WO2003040427A1

WO2003040427A1 - Thin film deposition by laser irradiation

Info

Publication number: WO2003040427A1
Application number: PCT/SG2002/000237
Authority: WO
Inventors: Dongjiang Wu; Ming Hui Hong
Original assignee: Data Storage Institute
Priority date: 2001-10-16
Filing date: 2002-10-16
Publication date: 2003-05-15

Abstract

A method for depositing a thin film material onto a laser-transmissible substrate. The substrate (182) has a first side (182a) for the thin film material to be deposited thereon and a second side (182b) opposite to the first side. The method comprises: providing a target (184) adjacent to the first side of the substrate; directing a laser beam (181) onto the target through the substrate from the second side; ablating the target using the laser beam whereby generating an ablated material from the target, and depositing the ablated material onto the first side of the substrate. The present invention maybe suitable for making thin films on to substrates, such as thin film electrodes, printed circuit boards and glass diffused resistors, etc.

Description

THIN FILM DEPOSITION BY LASER IRRADIATION

FIELD OF THE INVENTION

This invention relates to a method of depositing a thin film onto a substrate. In particular, it relates to a method of depositing a thin film onto a substrate by laser irradiation.

BACKGROUND OF THE INVENTION

In various industrial applications, there is a requirement to deposit a thin film on the surface of a substrate. For example, for manufacturing display panels, there is a need to deposit a thin film electric conducting material onto a glass substrate. A conventional method of depositing a thin film material onto a substrate is by using a laser. In certain applications, it is desirable to form a thin film on a predetermined portion of the substrate. In other applications, it is desirable to form a thin film onto the full area of the substrate.

One method of depositing a thin film metal material on to a substrate uses laser-thermal or photo decomposition of organometallic gases above the substrate surface. This continuous laser-thermal method is summarized in a book "Laser and Chemical Processing for Microelectronics" Cambridge, U. K., Cambridge

University Press, (1989) by Ibbs and Osgood. This method relates to laser induced ionization and metal particle attachment onto the surface of substrates. While the continuous laser-thermal method is presently used in certain applications, it suffers from certain problems. The metal atoms generated in the gas phase above the substrate tend to spread over the surface away from the decomposition region. Additionally, the process is relatively slow as compared to direct deposition method developed thereafter. Further, only those metals having an appropriate organometallic gas can be used. The above problems make rapid and precise metallization impractical for certain microelectronics and microstructuring applications. This method inherently has an associated environmental problem of requiring safe handling and disposal of some toxic organometallic gases and structures.

Another method of laser metallization makes use of direct laser-driven metal ablation from a target substrate to a working substrate, which is referred to as "direct deposition method". Variations of the micro-structuring by explosive laser deposition (MELD) method had previously been proposed for numerous metallization applications, such as deposition of a thin metal line on a semiconductor substrate as an electrically conducting path as disclosed in US Patent 5,246,885 to Braren et al, and a process of laser-driven, direct deposition of electrically conducting films onto a substrate as disclosed in US Patent 6,159,832 to Meyer et al. There are still certain disadvantages in these methods. Firstly, it is difficult to form a large area of electrically conducting film on the substrate due to structural restrictions of the system set up. Secondly, the electrically conducting film does not have a good adhesion to the substrate as the thin film is only attached to the surface of the substrate. Thirdly, the methods require an ultrafast laser, which is not reliable enough in commercial applications. Further, in the method under US patent 6,159,832, the target must be in a thin film form which requires special preparation, and the substrate needs to be heated up during the process, therefore the operation is complicated and the cost is relatively high.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, a method for depositing a thin film material onto a laser-transmissible substrate is provided. The substrate has a first side for the thin film material to be deposited thereon and a second side opposite to the first side. The method includesproviding a target adjacent to the first side of the substrate. A laser beam is directed onto the target through the substrate from the second side to ablate the target. The target then generates an ablated material, which is then deposited onto the first side of the substrate. The term "ablated material" refers to the laser induced plasma from the target, and the terms "deposited" and "depositing" should be understood as referring to attaching and/or doping a material into the portions underneath the surface of a substrate.

Preferably, the method further comprises focusing the laser beam onto the target.

Preferably, the method further comprises, during ablating the target using the laser beam, effecting a relative movement between the laser beam and the substrate whereby forming a trace of the ablated material onto the first side of the substrate.

Preferably, the laser beam ablates the target continuously during the relative movement whereby forming a continuous trace of the ablated material onto the first side of the substrate.

