Method for forming a metal silicide using a solution containing gold ions and fluorine ions

Abstract

A subject matter of the invention is a process for the formation of nickel silicide or of cobalt silicide, comprising the stages consisting in: exposing the surface of the silicon-comprising substrate with an aqueous solution comprising from 0.1 mM to 10 mM of gold ions and from 0.6 M to 3.0 M of fluorine ions for a duration of between 5 seconds and 5 minutes, depositing by an electroless route, on the activated substrate, a layer essentially composed of nickel or of cobalt, applying a rapid thermal annealing at a temperature of between 300 C. and 750 C., so as to form the nickel silicide or the cobalt silicide. The aqueous solution comprises a surface-active agent chosen from the compounds comprising at least one anionic or nonionic polar group and an alkyl chain comprising from 10 to 16 carbon atoms. This process essentially has applications in the manufacture of NAND memories and photovoltaic cells.

Claims

1. A process for forming nickel silicide or cobalt silicide on a silicon-comprising substrate, said process comprising the steps of: bringing a surface of the silicon-comprising substrate into contact with an aqueous solution comprising gold ions and fluoride ions in order to form particles of metallic gold on said surface, wherein the aqueous solution comprises a surface-active agent selected from the group consisting of compounds comprising at least one anionic or one nonionic polar group, and one alkyl chain comprising from 10 to 16 carbon atoms depositingby an electroless routea layer essentially composed of nickel or of cobalt, on the obtained surface covered with particles of metallic gold, wherein the thickness of the layer is between 10 and 45 nm, and the variation in the thickness of the layer is less than 10%, and applying a rapid thermal annealing at a temperature of between 300 C. and 750 C., so as to form nickel silicide or cobalt silicide.

2. The process as claimed in claim 1, wherein the surface of the silicon-comprising substrate is brought into contact with the aqueous solution for a duration of between 5 seconds and 5 minutes.

3. The process as claimed in claim 1, wherein the aqueous solution comprising fluoride ions is obtained by a step of incorporating hydrofluoric acid (HF), NH.sub.4F, or one of their mixtures, into water.

4. The process as claimed in claim 1, wherein the aqueous solution comprising gold ions is obtained by a step of incorporating chloroauric acid (HAuCl.sub.4) into water.

5. The process as claimed in claim 1, wherein the aqueous solution comprises: from 0.1 mM to 10 mM of gold ions, and from 0.6 M to 3.0 M of fluoride ions.

6. The process as claimed in claim 1, wherein the alkyl chain comprises from 10 to 14 carbon atoms.

7. The process as claimed in claim 1, wherein the layer essentially composed of nickel or of cobalt is a nickel-boron layer that is obtained by a step of bringing into contact said surface covered with particles of metallic gold with an aqueous solution comprising a nickel salt, a boron-based reducing agent, a stabilizing agent for nickel, and a polyamine, the pH of the solution being between 9 and 12 and the temperature of the aqueous solution being between 50 C. and 90 C.

8. The process as claimed in claim 1, further comprising a step of chemical cleaning of nickel silicide or cobalt silicide, so as to remove nickel or cobalt which has not migrated into the silicon at the end of the rapid thermal annealing stage.

9. The process as claimed in claim 1, wherein the rapid thermal annealing temperature ranges from 350 C. to 450 C.

10. The process as claimed in claim 1, wherein the surface-active agent has a molecular weight of between 100 g/mol and 1500 g/mol.

11. The process as claimed in claim 1, wherein the surface-active agent comprises an anionic polar group.

12. The process as claimed in claim 10, wherein the surface-active agent is sodium dodecylsulfate.

13. A process for the manufacture of a NAND memory comprising a process for the formation of nickel silicide or of cobalt silicide as defined in claim 1.

14. A process for the manufacture of photovoltaic cells comprising a process for the formation of nickel silicide or of cobalt silicide as defined in claim 1.

15. A process according to claim 1, wherein the thickness of the layer is between 10 and 25 nm.

16. A process according to claim 1, wherein the variation in the thickness of the layer is less than 5%.

17. A process for forming a layer that is essentially composed of nickel or of cobalt on a silicon-comprising substrate, said process comprising the steps of: exposing a surface of the silicon-comprising substrate with an aqueous solution comprising gold ions, fluoride ions and a surface-active agent chosen in the group consisting of compounds comprising at least one anionic or nonionic polar group, and one alkyl chain comprising from 10 to 16 carbon atoms, so as to activate the said surface of the silicon-comprising substrate, and depositingby an electroless routeon the activated surface of the silicon-comprising substrate, a layer essentially composed of nickel or of cobalt that has a thickness between 10 and 45 nm, the variation in the thickness of the layer being less than 10%.

