# Patent

## Title
Electroless deposition of continuous platinum layer

## Abstract

A method for providing an electroless plating of a platinum containing layer is provided. A Ti3+ stabilization solution is provided. A Pt4+ stabilization solution is provided. A flow from the Ti3+ stabilization solution is combined with a flow from the Pt4+ stabilization solution and water to provide a diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution. A substrate is exposed to the diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution.

## Background
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method of forming semiconductor devices on a semiconductor wafer. More specifically, the invention relates to depositing platinum containing layers to form semiconductor devices.
In forming semiconductor devices, thin layers of platinum may be deposited. Such a deposition may be provided by electroplating.
SUMMARY OF THE INVENTION
To achieve the foregoing and in accordance with the purpose of the present invention, a method for providing an electroless plating of a platinum containing layer is provided. A Ti3+ stabilization solution is provided. A Pt4+ stabilization solution is provided. A flow from the Ti3+ stabilization solution is combined with a flow from the Pt4+ stabilization solution and water to provide a diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution. A substrate is exposed to the diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution.
In another manifestation of the invention, a solution for electroless deposition of platinum is provided. The solution comprises Ti3+ ions, Pt4+ ions, NH4+ ions, citrate, and gluconate or tartarate ions. A ratio of Ti3+ to Pt4+ ion is between 100:1 to 2:1.
In another manifestation of the invention, a method for providing an electroless plating of a platinum layer is provided. A solution for electroless deposition of platinum is provided. The solution comprises Ti3+ ions, Pt4+ ions, wherein a ratio of Ti3+ to Pt4+ ion is between 100:1 to 2:1, NH4+ ions, citrate and gluconate or tartarate ions. A substrate is exposed to the solution for electroless deposition of platinum.
These and other features of the present invention will be described in more details below in the detailed description of the invention and in conjunction with the following figures.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
FIG. 1 is a flow chart of an embodiment of the invention.
FIG. 2 is a schematic view of a system that may be used in an embodiment of the invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention.
Electroless deposition of platinum has been accomplished using hydrazine and other hydrogen containing compounds as reducing agents. In addition to the environmental concerns associated with these hydrogen containing reducing agents, the oxidation reaction of these species involves the generation of N2 gas, which can be incorporated in the deposit. This impacts the purity of the deposited film, as well as quality of the coatings. Additionally, the hydrazine-platinum electrolyte requires operation at an elevated temperature and high pH for practical applications. Such requirements are undesirable for back end metallization of semiconductor interconnects, as the dielectric materials are prone to damage at high pH or temperature.
An embodiment of the invention provides an electroless plating bath containing Ti3+ for depositing Pt4+, where the Pt4+ is reduced from solution, while Ti3+ is oxidized to a higher more stable oxidation state of Ti4+. Ti3+ has significant benefits over hydrazine and other hydrogen containing reducing agents. Replacing hydrazine with Ti3+ metal ion reducing agent eliminates the toxicity and volatility that is inherent to hydrazine and makes the plating bath more environmentally friendly. Additionally, no gas evolution (i.e. N2) or side reaction is observed at the electrode. This results in a smooth, continuous, pure Pt film. The Ti3+ metal ion containing plating bath can also be operated over a wide temperature and pH range. The ability to deposit pure platinum film selectively at room temperature and relatively low pH makes its application in back end interconnect metallization particularly attractive, since conventional electrolytes operate at high pH and temperature which causes pattern collapse.
The Ti3+ metal ion reducing agent containing bath, used in an embodiment of the invention, is operable below room temperature and with a low pH. This is not possible with the hydrazine and other reducing agent containing electrolyte. The extended window of operation makes this bath attractive for application as a copper capping layer in interconnects metallization where low pH and low temperature are desired to prevent pattern collapse.
Formation of Pt electrodes for memory applications using plasma etching is difficult. An embodiment of the invention enables selective patterning of Pt electrodes in semiconductor manufacturing without using plasma etching. The cost and complexity associated with maintaining a high temperature during plating can also be reduced due to near room temperature operation of the Ti3+ metal ion reducing agent electrolyte.
FIG. 1 is a high level flow chart of an embodiment of the invention. In this embodiment, a Ti3+ stabilization solution is provided (step 104). A Pt4+ stabilization solution is provided (step 108). A flow from the Ti3+ stabilization solution is combined with a flow from the Pt4+ stabilization solution and water to provide a diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution (step 112). A wafer is exposed to the diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution (step 116). The diluted mixture is collected and may be reactivated for future use or disposed (step 120).