Alternatively, the laser beam ablates the target intermittently during the relative movement whereby forming a plurality of separate traces of the ablated material onto the first side of the substrate.

Preferably, the method further comprises, before directing a laser beam onto a target through the substrate, housing the target and the substrate in a hermetically sealed chamber.

Preferably, the hermetically sealed chamber is a vacuum chamber.

Alternatively, the hermetically sealed chamber is filled of inert gas. Preferably, the laser beam has a wavelength between about 300 nm and about 800 nm, and more preferably, the laser beam has a wavelength of about 532 nm.

Preferably, the laser beam is a pulsed laser beam.

Alternatively, the laser is a continuous wave laser beam.

In accordance with a second aspect of the present invention, an apparatus is provided, the apparatus comprising a laser source to generate a laser beam; a substrate through which the laser beam passes; and a target that is ablated by the laser beam to generate an ablated material, wherein the ablated material is deposited onto the substrate.

The present invention maybe suitable for making thin films on to substrates, such as thin film electrodes, printed circuit boards and glass diffused resistors, etc..

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic drawing of a system for forming a thin film onto a substrate according to a first embodiment of the present invention;

Fig. 2 is a schematic drawing of a system for forming a thin film onto a substrate according to a second embodiment of the present invention; Fig. 3A is an enlarged view showing part of the system of Fig. 1 with the plasma generation during deposition;

Fig. 3B is an enlarged view showing the plasma flying towards the substrate and forming a layer of material thereon;

Fig. 4A is a perspective view showing a spot pattern formed on the substrate, in which no relative movement between the substrate and the laser beam is effected; Fig. 4B is a perspective view showing a trace formed on the substrate, in which relative movement between the substrate and the laser beam is effected along one direction;

Fig. 4C is a perspective view showing a continuous trace formed on the substrate;

Fig. 4D is a perspective view showing a plurality of separate traces formed on the substrate;

Figs. 4E and 4F are perspective views showing respectively a solid area formed on the substrate; Fig. 5 is a partially cross-sectional view showing a substrate after thin film deposition;

Fig. 6A is a chart showing the experimental result of resistivity change against the repetition rate of an Al target;

Fig. 6B is a chart showing the experimental result of resistivity change against the beam scanning speed of an Al target; and

Fig. 7 is a flow chart illustrating the method according to the present invention.

DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS

As shown in Fig. 1, a system for forming a thin film onto a substrate 182 according to the one embodiment of the present invention comprises a laser source 11 , such as a Nd:YAG laser, which generates a laser beam 113. The substrate may be made of a material which is transparent to the laser beam 113, such as glass. The system comprises a beam attenuator 12 for attenuating the laser beam, a beam sampler 13 for splitting the laser beam, a first mirror 15 and a second mirror 16 for directing the laser beam to a desired location, and a galvanometer 18 for controllably moving the laser beam. The laser beam 113 is directed through the beam attenuator 12, the beam sampler 13, the first mirror 15, the second mirror 16, into the galvanometer 18. The output beam 181 from the galvanometer 18 is focused by a lens 189 and further directed onto a target 184 through the substrate 182. The substrate 182 has a first side 182a and an opposite side 182b.

A stage 185 is provided to hold the target 184 and the substrate 182 thereon. A measuring and adjusting device such as one or more micrometers 183 is/are provided and positioned between the substrate 182 and the stage 185 for measuring and adjusting the distance between the substrate 182 and the target 184. A sampling beam 133 split from the sampler 13 is directed into an energy meter 14 for measuring the energy of the laser beam 113. Output signal 141 from the energy meter 14 is directed to a computer 19 for monitoring and controlling the irradiation of the laser beam 113. Links 114 and 116 may be provided between the laser source 11, the galvanometer 18 and computer 19 for controlling purposes.

The substrate 182 and the target 184 are arranged in such a way that the target 184 is placed adjacent to, i.e. below the first side 182a of the substrate 182, on which the thin film material is to be deposited. A hermetically sealed chamber 190 may be provided to house the substrate 182 and the target 184. The hermetically sealed chamber 190 may be a vacuum chamber, or a chamber filled of inert gas. The laser fluence of the laser beam 113 is selected as greater than the ablation threshold of the target 184. The term "laser fluence" is defined as the laser pulse energy per irradiation area, and the term "ablation threshold" is defined as the level above which the target starts to be ablated.

