BACKGROUND OF INVENTION

Field of Invention

The present invention relates to a process for cutting off the so-called

electromagnetic interference (EMI, hereinafter), much criticized nowadays, by

means of coating of a single metal or an alloy of such materials of good

conductivity as Al, Cu, Ag, Au, Ni, etc., on non-conductive plastic generally

used for cases of eletronics goods such as computers, making use of a vacuum

sputtering process with the use of plasma which forms a thin coating, said

process being an improvement with excellent adhesiveness, and with less of the

disuniformity in thickness of the coating layer caused by irregularity of the

surface of the basic material that is coated on.

Description of Prior Art

As for the conventional art regarding blocking EMI a process is in use,

wherein non-conductive plastic is plated with Ni or Cu by a non-electrolytic

plating process but, by this method, not merely is a selective overlaying of a

single surface difficult but serious environmental pollution arises calling

forth a speedy development of alternative technologies. In the industrially

more advanced countries, a vacuum depositing technique has widely been

industrialized and technologies for EMI blocking are being studied in earnest,

but no greatly satisfactory achievements are reported on in terms of tight

adhesion or adjustment of uniformity of thickness.

In the present invention coating by magnetic sputtering is utilized

sputtering being used the most widely in the electronics industry these days.

The greatest advantage of the sputtering technique is that the deposition

process is performed at low temperature and the chemically quantitative ratios

are easily adjustable, whereupon the technique is extremely suitable in coating

material for electronics products, where surface dispersion is of much concern.

Because plasma etching which controls the shape of the basic material also can

be done in the same vacuum container without disruption of the vacuum, it is

esteemed as an indispensable technique in high-density integrated circuits.

In preparation of the thin coating of electrically resistive material, contact

material, magnetic material, memory discs, si for solar cells, and in

preparation of the thin coating of superconductors, of which much concern

arises these days, the sputtering technique is in wide use. In the field of

optical industry, too, it serves as vital technique in the reflexive or

transparent coating of laser, colored glass for architecture, lenses for

spectacles or cameras, etc., etc. and in many other fields of industry such as

chemical and decorative work also, but it has not ever been used in a process

for coating for blocking EMI.

SUMMARY OF INVENTION

The present invention relates to a process for laminating by the use of a

magnetic sputtering technique, the basic material of plastic, with a thin layer

having tight adhesiveness and good uniformity and blocking EMI effectively.

The techniques of vacuum deposition utilized plasma include the sputtering,

ion-plating, PACVD (plasma assisted chemical vapor deposition), etc., against

heat, with conductive metals the sputtering is the most appropriate technique,

and in the present invention also sputtering is mainly used.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a schematic diagram showing, in rough, exchange of the momentum

generated by the sputtering.

Fig. 2 is a graph showing a yield of the sputtering in accordance with

incidence energy of the Ar ⁺ ion.

Fig. 3,(a) represents the incidence angle of the projecting Ions against

the target.

Fig. 3,(b) is a graph showing the variance of the sputtering yield by the

varied angles of incidence.

Fig. 4 is a schematic diagram of a tri-polar sputtering process.

Fig. 5 is a schematic diagram of the process for laminating with an alloy

utilizing the magnetic sputtering according to the present

invention.

Figs. 6, 7, 8 and 9 are another examples of the present invention showing

the process for laminating with the targets composed of different

metals components.

DETAILED DESCRIPTION OF THE INVENTION

Now referring to the accompanying drawing, the process for laminating

plastics with metals for blocking EMI by utilizing the magnetic sputtering in

accordance with the present invention is described in detail below.

The sputtering technique is a process of physical deposition. The

principle is as follows: when a certain pressure and electric power are given

inside a vacuum chamber a plasmatic discharge of electricity occurs around a

target(l), whereupon the cations that existed in the area of the discharge beat

the surface of said target!1) by electrical force.

At this time the kinetic energy of the beating cations is transmitted to the

atoms(5) that exist on larger than the binding energy of the atoms of the

beaten target(l) the atoms of said target(l) are emitted. This phenomenon of

sputtering is comparable to a collision of elastomers, and can be illustrated

simply as in Fig. 1. In short, when ions(2) which have a mass Mi are shot

toward said target ato s(5) which have a mass Mt and a velocity Vi, the

velocity Vi' of said ions after the collision will be, according to the law of

momentum conservation;

Vi' = {(Mi - Mt) / (Mi + Mt)} Vi (1)

and Vt' the velocity of the emitted target atoms, can be represented as the

equation;

Vt' = {(2Mi) / (Mi + Mt)) Vi ^• (2),

Provided that the ions' incidence angle against the surface of the target is

90 degrees, and the colliding beads are all to be completly elastomers.

