - Ί since then, as will be explained more precisely, a plurality of favourably combinable processes can be carried out in direct sequence without it being necessary to interrupt the high-vacuum conditions. The semi5 conductor substrates to be coated, mostly silicon wafers having already partly finished circuits, are withdrawn from at least one supply chamber 8, 9 or deposited therein.

By contrast with the layers produced by means of 10 known processes, the titanium nitride layers deposited by means of the process according to the invention are already characterised directly after the deposition by a substantially better barrier effect. The latter is based, inter alia, on the incorporation of carbon, as a result of which the layers combine the known, advantageous barrier effects of titanium nitride and titanium carbide. Consequently, it is not necessary, in particular, to oxidise the layers partially by air contact for the purpose of improving the barrier effect, as is the case with the sputtering process. However, the titanium nitride layers produced in accordance with the invention have a dense structure having fewer pores, and thus offer fewer diffusion paths. By means of suitable choice of the parameters of the deposition process, it is possible to produce layers having minimum mechanical stresses in conjunction with a dense, polycrystalline structure, as a result of which the reliability as a barrier layer increases, the adhesion of the titanium nitride layer itself is improved, and the compatibility with other layers is. increased. Due to these improved layer characteristics, it is possible to integrate the process with other preceding or subsequent processes in the same highvacuum facility; a particular advantage resides in this by comparison with all known processes.

The process has substantial advantages by comparison with the prior arts As a result of the elimination of dangerous or corrosive starting compounds, the mode of procedure is substantially facilitated by comparison with conventional - 8 CVD processes, the service life of the equipment employed is increased and a high degree of safety is guaranteed.

By carrying out the process in a facility which consists of a high-vacuum-tight lock 7 and at least one high-vacuum-tight deposition chamber 1-6, reoxidation of the silicon surface before the deposition is avoided, the take-up of humidity by the layers is reduced, and their adhesion and the transition resistance to the underlying silicon are thereby improved. Furthermore, as a result of this and due to the improved layer characteristics explained above, the in-situ integration of the process together with preceding and/or subsequent process steps is facilitated, preferably when a high-vacuum facility having a plurality of chambers that can also be used for different processes is employed. The following process combinations are possible, for example, without interrupting the high-vacuum conditions: previous deposition of a metallic contact layer, consisting of titanium for example, in chamber 1, deposition in accordance with the invention of a titanium nitride layer in chamber 2, subsequent heat step in chamber 3 and/or application of a further layer such as tungsten for the purpose of filling contact holes, or of a different conductive or non-conductive layer in possible further chambers 4-6.

Such and other process combinations lead, on the one hand, to improved characteristics of the circuits, for example to lower and reproducible contact resistances and, on the other hand, to a higher degree of process safety and lower treatment times.

Preferably through the choice of the starting substances, and possibly supported by non-thermal excitations, the process facilitates the undertaking of titanium nitride depositions at temperatures far below 450eC. Consequently, it can also be used when the substrates to be coated already have aluminium.

Exemplary embodiment: A high-vacuum facility represented in FIG. 1 and having a plurality of chambers 1-6 and a high-vacuum lock 7, which, for example, attains a base pressure below - 9 5 x 107 mbar at a leak rate below 5 x 106 mbar x 1 x s'1, is used for the process according to the invention. A semiconductor substrate to be coated, preferably a so-called silicon wafer having already partially produced integrated circuits, is brought from a supply chamber 8, 9 into a chamber 2 suitable for CVD processes. Furthermore, this chamber advantageously possesses at least two electrodes for generating a plasma, and a device which facilitates optical excitation of introduced gases, for example an appropriate window. A nitrogen-containing organic titanium compound from one of the classes of substances already mentioned and having an evaporation temperature in the range from 25 to 120’C is introduced into the chamber by means of a carrier gas (e.g. H2, N2, He) or by suction. Furthermore, H2, N2 and NH3 can be introduced as process gases into the chamber. The deposition of the titanium nitride layer takes place in the temperature range from 200 to 550’C at a pressure of 0.1 to 100 mbar with the use of at least one of the types of excitation already mentioned.

