WO2008147095A1

WO2008147095A1 - Method and apparatus for plasma ion implantation

Info

Publication number: WO2008147095A1
Application number: PCT/KR2008/002957
Authority: WO
Inventors: Sun-Soon Park; Moo-Hyun Cho; Sang-Jung Kim; Hyo-Yol Liu
Original assignee: Dawonsys Co., Ltd.
Priority date: 2007-05-29
Filing date: 2008-05-27
Publication date: 2008-12-04
Also published as: KR100857845B1

Abstract

A plasma ion implantation method is capable of implanting ions into a surface of an object with no generation of sparks. The plasma ion implantation method includes the steps of : positioning the object and a conductive grid electrode within a vacuum chamber, the object having an ion implantation surface spaced apart a predetermined distance from the grid electrode; forming plasma around the object and the grid electrode received within the vacuum chamber; applying a negative voltage to the grid electrode so that plasma ions can make reciprocating movement within an ion matrix sheath formed around the grid electrode; and restoring the negative voltage applied to the grid electrode to an original state in such a way that some of the ions are rectilinearly moved to impinge against the surface of the object for implantation into the surface of the object.

Description

[DESCRIPTION] [Invention Title]

METHOD AND APPARATUS FOR PLASMA ION IMPLANTATION [Technical Field]

<i> The present invention relates to a surface treatment method and apparatus and, more particularly, to a surface treatment method and apparatus that makes use of plasma ion implantation. [Background Art]

<2> A method of changing surface properties by implanting ions into a surface of an object such as a metal body, a semiconductor, a ceramic body, a polymer body or the like is commercially employable in many different fields. If ions are implanted into a surface of an object, it becomes possible to improve surface properties of the object, including friction resistance, wear resistance, corrosion resistance and hardness, and also possible to impart conductivity to a non-conductor such as polymer or the like.

<3> There are known various kinds of methods by which ions accelerated with high energy are implanted into a surface of an object to be surface-treated. Typical ion implantation methods include a method of extracting ions generated in a plasma generation apparatus, focusing the ions to form an ion beam, accelerating the ion beam and irradiating the accelerated ion beam on a surface of an object. An ion beam implantation apparatus used in this method is structurally complex and is hard to operate. For that reason, the ion beam implantation apparatus has been restrict ively used for a special purpose, despite the fact that it can enjoy an excellent surface property improvement effect provide by ion implantation. In particular, the ion beam implantation apparatus has not been extensively used in commercial applications because it suffers from reduced productivity and increased costs when implanting ions into an object with a large surface area.

<4> Representative references directed to this prior art technology include U.S. Patent No. 4,764,394 entitled "METHOD AND APPARATUS FOR PLASMA SOURCE ION IMPLANTATION" and issued to John R. Conrad in 1988 (reference document 1) and Korean Patent No. 10-0442309 entitled "CONTINUOUS SURFACE TREATMENT APPARATUS AND METHOD OF FLIM-LIKE POLYMER and issued to Yong R. Choi in 2003 (reference document 2). The disclosure of reference documents 1 and 2 are incorporated herein by reference in its entirety.

<5> Reference document 1 discloses a plasma source ion implantation method and apparatus that reduces the drawbacks inherent in the ion beam irradiated plasma ion implantation method noted above. Fig. 1 is a schematic view for explaining the plasma source ion implantation method disclosed in reference document 1, and Fig. 2 is a diagram illustrating a voltage distribution in the vicinity of an object surface when ions are implanted by the plasma source ion implantation method. In the plasma source ion implantation method, an object 20 to be surface-treated by ion implantation is placed on a table 16 within a chamber 11 filled with plasma, and negative high voltage pulses are applied to the object 20. By doing so, ions are accelerated in a transition plasma sheath generated around the object surface. The ions thus accelerated impinge against the object surface applied with high voltage. This principle is employed in the plasma source ion implantation method. Referring to Fig. 1, a vacuum pump 13 for generating vacuum within the chamber 11 and a gas supply device 14 for supplying a plasma generation gas into the chamber 11 are connected the chamber 11. A table 16 for supporting an object is arranged inside the chamber 11 and a pulse generator 15 for applying negative high voltage pulses to the object is connected to the table 16. Reference numeral 12 designates a plasma generator for generating plasma with the gas supplied into the chamber 11.

