RU2356964C1 - Manufacturing method of sputtering targets from cast disilicide of refractory metal and facility for its implementation - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to nonferrous metals metallurgy field and can be used at manufacturing of sputtering metallic targets for application of thin-film metallization super large scale integration circuit of different purposes in microelectronics. Manufacturing method of sputtering target consists in usage of liquid-phase synthesis of silicon components and refractory metal for receiving of stoichiometric disilicides of refractory metals optimal structure and as-cast composition (not in the form of powders). Then it is implemented magnetron sputtering of framework-specific target, providing films formation of disilicide with required composition. Method proposes, that in disilicide ingots silicon content corresponds to eutectic concentration. Facility for method implementation contains target from silicon components and refractory metal, additionally, sputtered cylindrical or rectangular plate of target corresponds mosaic structure, gathered from ingots of high-clean disilicide of refractory metal, fixed on the copper basis by means of ultrasonic brasing. Facility also can consist of one or several disilicide ingots of refractory metal of cylindric section, if sputtering is implemented by point source (laser, electronic beam).

EFFECT: improvement and increasing of reliability of barrier and conducting films of disilicide of refractory metal (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Co) by means of usage for sputtering high purity cast materials.

4 cl, 4 tbl, 2 dwg

Description

The invention relates to the field of production of sprayable metal targets for microelectronics. Refractory metal disilicides are of considerable interest due to their great potential as a material of low-resistance contacts, gate electrodes, interconnects, and diffusion barrier layers between a silicon substrate and metallization from aluminum alloys in integrated circuits.

Currently, two main industrial technologies for producing silicide films are used. The first is the deposition of pure metal films on the Si surface, followed by high-temperature annealing (up to 1000 ° C), resulting in silicide layers. However, when using this technology in the production of ultra-large integrated circuits (VLSI) with submicron sizes, problems may arise associated with the degradation of fine-lying pn junctions due to diffusion of impurity atoms into the silicide film during annealing and, as a result, an increase in contact resistance. The second technology is magnetron sputtering of targets made of silicides. For the manufacture of such targets, powder metallurgy technology is usually used, as a result of which uncontrolled contamination of the sprayed targets and the deterioration of the electrophysical properties of the resulting films are possible.

Closest to the claimed object are US patents No. 6562207 from 05/13/2003, No. 5418071 from 01/01/2001 and No. 6300265 from 06/22/2000 (prototypes), which are essentially the same in the method of producing sputtering targets, which consists in using powder metallurgy methods: mixing powders initial components and pressing them at a temperature, pressure and exposure sufficient to form single-phase microstructures. However, the synthesis of refractory compounds (for example, stoichiometric disilicide of refractory metals) by powder metallurgy methods has a number of significant drawbacks.

- Since the products are obtained using a series of operations, including mixing the component powders, pressing and annealing the mixture, uncontrolled impurities accumulate in the material, i.e. contamination of targets with impurities is predetermined by the manufacturing method itself.

- Due to the low speed of diffusion processes, equilibrium is not achieved in real time, therefore, the components in the sputtering target are unevenly distributed, and the microstructure is multiphase, which leads to the formation of films with structural and chemical heterogeneity.

- It is difficult to obtain a high density of the material, and the densities proclaimed in the patents at the level of 85-95% and higher of the theoretical density are not real, since the latent porosity and the release of gas impurities into the target volume when it is heated to 250-300 ° C are not taken into account, and sputtering of such targets is accompanied by uncontrolled instability of the electrophysical properties of the deposited thin films.

- When spraying metal-ceramic targets, the plasma flow of finely dispersed ionized particles of the applied substance is accompanied by a flow of large neutral particles deposited on the surface of the deposited films and dramatically reducing the quality of the films due to the formation of “punctures” and other defects.

- The synthesis of ceramic-metal compounds by powder technology using extreme pressure, vacuum, and holding is associated with significant hardware and technological complication and cost of the entire process of manufacturing targets.