As shown in Fig. 3A and Fig. 3B, in operation, the focused laser beam 181 is directed onto the target 184 through the substrate 182 from its second side 182b. The target 184 is therefore ablated and the target plasma 311 flies towards the substrate 182 at a speed of about 10,000 m/s or higher, which results in the deposition of the ablated material onto the surface and into the first side 182a of the substrate 182. The material 33 ablated from the target 184 is therefore formed on the first side 182a of the substrate 182 in a "dot" pattern 41 , as shown in Fig. 4A. Relative movement may be effected between the focused laser beam 181 and the substrate 182 during the above ablation process, by either fixing the substrate 182 and the target 184 at a set position but moving the focused laser beam 181, or fixing the irradiation of the laser beam 13 but moving the substrate 182 and the target 184. In the former case, the substrate 182 and the target 184 are fixed on the stage 185, and the focused laser beam 181 is movable by the galvanometer 18, as illustrated in Fig. 1. In the latter case, a laser beam 281 output from a lens 21 is directed onto the target 184 through the substrate 182 without a galvanometer, and the substrate 182 and the target 184 are fixed onto an X-Y stage 22 and are movable theretogether, as illustrated in Fig. 2. By properly controlling the galvanometer 18 (Fig. 1) or the X-Y stage 22 (Fig. 2), or both, relative movement between the substrate 182 and the laser beam 13_. may be effected, either in a one-dimension manner so that a single "line" pattern 43 of the ablated material onto the substrate 182 may be formed, as illustrated in Fig. 4B, or in a two-dimension manner so that a continuous trace 45 of the ablated material may be formed along the X-direction, Y-direction, or combination thereof, onto the substrate 182, as illustrated in Fig. 4C.

During the process, one or more dwells (short stops or pauses during machine operation) may be inserted, so that the laser beam may be irradiated in an intermittent manner, instead of a continuous manner. Accordingly, a plurality of separate traces including one or more rectilinear traces 47a curvilinear traces 47b (only one is shown as an example) of ablated material may be formed onto the substrate 182 in a predetermined pattern, as illustrated in Fig. 4D. It should be appreciated that a substrate with traces formed according to a designed pattern may be a printed circuit board.

It should also be appreciated that the galvanometer and the X-Y stage may both be present in the system and contribute to the relative movement between the substrate and the laser beam. The continuous trace or the plurality of separate traces of the ablated material may be formed side-by-side with their boundary portions overlapped with a predetermined overlap rate, so that a solid area 48 of the ablated material can be formed on the substrate 182, as shown in Fig. 4E. Alternatively, the solid area may also be formed by a continuous trace 49 which is deposited on the substrate by moving the X-Y stage and/or the galvanometer in a reciprocating manner, as shown in Fig. 4F.

The dashed arrow lines in Figs. 4B - 4F illustrate the movement directions of the laser beam relative to the substrate for forming the respective traces of the ablated material onto the substrate.

The overlap rate η is defined as the dimension of the overlapped portions over the dimension of the individual "dots" or which form a solid trace, or the individual traces which form a solid area can be explained according to the following formula:

77 = 1 - fxd

where v is the relative movement speed between the substrate and the laser beam (beam scanning speed); is the laser pulse repetition rate, and d is the diameter of the focused laser beam. In order to form a solid trace of the ablated material with desired property on the substrate, the overlap rate is preferably at least 5%. In order to form a solid area on the substrate, the overlap rate of the solid traces is also preferably at least 5%.

Factors affecting the deposition includes laser fluence, target-substrate distance, relative movement speed between the laser beam and the substrate as well as the laser pulse repetition rate. The laser fluence may vary depending on different types of target materials. For example, the laser fluence for a copper target is higher than that of an aluminum target because the melting point of copper is 1358K but that of the aluminum is 933K. For an aluminum target, for example, the laser repetition rate may be selected between about 600 - about 2400 Hz based on a relative movement speed of about 4 mm/s. The relative movement speed may be selected from about 1 - about 7 mm/s based on a repetition rate of about 1000 Hz. Variant materials may be used as the target material, such as metals or metalic oxide. Examples of which may be Al, Cu, Ti, Cr, W, Sn, Zn, Ru or Ni. The target may be in the form of a solid block, or powder. Depending on the selections of the above factors, the ablated material 51 may be formed onto the surface of the first side 182a of the substrate 182, and/or doped thereinto, as illustrated in Fig. 5. Corresponding experimental results of the resistivity change against the repetition rate and the scanning speed are shown in Fig 6A and Fig. 6B. Thin films formed according to the present invention achieve a resistivity level of about 1.5 Ω/Sq. In comparison, a commercialized ITO electrode has a resistivity level of about 58.7 Ω/Sq. This result shows that thin film obtained according to the present invention has a very good conductivity.