In an example, a Ti3+ stabilization solution is provided in a Ti3+ stabilization solution source (step 104). A Pt4+ stabilization solution is provided in a Pt4+ stabilization solution source (step 108). FIG. 2 is a schematic view of a system 200 that may be used in an embodiment of the invention. The system comprises a Ti3+ stabilization solution source 208 containing a Ti3+ stabilization solution, a Pt4+ stabilization solution source 212 containing a Pt4+ stabilization solution, and a deionized water (DI) source 216 containing DI. A flow 220 from the Ti3+ stabilization solution source 208 is combined with a flow 224 from the Pt4+ stabilization solution source 212 and a flow 228 from the DI water source 216 to provide a diluted mixture 232 of the Ti3+ stabilization solution and the Pt4+ stabilization solution (step 112). A wafer 236 is exposed to the diluted mixture 232 of the Ti3+ stabilization solution and the Pt4+ stabilization solution (step 116). The diluted mixture 232 is collected (step 120). A disposal system 240 may be used to dispose the diluted mixture 232. An alternative embodiment provides the collection of the diluted mixture 232, which is reactivated.
In this example, the Ti3+ stabilization solution comprises a TiCl3 solution in diluted hydrochloric acid with or without citric acid or trisodium citrate. The Ti3+ stabilization solution may further comprise NH4OH. The Pt4+ stabilization solution comprises H2PtCl6, trisodium gluconate or gluconic acid, and ammonium hydroxide.
In one embodiment, the flow 220 of the Ti3+ stabilization solution is combined with the flow 224 of the Pt4+ stabilization solution and the flow 228 of DI water, to form a diluted mixture of 0.05M TiCl3, 0.32M NH4OH, 0.002M H2PtCl6, 0.15M Na3Citrate, and 0.025M Na3Gluconate. The diluted mixture has a pH of between 9-10 and a temperature of about 20° C.
The Ti3+ stabilization solution provides a stable Ti3+ solution that has a shelf life of several months without degrading. The high concentration allows the Ti3+ stabilization solution to be stored in a smaller volume. In addition, the Pt4+ stabilization solution provides a stable Pt4+ solution that has a shelf life of several months without degrading. The high concentration allows the Pt4+ stabilization solution to be stored in a smaller volume. The solutions are combined and diluted just prior to exposing the wafer to the diluted mixture, since the diluted mixture does not have as long a shelf life as the stabilization solutions.
This embodiment of the invention provides a platinum containing layer with a thickness of between 1 nm and 30 nm. Preferably, the platinum containing layer is pure platinum. Because the platinum containing layer is relatively thin, a dilute bath is sufficient. In one embodiment, the wafer is exposed to a continuous flow of the diluted mixture. In another embodiment, the wafer is placed in a still bath of the diluted mixture for a period of time. Since the concentration of platinum and titanium is very low in the diluted mixture, in one embodiment, the diluted mixture may be disposed (step 120) after being exposed to the wafer, since the low concentration means that only a small amount of platinum and titanium is discarded. In another embodiment, the diluted mixture is recycled after being exposed to the wafer. The recycling may be accomplished through reactivation of the dilute mixture.
Generally the solution mixture used for plating has Ti3+ and Pt4+ ions at a Ti3+ to Pt4+ ion ratio between 100:1 to 2:1. More preferably, the solution mixture used for plating has Ti3+ and Pt4+ ions at a Ti3+ to Pt4+ ion ratio between 50:1 to 4:1. In addition, the solution mixture has a ratio of citrate to Ti3+ is between 30:1 to 2:1. More preferably, the solution mixture has a ratio of citrate to Ti3+ is between 15:1 to 3:1. Preferably, the solution mixture has a ratio of NH4+ to Ti3+ is between 12:1 to 3:1. In addition, the solution mixture has citrate from Na3Citrate or citric acid and Gluconate from Na3 Gluconate or Gluconic acid. In addition, the Pt4+ ions come from H2PtCl6. The Ti3+ ions come from TiCl3. The NH4+ ions come from NH4OH. Without being limited by theory, it is believed that ammonia ligands help to provide a lower temperature and lower pH platinum deposition.
Generally, a wafer or other plating surface is exposed to the solution mixture at a temperature between 10° to 40° C. A plating surface is a surface on which the platinum containing layer is selectively deposited. Such selective deposition may use a mask to protect surfaces where deposition is not desired. Preferably, the solution mixture has a pH from 6 to 10. Preferably, the solution mixture provides Ti3+ with a concentration between 5-300 mM. More preferably, the solution mixture provides Ti3+ with a concentration between 25-75 mM. Preferably, the solution mixture provides Ti3+ with a concentration between 25-75 mM. Most preferably, the solution mixture provides Ti3+ with a concentration between 30-60 mM. The lower temperature and lower pH provide a deposition with less damage to layers provided by the semiconductor fabrication process. In addition, such a process does not require any activation step that might attack and damage the copper substrate. In addition, such a process does not create a gas byproduct.
Preferably, the solution mixture is boron free. Preferably, the solution mixture is phosphorus free. Preferably, the solution mixture is hydrazine free. Preferably, the solution mixture is formaldehyde free. It has been found that providing a solution mixture that is boron, phosphorus, hydrazine, and formaldehyde free allows for a more pure plating that does not have impurities provided by using boron-containing reducing agents, phosphorus-containing reducing agents, hydrazine, or formaldehyde. In addition, avoiding using hydrazine, provides a safer and more environmentally friendlier process.
In other embodiments, the source of Ti3+ is Ti2(SO4)3 or other soluble salts of Ti3+. Trisodium citrate or citric acid can be displaced by disodium salts of the isomers of tartaric acid. Trisodium gluconate or gluconic acid can be replaced with methoxyacetic acid or other carboxylic acid ligands.
In one embodiment, the deposited platinum containing layer is at least 99.9% pure platinum. More preferably, the deposited platinum containing layer is pure platinum.
While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and various substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and various substitute equivalents as fall within the true spirit and scope of the present invention.