18. A process according to claim 17, wherein the thickness of the layer is between 10 and 25 nm.

19. A process according to claim 17, wherein the variation in the thickness of the layer is less than 5%.

Description

(1) A better understanding of the invention will be obtained on reading the description of the following nonlimiting examples, made with reference to the appended figures.

(2) FIG. 1 represents two scanning electron microscope images showing the gold nanoparticles on the polycrystalline silicon surface which were obtained A) in the absence of surface-active agent and B) in the presence of surface-active agent.

(3) FIG. 2 represents an image obtained with a scanning electron microscope showing the deposition of a very thin layer of the nickel alloy (<20 nm) after activation with the solution of the invention.

(4) FIG. 3 is an image obtained with a scanning electron microscope showing the layer of nickel silicide NiSi (<40 nm) obtained after thermal annealing (350 C./1 min) and cleaning from the nickel which has not reacted or not migrated.

(5) The examples which follow were carried out on the laboratory scale.

(6) Unless otherwise indicated, these examples were carried out under standard temperature and pressure conditions (approximately 25 C. under approximately 1 atm) in ambient air and the reactors used were directly obtained commercially without additional purification.

Reference Example 1

Activation of a Substrate Covered with a Layer of Polycrystalline Silicon Starting from a Solution Comprising a Noble Metal Salt and Hydrofluoric Acid

(7) a) Cleaning the Surfaces:

(8) Depending on the origin of the substrate and on his requirements, a person skilled in the art will know how to adapt a protocol for cleaning the surface. In our case, no cleaning was necessary since the activation solution is also a slow etching solution. In this example, the substrate used is a silicon coupon with side lengths of 1 cm2 cm and with a thickness of 750 m covered with a layer of silicon dioxide (SiO.sub.2) having a thickness of approximately 90 nm, itself covered with a layer of polycrystalline silicon with a thickness of approximately 190 nm.

(9) b) Activation of the Surface of the Substrate:

(10) b1) Preparation of the Activation Solution:

(11) 50 ml of a mixture of hydrochloric acid at 2.5% by weight (1.5 M) and 15 mg of the chosen noble metal salt are prepared at ambient temperature in a clean PTFE beaker. Table 1 gives information on the type and the amounts of noble metal salt used.

(12) b2) Activation Treatment on the Surface of the Substrate:

(13) The substrate described in stage a) is immersed for a given time (see table 1) in the mixture prepared in stage b1). The substrate thus treated is copiously rinsed with deionized water and dried under a stream of nitrogen.

(14) c) Deposition of a Layer of NiB Metal by an Electroless Process:

(15) c1) Preliminary Preparation of the Electroless Solution:

(16) 31.11 g of nickel sulfate hexahydrate (0.118 mol), 44.67 g of citric acid (0.232 mol), 52.26 g of N-methylethanolamine (0.700 mol) and 2.5 ppm of polyethyleneimine (PEI) with an M.sub.n=600 g/mol are introduced, in order, into a 1 liter container and a minimum amount of deionized water. The final pH was adjusted to 9 with the base and the total volume was adjusted to 1 liter with deionized water.

(17) One volume of a reducing solution is added to 9 volumes of the preceding solution, immediately before the following stage. This reducing solution comprises 28 g/l of dimethylamine-borane (DMAB; 0.475 mol) and 60.00 g of N-methylethanolamine (0.798 mol).

(18) c2) Formation of the Layer of NiB Alloy on the Layer of Polycrystalline Silicon:

(19) A layer of NiB nickel alloy was deposited on the surface of the substrate treated in stage b) by immersing it in the electroless solution prepared previously and brought to 65 C., for a period of 30 seconds to 5 minutes, according to the final thickness desired.

(20) d) Formation of the Nickel Silicide:

(21) The sample obtained at stage c), with the nickel alloy on it, is subjected to rapid thermal annealing (RTA) at 350 C. for 1 minute. The operation can be carried out with a tubular oven or a heating plate.

(22) Results:

(23) TABLE-US-00001 TABLE 1 Concen- Acti- Deposition tration HF vation of nickel Diffusion of Minimum of noble concen- time by an the nickel thickness of Noble metal metal salt tration (in electroless into the the nickel Entry salt (ppm) (M) sec.) process silicon layer (nm) 1 (NH.sub.4).sub.2PdCl.sub.4 240 1.5 30 Yes Nonuniform 200 diffusion 2 CuSO.sub.4 240 1.5 30 Yes No 50 3 AgAc 240 1.5 30 Yes No 60 4 HAuCl.sub.4 240 1.5 30 Yes Yes 50

(24) Comments:

(25) This first series of tests makes it possible to underline the importance of the nature of the noble metal used for the migration of the nickel into the silicon. The palladium(II) solutions are capable of activating the surface but give conformal nickel thicknesses which are much greater than those targeted. Furthermore, it is observed that the silver and copper salts do not make it possible to bring about the migration of the nickel into the silicon at a thermal annealing temperature of 350 C. Finally, the gold(III) salt makes it possible to obtain a thin layer of nickel-boron alloy but also the uniform migration of the latter into the silicon.