According to the formulae above, after the collision of the projecting ions,

said ions will penetrate the surface of said target when the velocity thereof

is Mi > Mt, and they will move in a direction opposite to that of the

projection when it is Mi < Mt. But in actual sputtering, it is hardly possible

to represent the motion of the emitted atoms as simple equations as were given

above, the colliding energy of the projecting ions is propagated to the other

atoms existing around, divided into the primary and Low Energy Knockon to give

emission atoms, as shown in Fig. 1. In conventional sputtering deposition,

the kinetic energy of emitted atoms is about 10-40eV, reportedly about 50-100

times as high as the kinetic energy of the depositing atoms in vacuum

deposition.

For understanding phenomena of the sputtering it is required to look into

regarding the sputtering yield, which means the number of target atoms emitted

by one ion being projected toward the surface of the target from the plasma,

and which is represented as the following equation;

S = K{(Mi - Mt) / (Mi + Mt)2 (I / Uo) a (Mt / Mi)} (3)

Wherein K stands for a constant ranging from 0.1 to 0.3; E, energy of the

incidence ion; Uo, the binding energy of the target atoms; and (Mt/Mi), a

simple increasing function of Mt/Mi.

In Mt/Mi = 2, as a typical ratio quantity the a is about 0.3. Fig. 2 shows

the various sputtering yields of the target materials according to the

incidence energy of the Ar* ion, it teaches that it varies depending upon the

sorts of target materials. The sputtering yield also varies depending upon the

incidence angles of the projecting ion as shown in a graph of Fig. 3,(b) and is

exhibited as the sec Θ of the incidence angle θ shown in Fig 3,(a). When

the incidence angle increases, however, it begins to depart from the functions

of Sec θ; and, after showing the maximum value of the sputtering yield, its

value drops to 0 at =_τr/2 where in the incidence angle and the surface of the

target is in equilibrium. Most ions are projected perpendicular to the

surface of the target, only a very small amount of ions are projected in

smaller angles by an effect of the dispersion of gas.

The simplest sputtering system is a plain bipolar device as described above

wherein l-20mA/cm ² and the pressure is about 10 torr. However such plain

bipolar method has a defect that the speed of deposition is too slow.

Therefore its industrialization is accompanied by much difficulties. Thus

after the development of such plain bipolar technique much research has been

done to increase the deposition speed, and a tripolar sputtering technique has

been developed as a result. Such tripolar method is of that another pole is

added to the two to adjust the thermal electron release source and the flow of

the released thermal electrons.

Fig. 4 shows a scheme of the tripolar method. As the soure releasing

thermal electrons a tungsten filament is used, and such improved method is

possible to increase the speed of deposition since the density of plasma can be

raised by means of releas of thermal electons.

In addition to said tripolar process there also are other sputtering methods

as follows: The radio frequency sputtering making use of nonconductive

targets; the reactive sputtering making use of conductive targets and inducing

reactive gas, which forms a nonconductive thin layer; and the magnetic

sputtering.

Of said sputtering methods, applying plasma, the one most suitable for

forming conductive thin coatings over plastic for blocking EMI, is that of the

magnetic sputtering.

The adhesive force of the thin EMI-blocking layer over plastic depends

largely on the process for pre-treatment of the basic material, and in the

process of the present invention the following two types of pre-treatment

processes are given to the basic material:

(1) Removal of fat with a neutral detergent → washing with water → removal

of fat with a neutral detergent → washing with water —» drying in warm air;

(2) Removal of fat with a neutral detergent → washing with water → Removal

of fat with NaOH(40%, 30~50 ^* C) → washing with water → drying in warm air.

It was proved in both cases that adhesive force of adhesive tapes was

excellent by test and in actual production lines the removal of fat with a

neutral detergent of (1) above, is widely utilized since the process is fairly

simple.

After the pre-treatment of the basic material is finished the formation of

the metal depositing layer by the sputtering, which is the main objective of

the present invention, is undertaken.