## Claims

1. A method for providing an electroless plating of a platinum containing layer, comprising:
providing a Ti3+ stabilization solution;
providing a Pt4+ stabilization solution;
combining a flow from the Ti3+ stabilization solution with a flow from the Pt4+ stabilization solution and water to provide a diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution; and
exposing a substrate to the diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution.

2. The method, as recited in claim 1, wherein exposing the substrate to the diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution, comprises:
providing a solution temperature between 10° to 40° C., inclusive; and
providing a pH of between 6 to 10, inclusive.

3. The method, as recited in claim 2, wherein exposing the wafer to the diluted mixture of the Ti3+ stabilization solution and the Pt4+ stabilization solution provides Ti3+ with a concentration between 25-75 mM.

4. The method, as recited in claim 3, further comprising disposing the diluted mixture.

5. The method, as recited in claim 4, wherein the platinum containing layer is 99.9% pure platinum.

6. The method, as recited in claim 3, further comprising reactivating the diluted mixture.

7. The method, as recited in claim 3, wherein the Ti3+ stabilization solution comprises a solution of TiCl3 and HCl.

8. The method, as recited in claim 7, wherein the Pt4+ stabilization solution comprises a solution of H2PtCl6 and ammonium hydroxide and trisodium gluconate or gluconic acid.

9. The method, as recited in claim 8, wherein the Ti3+ stabilization solution further comprises NH4OH.

10. The method, as recited in claim 9, wherein the Pt4+ stabilization solution has a shelf life of over a month.

11. The method, as recited in claim 10, wherein the Ti3+ stabilization solution has a shelf life of over a month.

12. The method, as recited in claim 9, wherein the diluted mixture is boron, phosphorus, hydrazine, and formaldehyde free.

13. The method, as recited in claim 1, wherein the diluted mixture is boron, phosphorus, hydrazine, and formaldehyde free.

14. A solution for electroless deposition of platinum, comprising:
Ti3+ ions;
Pt4+ ions, wherein a ratio of Ti3+ to Pt4+ ion is between 100:1 to 2:1; and
NH4+ ions and citrate or gluconate or tartarate ions.

15. The solution, as recited in claim 14, wherein the solution has a pH between 6 and 10, inclusive.

16. The solution, as recited in claim 15, further comprising ions.

17. The solution, as recited in claim 16, wherein the concentration of Ti3+ ions is 25-75 mM.

18. A method for providing an electroless plating of a platinum layer, comprising:
providing a solution for electroless deposition of platinum, comprising:
Ti3+ ions;
Pt4+ ions, wherein a ratio of Ti3+ to Pt4+ ion is between 100:1 to 2:1; and
NH4+ ions, citrate and gluconate or tartarate ions; and

exposing a substrate to the solution for electroless deposition of platinum.

19. The method, as recited in claim 18, wherein the providing the solution, provides the solution at a pH of between 6 to 10, inclusive, and at a temperature between 10° to 40° C., inclusive.

20. A solution for electroless deposition of platinum, comprising:
Ti3+ ions;
Pt4+ ions; and
NH4+ ions and citrate or gluconate or tartarate ions.

21. The solution, as recited in claim 20, wherein the solution has a pH between 6 and 10, inclusive.

22. The solution, as recited in claim 21, further comprising Cl− ions.

23. The solution, as recited in claim 22, wherein the concentration of Ti3+ ions is 25-75 mM.

24. The solution, as recited in claim 20, wherein the platinum ions are Pt4+ ions.

25. The solution, as recited in claim 24, wherein a ratio of Ti3+ to Pt4+ ion is between 100:1 to 2:1.

26. The solution, as recited in claim 20, wherein the solution is boron, phosphorus, hydrazine, and formaldehyde free.