The technical task is to improve the quality and stability of the electrophysical properties of transition metal silicides by producing cast targets with 100% density. This is achieved by the fact that deep purification from impurities and liquid-phase synthesis and the production of cast silicides is carried out through the use of vacuum electron beam zone melting, vacuum induction melting and electromagnetic melting of a mixture of components in a vacuum or gas atmosphere in suspension.

The proposed method for producing cast disilicides of refractory metals makes it possible to synthesize bulk samples of much higher purity compared to powder ones, since high-purity single crystals of refractory metals grown by electron beam melting and high-purity single-crystal silicon, and their liquid-phase synthesis, are used as starting materials produced by the crucible-free method using smelting in suspension (in an electromagnetic field). As a result of these vacuum-metallurgical processes, it is possible to obtain ingots of high-purity compounds with high density. However, even the indicated method for producing ingots does not avoid the problems associated with the nature of cast silicides, expressed in their increased fragility and hardness. Therefore, the casting of silicides is carried out in special molds, which make it possible to obtain samples with the given shape, dimensions and surface quality, which require only minimal finishing.

DETAILED DESCRIPTION OF THE INVENTION

The literature presents data on the microstructure of silicides of various compositions, their properties, production technology and applications. More than half of all elements of the periodic system of elements react with silicon to form one or more silicides. Table 1 shows data on the temperature and concentration of silicon in the eutectic for disilicides of refractory metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Co).

Table 1. Temperatures and eutectic concentrations in metal-silicon systems. Silicide TiSi ₂ ZrSi ₂ Hfsi ₂ VSi ₂ NbSi ₂ TaSi ₂ CrSi ₂ MoSi ₂ WSi ₂ FeSi ₂ CoSi ₂ Nisi ₂ T ° C 1350 1350 1300 1385 1295 1385 1300 1410 1440 1308 1195 966 ° C At.% Si 86.0 90.0 91.8 97.0 95.0 99.0 87.0 97.0 99,2 73.0 23.0 46.0

From the phase diagrams of metal-silicon systems, it is possible to determine the temperature and concentration of components in the eutectic, and then the mass of the starting components (silicon and metal) and the temperature of the formation of disilicides of the desired stoichiometric composition by synthesis in the liquid phase.

The electrical resistivity of disilicides is the most important criterion in assessing the possibility of their use as conductive VLSI systems. The submicron sizes of the elements determine extremely high requirements for purity, as well as for the chemical and structural uniformity of the applied disilicide films. In this regard, obtaining high-purity starting metals in accordance with the results of studying the physicochemical laws of refining was one of the main tasks. Refractory metals at high temperatures are characterized by active interaction with gases: oxygen, water vapor, nitrogen, carbon monoxide, hydrocarbon, etc. The main methods for controlled reduction of the content of interstitial impurities in metals are high-temperature annealing and melting in high vacuum, during which the dissolved impurity atoms diffuse to the surface of the sample and desorbed from it. The combination of high vacuum and high temperatures in the absence of "polluting" materials in contact with the melt are necessary conditions for the deep cleaning of refractory metals. Most fully meets these requirements electron beam melting (EBM) in a deep vacuum. This process is optimal for deep refining and obtaining high-purity polycrystalline metal ingots. A specific variation of this method is electron beam zone melting (ELSP), which allows one to obtain single crystals of high-purity metals.

Induction heating was used in the process chain twice: (1) for sintering and (2) for melting. Sintering consists in loading pieces of silicon and metal into a quartz crucible blown with argon, then heating is performed until the components are melted and then the heating is stopped. Get a compact ingot (bead), consisting of a mixture of silicides, metal and silicon. Then the ingot is subjected to melting in a state suspended in an electromagnetic field.

The main advantages of levitation:

- intensive mixing of the melt, providing a uniform distribution of the components of the melt in the volume,

- lack of contact with refractory material,

- high crystallization rates,

- the possibility of obtaining an ingot of a given shape,

- the ability to conduct melting in a vacuum or any gas atmosphere,

- controlled melting time.