Fig. 5 shows the circuit diagram of the magnetic sputtering device, in

which the target 1 and the basic plastic material 9 are set opposite to each

other in the vacuum chamber 23; poles N and S of magnets 16 are alternatingly

positioned on said targets l; the anode 8 is connected with basic plastic

material 9 and the cathode 7 to the target. Using said magnetic sputtering

device, when power is supplied the plas ic discharge occurs wherein ions 10 hit

said target 1 to emit conductive deposition ion 12 and the resulting ion 12 is

coated over said basic plastic material 9.

Since under the same pressure the probability for electrons to hit neutral

atoms rises in proportion to the moving distance of the eletrons, their spiral

movement around the target increases the range of their movement greatly and

enhances the probability of their ionization to exhibit a fast speed for

deposition; and the highest ionization probability and high density of plasma

band exhibit at the region where electric and magnetic force lines cross one

another perpendicularly to occur the regional sputtering phenomena.

In the present invention a single metal such as Al, Ag and Ni or different

multiple metals, such as Al-Ag, Ag-Cu, Cu-Au are used as the metal target.

Now referring to accompanying drawings examples of each thin layer is

embodied in detail as follows;

Example of thin layer of an alloy Al-Ag in accordance with the present

invention is illustrated in Figures 6 and 7. Either an Al metal target 18 and

Ag metal target 19 are, as shown in Fig. 6, aligned for a certain length in the

vacuum chamber 23, and ions of metal elements are deposited by sputtering over

the surface of the basic material to form alloy thin layer by moving a moving

device 17 of base material moving parallelly so as to face the target, or in a

non-parallel process, a target is composed of Ag 19 and Al 18, as shown in Fig.

7, and the basic material 9 moves to form alloy thin layer opposite the

resulting target to form a mixed metal thin layer.

The thin layer coated with alloys such as Al-Ag, Cu-Au, etc. has advantages

as follows:

The vacuum can be maintained without disruption since the metal ions are

mixed and coated while the base material moves in the same vacuum chamber; the

rate of Ag being contained in the alloy thin layer can be adjusted with ease

because the locations of Ag target 19 and the basic material 9 can be adjusted

optionally; and the possibility of disruption of adhesion on the interface of

Al-Ag which could tend to happen in the conventional multi-layer coating can be

removed.

While in the coating of a single metal Al, the deposition is carried out

with 4.5w/cm ² target power for 5 minutes to give a conductivity with linear

resistance less than 1 Ω/cm, it is possible, in the case of a thin coating of

Al-Ag alloy, to lower the linear resistance to 0.7 Ω/cm with less thick layer

than that of a single Al coating, and it is also possible to deposit the Al-Ag

alloy without any disruption of the vacuum.

In the present invention, the role of Ag is to raise the conductivity while

decreasing the thickness of the coating, it can minimize the variation by heat

of plastic to give an EMI-blocking thin coating with excellent adhesion and in

a result, the remaining stress of the coating can be minimized. In the case

of Ag a very thin coating less than 0.5;um, was formed because the high prices

of the material itself. In the present invention, Al-Ag thin layer displayed

an optimal thickness and excellent properties.

The coating of a single metal, Cu, displayed properties similar to Al, and

the good conductive thin coating layer of less than 1 Ω/cm is given by

deposition with 4.7w/cm ² target power for five minutes.

The coating over plastic by the magnetic sputtering method described above

was excellent in adhesion. Results of a test of adhesion comparing the Al

coatings by the conventional vacuum overlaying technique with that of the

sputtering of the present invention are given in table 1.

Table 1 : 3/*m Al thin layer deposited on plastic for a computer case.

The advantage of the coating by the magnetic sputtering technique of the

present invention is to be found in its quite high speed of deposition and the

little rise in temperature of the basic material, making it convenient to form

deposition coating on basic material of plastic or polymer series generally

weak to heat, while it is also possible to adjust the ingredients of the

depositing layer at will thanks to the use of multiple targets.

The deposition velocity of 10,000A/min is obtained, by the magnetic

sputtering technique of the present invention, and the forming of a thin

coating layer on the basic material is possible, with out causing generation of

heat, by controlling the flow of the electric current to both poles or the

basic material, the magnetic sputtering technique of the present invention is a

very good process for forming EMI-blocking thin layers on plastic used for

production of cases for computers and other electronics goods parts.