Example 2

Activation of a Substrate Covered with a Layer of Polycrystalline Silicon Starting from a Solution According to the Invention Comprising a Gold(III) Salt, Hydrofluoric Acid and a Surface-Active Agent

(26) a) Cleaning of the Surfaces:

(27) This stage is identical to stage a) of example 1.

(28) b) Activation of the Surface of the Substrate:

(29) b1) Preparation of the Activation Solution According to the Invention:

(30) 50 ml of a mixture of hydrofluoric acid at 2.5% by weight, 240 ppm of gold(III) hydrochloride and a surface-active agent (see table 2) were prepared at ambient temperature in a clean PTFE beaker.

(31) b2) Activation Treatment of the Surface of the Substrate:

(32) The substrate described in stage a) is immersed for a given time (see table 2), in this case 30 seconds, in the mixture prepared in stage b1). The substrate thus treated is copiously rinsed with deionized water and dried under a stream of nitrogen.

(33) c) Deposition of a Layer of NiB Metal by an Electroless Process:

(34) c1) Preliminary Preparation of the Electroless Solution:

(35) The electroless solution used and the operating conditions are identical to those of stage c1), example 1.

(36) c2) Formation of the Layer of NiB Nickel Alloy on the Layer of Polycrystalline Silicon:

(37) A layer of NiB metal alloy was prepared on the surface of the substrate treated in stage b) by immersing it in the electroless solution prepared previously and brought to 65 C., for a period of 30 seconds to 5 minutes, according to the final thickness desired. The duration of immersion in the electroless solution is determined so as to obtain a minimum nickel thickness with a good uniformity and conductivity.

(38) The thickness of the layer is measured by scanning electron microscopy by taking a cross section of the sample.

(39) The method which makes it possible to measure the conductivity and the uniformity is the four-point measurement method known to a person skilled in the art.

(40) The results are presented in table 2, where the minimum thickness is obtained with different stabilizing agents.

(41) TABLE-US-00002 TABLE 2 Thickness HAuCl.sub.4 HF Surfactant of the concentration concentration Nature of the concentration nickel layer Test No. (mM) (M) surfactant (g/l) (nm) 1 0.7 mM 1.5 None / 50 comparative 2 0.7 mM 1.5 Polyvinyl- 3 45 comparative pyrrolidinone (PVP) 3500 g/mol 3 0.7 mM 1.5 Brij 35* 3 40 1225 g/mol 4 0.7 mM 1.5 Sodium dodecyl 3 20 sulfate (SDS) 288 g/mol *Brij 35: polyoxyethylene glycol dodecyl ether

(42) This second series shows the positive impact of a surfactant of low molecular weight in stabilizing the gold nanoparticles formed after an oxidation/reduction reaction with the polycrystalline silicon. By adding a stabilizing agent to the activation bath, the size of the metallic gold particles can be reduced while increasing their density. This distribution of the gold particles makes it possible to deposit a thinner nickel layer. The best result is obtained with an anionic surfactant (SDS), which makes possible electrosteric stabilization of the nanoparticles on the surface of the polycrystalline silicon, whereas nonionic surfactants, which bring about only steric stabilization (Brij 35), do not make it possible. The SDS molecule stabilizes the gold nanoparticles and increases their concentration on the surface of the polycrystalline silicon (see FIG. 1B).

(43) d) Formation of the Nickel Silicide:

(44) As in stage d), example 1, the sample obtained in stage c), covered with nickel alloy, is subjected to rapid thermal annealing at 350 C. for one minute. The sample is subsequently treated with a chemical solution in order to remove the unreacted nickel.

(45) A section of the sample from test 4 is observed with a scanning electron microscope before and after thermal annealing and chemical etching. The thickness of 20 nm of the nickel alloy layer obtained after activation and electroless deposition is shown in FIG. 2. The thin and very homogeneous layer of nickel silicide formed after thermal annealing is presented in FIG. 3.

(46) The layer of nickel silicide (NiSi) is very homogeneous with a thickness two times greater than that of the initial layer of nickel alloy. Perfect diffusion with a stochiometric reaction between the nickel and silicon (Ni:Si=1:1) is obtained with a good uniformity of the resulting layer.