The method of forming a thin film onto a substrate according to the present invention may be illustrated in Fig. 7. In a first block 71 , a target is provided adjacent to the first side of the substrate. In the next block 73, a properly configured laser beam is directed onto the target through the substrate. The target material is therefore ablated and the ablated material is generated from the target at block 75 and further at block 77, the ablated material is deposited onto the first side of the substrate at the desired location.

During the ablation process, a relative movement between the substrate and the laser beam may be effected so that one or more traces of the ablated material may be formed on the substrate.

Claims

1. A method for forming a thin film onto a substrate, comprising: providing a target adjacent to a first side of the substrate; directing a laser beam onto the target through the substrate from a second side of the substrate; ablating the target using the laser beam to generate an ablated material from the target, depositing the ablated material onto the first side of the substrate.

2. The method of claim 1 , wherein the ablated material forms a thin film onto the first side of the substrate.

3. The method of claim 2, wherein the thin film has a resistivity level of no more than about_.1.5 Ω/Sq.

4. The method of claim 1 , further comprising focusing the laser beam onto the target.

5. The method of claim 1 , further comprising, during ablating the target using the laser beam, effecting a relative movement between the laser beam and the substrate forming a trace of the ablated material onto the first side of the substrate.

6. The method of claim 5, wherein the relative movement further comprising at least one rectilinear movement.

7. The method of claim 5, wherein the relative movement further comprising at least one curvilinear movement.

8. The method of claim 5, wherein the relative movement further comprising at least one reciprocating movement.

9. The method of claim 5, wherein the laser beam ablates the target continuously during the relative movement forming a continuous trace of the ablated material onto the first side of the substrate.

10. The method of claim 5, wherein the laser beam ablates the target intermittently during the relative movement forming a plurality of separate traces of the ablated material onto the first side of the substrate.

11. The method of claim 5, wherein the trace of ablated material covers a partial area of the first side of the substrate.

12. The method of claim 5, wherein the trace of ablated material covers the full area of the first side of the substrate.

13. The method of claim 9, wherein the continuous trace overlaps itself for covering a predetermined area of the first side of the substrate.

14. The method of claim 10, wherein the plurality of separate traces overlap with each other for covering a predetermined area of the first side of the substrate.

15. The method of claim 1 , further comprising, before directing a laser beam onto a target through the substrate, housing the target and the substrate in a hermetically sealed chamber.

16. The method of claim 15, wherein the hermetically sealed chamber is a vacuum chamber.

17. The method of claim 15, wherein the hermetically sealed chamber is filled of inert gas.

18. The method of claim 1 , wherein the target is metal.

19. The method of claim 18, wherein the metal includes Al, Cu, Ti, Cr, W, Sn, Zn, Ru or Ni.

20. The method of claim 18, wherein the metal is in a form of solid block.

21. The method of claim 18, wherein the metal is in a form of powder.

22. The method of claim 1 , wherein the target is metallic oxide.

23. The method claim 1 , wherein the substrate is transparent to the laser beam.

24. The method claim 23, wherein the substrate is glass.

25. The method of claim 1 , wherein the laser fluence is greater than the ablating threshold of the target.

26. The method of claim 1 , wherein the laser beam has a wavelength between about 300 nm and about 800 nm.

27. The method of claim 26, wherein the laser beam has a wavelength of about 532 nm.

28. The method of claim 1 , wherein the laser beam is a pulsed laser beam.

29. The method of claim 1 , wherein the laser beam is a continuous wave laser beam.

30. The method of claim 1 , wherein the ablated material flies towards the substrate at a speed of at least about 10,000 m/s.

31. An apparatus comprising: a laser source to generate a laser beam; a substrate through which the laser beam passes; and a target that is ablated by the laser beam to generate an ablated material, wherein the ablated material is deposited onto the substrate.

32. An apparatus for depositing a thin film onto a substrate as recited in claim 31 , wherein the substrate includes a first surface and a second surface, wherein the first surface is adjacent to the target, and wherein the ablated material is deposited on the first surface of the substrate.

33. An apparatus or depositing a thin film onto a substrate as recited in claim 31 , wherein the ablated material forms a thin film on the first surface of the substrate.