For example, as a nitrogen-containing organic titanium compound from the 1st class of substances Ti[N(CH3)2]4 can be decomposed thermally using H2 and/or NH3 as reducing agent: 2 Ti[N(CH3)2]4 + 3H2 - 2 TiN + 6 HN(CH3)2 + 2C2H6 By adding N2 or NH3, the deposition of a titanium-rich titanium nitride layer can be suppressed.

As further embodiments, instead of or in addition to thermal excitation it is possible to use plasma excitation by igniting a plasma between two electrodes, or optical excitation of the nitrogen-containing organic titanium compound, through which the N-C bond is for example purposely split.

Claims

1. Patent Claims 1· Process for producing a titanium nitride layer for circuits integrated on the very largest scale by means of chemical vapour deposition in a high-vacuum 5 chamber, characterised in that a nitrogen-containing organic titanium compound which is thermally excited is used as the starting substance, and hydrogen and/or ammonia is added as reducing agent.

2. Process for producing a titanium nitride layer 10 for circuits integrated on the very largest scale by means of chemical vapour deposition in a high-vacuum chamber, characterised in that a nitrogen-containing organic titanium compound which is excited optically or by a plasma or by a combination of at least one of these 15 types of excitation with thermal excitation is used as the starting substance.

3. Process according to Claim 2, characterised by an addition of hydrogen and/or ammonia as reducing agent.

4. Process according to one of Claims 1 to 3, 20 characterised in that the nitrogen-containing organic titanium compound is selected from one of the following classes of substances: Ti(NR 2 ) A where R = alkyl, aryl, CF a Ti(NHR) 4 where R = alkyl, aryl, CF 3 25 Ti(NR 2 ) 2 R'2 where R = alkyl and R' = alkyl, aryl, CF 3

5. Process according to one of Claims 1 to 4, characterised in that the deposition is carried out at a temperature in the range from 200 to 550°C.

6. Process according to one of Claims 1 to 5, 30 characterised by an addition of nitrogen and/or ammonia.

7. Process according to one of Claims 1 to 6, characterised in that mechanical, electrical or chemical characteristics of the deposited titanium nitride layer are adjusted by choosing the nitrogen-containing organic 35 titanium compound and/or suitable process parameters.

8. Process according to one of Claims 2 to 7, characterised by optical excitation of the nitrogencontaining organic titanium compound for the purpose of splitting of the nitrogen-carbon bond. ib 911058 - 12

9. Process according to one of Claims 1 to 8, characterised by the choice of the process parameters within the following ranges: Temperature : 200-550’C 5 Pressure : 0.1-133 mbar RF power of the plasma excitation : 0-800 W Electrode spacing : 0.3-1.5 cm Flow of carrier gas : 0-600 seem N 2 , H 2 or He 10 Flow of N 2 or NH 3 : 0-300 seem Flow of H 2 : 100-1,000 seem Evaporation temperature : 25-120’C Deposition rate : fz 500 nm/min.

10. Process according to one of Claims 1 to 9, 15 characterised by a directly preceding deposition of a conductive layer while maintaining the high-vacuum condition for the semiconductor substrate.

11. Process according to one of Claims 1 to 10, characterised by a directly subsequent deposition of a 20 conductive or non-conductive layer while maintaining the high-vacuum condition for the semiconductor substrate.

12. Apparatus for carrying out the process according to one of Claims 1 to 11, consisting of a multi-chamber high-vacuum facility, whose chambers (1-6) are connected 25 by a high-vacuum lock (7) and facilitate the combination of a plurality of different processes without interruption of the high-vacuum conditions.

13. A process for producing a titanium nitride layer for circuits integrated on the very largest scale by means of chemic-al vapour deposition in a high-vacuum chamber, according to any preceding claim substantially as hereinbefore described.

14. An apparatus according to claim 12 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.