The listed advantages of the method are especially important when working with refractory and highly reactive metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Co.

To prepare silicides, the starting metals were subjected to double vacuum refining by electron beam melting, as a result of which metals with a purity of 99.99% -99.999% were obtained. Used silicon of similar purity. The initial samples weighing 20 g were prepared from a mixture of pieces of silicon and metal (Table 2), which was first melted in quartz crucibles, and the obtained ingot was melted in suspension. After a short exposure in vacuum, the melt was crystallized in a copper mold, obtaining an ingot of the desired shape and with smooth surfaces. Depending on the design of the target, ingots of cylindrical (for laser sputtering), rectangular (for magnetron sputtering) and rhombic (for targets with a “lock”) section were obtained.

Table 2. Data for liquid-phase synthesis of disilicides. Silicide T _pl , ° C m.% Si In eutectic The content of components, g metal compound metal silicon TiSi ₂ 1665 1540 54 9.2 10.8 ZrSi ₂ 1865 1600 38 12,4 7.6 VSi ₂ 1900 1750 52 9.6 10,4 NbSi ₂ 2500 1930 40 12.0 8.0 TaSi ₂ 3000 2200 23 15.4 4.6 CrSi ₂ 1890 1550 fifty 10.0 10.0 MoSi ₂ 2650 2100 37 12.6 7.4 WSi ₂ 3410 2165 25 15.0 5,0 FeSi ₂ 1539 1220 fifty 10.0 10.0 Nisi ₂ 1453 1150 fifty 10.0 10.0 CoSi ₂ 1496 1326 fifty 10.0 10.0

Refractory metal disilicides ingots 4 × 10 × 40 mm in size, obtained by levitation methods, were subjected to insignificant mechanical finishing: removal of the profitable part and grinding of surfaces in order to remove surface defects. These ingots are components of a magnetron target. The elements were fastened to a copper base using ultrasonic soldering. By this method, various targets were made from stoichiometric disilicides of the above metals with a diameter of 76 - 152 mm (Figs. 1 and 2), rectangular targets measuring 20 × 114 × 260 mm for the Magna installation, targets for laser ablation and electronic evaporation. It should be noted that, as a result, it was the stoichiometric disilicides that were obtained, which was confirmed by x-ray phase analysis. A local X-ray spectral analysis of thin sections cut from different parts of the ingot revealed a fine-grained structure and uniformity of cast samples for both components over the entire thin section area.

Table 3 shows the specific electrical resistivity of disilicides of refractory metals in the form of thin films and cast samples.

Table 3. Specific resistance of refractory metal disilicides. Disilicide Resistivity, μOhm · cm film cast target TiSi ₂ 13.0 16.9 ZrSi ₂ 40,0 75.8 Hfsi ₂ 150.0 62.0 VSi ₂ 67.0 66.5 NbSi ₂ 55.0 50,4 TaSi ₂ 60.0 46.1 MoSi ₂ 67.0 21.6 WSi ₂ 50,0 50,0 FeSi ₂ 350,0 250,0 Nisi ₂ 50,0 45.0 CoSi ₂ 30,0 16,0

An example implementation of the method

The implementation of the method was carried out, in particular, on ingots of stoichiometric tungsten disilicide, which is the most problematic in terms of producing thin films with reproducible characteristics. In order to study in more detail the features of producing cast disilicides and manufacturing composite targets from them, as well as studying the process of applying thin WSi ₂ films by magnetron sputtering, the films were applied in three ways: (1) by laser evaporation of cast

WSi ₂ , (2) magnetron sputtering of composite cast WSi ₂ targets, (3) magnetron co-sputtering of tungsten and silicon targets.

Silicon and tungsten of high purity (5N) were used as starting materials for the preparation of WSi ₂ . To prepare metal targets from high-purity W for magnetron sputtering and co-sputtering, metals were refined by electron beam melting with a floating zone in high vacuum. The content of impurities was controlled by spark spectrometry and atomic absorption methods.

Ingots of the required shape and size were obtained using high-frequency vacuum levitation of a mixture of the starting components. The essence of the method consists in suspending a solid sample weighing ~ 20 g in an electromagnetic field and melting it with induced electric currents.

In the case of thin films by laser ablation, rectangular WSi ₂ targets _of size 10 × 15 × 5 mm or cylindrical targets WSi _{2 of} size 5 × 15 mm were evaporated using a solid-state Nd laser with a wavelength of 1.06 μm as substrates for applying silicide films used Si (100), Si (111), MgO, ZrO ₂ and Al ₂ O ₃ . The temperature of the substrates was varied in the range of 100-750 ° C; spraying time 5-20 minutes, film thickness 15-200 nm.

Composite targets for magnetron sputtering were prepared from WSi ₂ ingots 4 × 10 × 40 mm ^{3 in} size mounted on a cylindrical or rectangular copper base (FIGS. 1 and 2). Magnetron sputtering included the cleaning of cast targets and substrates by sputtering in Ar. The films were obtained by sputtering composite cast targets WSi ₂ in an industrial Lada-35 installation at a vacuum of 2 × 10 ^-2 mm RT. Art. and DC power 0.26 kW. In parallel, two targets (tungsten and silicon) were co-magnetron sputtered, and films with stoichiometric WSi ₂ were obtained and studied. The film thickness was 200-250 nm. X-ray phase analysis of cast samples was performed using a Siemens D-500 diffractometer (Ψ = 25 °, Fe-07). A local X-ray spectral analysis of the samples was performed using JXA-5. The reaction between the film and the plate (substrate), as well as the distribution of the elements, was studied using ion sputtering and Auger electron spectroscopy on a JAMP-10S instrument. The phase composition of the WSi ₂ films was investigated using a D / max-RC Rigaku diffractometric complex in the Bragg geometry with an asymmetric angle of incidence. The angle of incidence of x-rays (CuK _α ) was 0.1-5 °. Electron microscopic studies were performed using JSM-35S and Stereoscan 240 microscopes. Both the elemental composition and the distribution of elements over the depth of 100 nm thick WSi ₂ films were performed using Rutherford backscattering of helium ions with an energy of 1.5 MeV.

A local X-ray spectral analysis of cast targets showed that the samples are homogeneous with a tetragonal structure. Phase analysis of films obtained by laser sputtering (ablation) of cast targets

WSi ₂ on silicon substrates (100) and (111) showed that amorphous WSi ₂ forms at temperatures up to 400 ° С. Above this temperature, crystallization occurs, resulting in a mixture of the tetragonal phase with a small amount of a semiconducting hexagonal phase. As the temperature rises further, the amount of the hexagonal phase decreases. At 700 ° C, the film becomes completely tetragonal with a metallic type of conductivity. At 750 ° C, a small amount of the second phase appears again (W ₅ Si ₃ ).

Table 4. Interplanar spacing d _hkl WSi _{2 of} cermet and cast targets in comparison with the data for films deposited on Si (100) substrates by laser and magnetron sputtering of cast targets Data Powder Cast Film Film [one] target target (ln) * (m.) ** hkl d d d d d 002
101
110
103
004
112
200
114
105
211
006
213
204 3,908
2,970
2,270
2,020
1,961
1,961
1,603
1,485
1,412
1,407
1,304
1,258
1,241 3,911
2,970
2,270
2,025
1,963
1,963
1,606
1,485
1,412
1,407
1.305
1,258
1,241 3,907
2,970
2,268
2,026
1,964
1,964
1,606
1,484
1,413
1,407
1,304
1,258
1,240 3,928
2,962
2,263
2,025
1,965
1,965
1,601
1,483
-
1,409
1.308
1,255
- 3,927
2,970
2,269
2,025
-
1,964
1,608
1,484
1,410
-
-
1,257
- * (ln) - laser spraying, ** (m.r.) - magnetron sputtering.
[1] - Powder Diffraction File, Int. Center for Diffraction Data, USA. 1987, card 11-0195.

A comparison of the interplanar spacings for the lines in the diffraction patterns of sintered and cast targets, as well as for films sprayed from cast targets by laser and magnetron methods, is given in Table 4. As can be seen, the phase compositions of the films correspond to the compositions of the target material. The films were single phase throughout the depth and consisted of a tetragonal WSi ₂ phase.

The specific resistivity of the WSi ₂ cast targets was ~ 50 μm · Ohm cm, which is lower than 70–120 μm · Ohm cm for a powder target. The electrical resistance of the contacts to the n-type conductive layers on the samples without a polysilicon sublayer under the WSi ₂ layers was irreproducible and had a spread of up to 20 m Ohm for 50 contacts. Samples with a polysilicon sublayer had a much smaller scatter in the value of electrical resistance. So, R _{Al-n +} = 360-400 kOhm for 50 contacts on the sample annealed at 900 ° С, while R _{Al-n +} = 1.4-1.8 kOhm for 50 contacts on the sample annealed at 1000 ° С. At the same time, R _Al-polySi = 600-700 kOhm for 100 contacts on samples annealed at 900 ° С; and on samples annealed at 1000 ° C, R _Al-polySi = 3.3-3.5 kOhm for 100 contacts. All n ⁺ -p connections along the surface perimeter of about 16000 μm ² and 2120 contacts had leakage currents lower than 0.04 nA at a voltage of 15 V. With increasing voltage, the leakage currents increased monotonically to 0.1 nA at 20 V; however, at 21-22 V the compounds had breakdowns.

So, obtaining cast disilicides of refractory metals of high purity of massive sizes is a well-known metallurgical problem, since all silicides of refractory metals are extremely fragile and hard. In addition, the known methods for producing silicides do not allow the production of silicides of a given stoichiometric composition with high purity of the final product, i.e. sprayed target. Therefore, cast silicides of high purity, subjected to vacuum processing in the molten state, having a high specific gravity and high purity by impurities, must satisfy all the basic requirements for sprayed materials in microelectronics. In the example implementation of the method, the advantages of applying WSi ₂ films by magnetron sputtering of cast composite WSi ₂ targets are shown. The conditions of magnetron sputtering of cast targets ensure the production of monophasic tetragonal WSi ₂ films with an optimal electrical resistivity of 50 μm · Ohm. Therefore, composite cast magnetron targets from high-purity refractory metal disilicides can be considered the optimal solution to the problem, which can solve two main problems : production of stoichiometric disilicides of refractory metals and achievement of high purity.

Claims (4)

1. A method for the production of a composite target from refractory metal disilicides (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Co), which consists in the liquid-phase synthesis of components (metal and silicon) by the method of their direct interaction in a melt using vacuum-metallurgical processes, characterized in that they use crushed single crystals of components that have undergone deep vacuum refining by repeated remelting, then the manufacture of refractory disilicide ingots is carried out using suspended smelting, p oizvodya solidification of the melt into copper molds the required shape. 2. A method for producing a composite target from refractory metal disilicides according to claim 1, characterized in that the refractory metal disilicide ingots have a eutectic silicon content, which provides films of this metal disilicide with an optimal structure and composition. 3. The device for implementing the method according to claim 1 or 2, comprising a magnetron target in the form of a sprayable cylindrical or rectangular plate made of refractory metal disilicide mounted on a copper base, characterized in that the sprayed target plate is a mosaic structure assembled from rectangular ingots refractory metal disilicide fixed on a copper base by ultrasonic soldering. 4. The device for implementing the method according to claim 1 or 2, characterized in that the sprayed target for laser or electron sputtering consists of one or more ingots of refractory metal disilicide of cylindrical or rectangular cross-section.

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