<6> The plasma source ion implantation method disclosed in reference document 1 is capable of greatly simplifying an ion implantation process and performing the ion implantation process in a cost-effective manner, as compared to the conventional irradiation type ion beam implantation method. However, the plasma source ion implantation method suffers from a problem in that the ion- implanted surface is rendered uneven or damaged by a high voltage spark resulting from the application of the negative high voltage to the object (target) of surface treatment submerged in the plasma. This problem is fatal in case of employing the ion implantation process for the purpose of mass production. The ions accelerated by an ion beam accelerator can be implanted with energy of several hundreds keV or more. In case of the plasma source ion implantation method, however, it is usually possible to perform the ion implantation with energy of lOOkeV or less. Under a plasma condition intended for industrial mass production, it is actually quite difficult to avoid generation of sparks even when a voltage is applied to generate ion energy of about 50keV.

<7> Particularly, it is difficult to directly apply the plasma source ion implantation method to a polymer film as a non-conductor because no voltage can be directly applied thereto. For the purpose of surface-treating the polymer film through ion implantation, it may be possible to use a modified method in which a polymer film is brought into contact with a flat surface of a metal electrode and high voltage pulses are applied to the metal electrode. In this instance, ion acceleration and implantation can be caused to occur using a capacitive voltage dividing phenomenon spontaneously generated between the plasma and the electrode. In case of implanting ions into the polymer film with the modified method, however, a spark initiation voltage is reduced due to the outgasing phenomenon of the polymer surface caused by impact of the ions. This poses a problem in that sparks are severely generated as compared to a case where ions are implanted into an object.

<8> Shown in Fig. 3 is a method of indirectly implanting ions into a surface of a non-conductor such as a polymer film or the like by use of a grid electrode. Referring to Fig. 3, a non-conductive surface treatment object 20 is placed on a table 16 housed within a chamber 11. A grid electrode 21 is provided in a position spaced apart a specified distance from a surface of the object. A gas for use in generating plasma is supplied from a gas supply device 14 in a state that suitable vacuum is established within the chamber 11 by means of a vacuum pump 13. As plasma is formed within the chamber 11 by the operation of a plasma generator 12, negative high voltage pulses are applied to the grid electrode 21. In this case, some of the ions accelerated within a plasma sheath formed in the grid electrode are implanted into the grid electrode, whereas others pass through the grid electrode and reach the surface of the non-conductive object placed on the table 16. If the ions reaching the surface of the non-conductive object have sufficiently great energy, they are implanted into the surface of the non-conductive object. This method may be called a grid-biased plasma source ion implantation method and essentially falls with the scope of the plasma source ion implantation method. In the grid-biased plasma source ion implantation method, if the interval between the grid electrode and the surface of the non-conductive object grows smaller, the energy of the ions reaching the object becomes greater, thereby assuring good ion implantation. However, if the interval is too small, a shadow of the grid electrode appears on the surface of the object, which deteriorates the uniformity of the treated surface.

<9> A method and apparatus for continuously surface-treating a polymer film with the grid-biased plasma source ion implantation method noted above is disclosed in reference document 2 cited earlier. Fig. 4 schematically shows the apparatus disclosed in reference document 2. Referring to Fig. 4, a polymer film 22 is continuously supplied to and discharged from a plasma chamber 11 by means of an unwinder and a winder not shown. A vacuum pump 13, a gas supply device 14 and a plasma generator 12 are mounted to the chamber 11. A cylindrical support drum 17 rotatably installed for winding, supporting and guiding the polymer film 22 continuously supplied from the unwinder. A conductive grid electrode 23 is arranged in a position spaced apart a specified distance from the outer circumferential surface of the support drum 17. A pulse generator 15 is provided to apply negative high voltage pulses to both the support drum 17 and the grid electrode 23.

<io> With the method and apparatus for realizing the grid-biased plasma source ion implantation method as shown in Fig. 4, the support drum 17 and the grid electrode 20 are all electrically connected to the pulse generator 15. Therefore, if a gap 40 exists between the outer circumferential surface of the support drum 17 and the polymer film 22 at an initially wound contact point as illustrated in Fig. 5, a problem is posed in that sparks are generated in the gap 40 during application of high voltage pulses, thus causing damage to the polymer film.

<ii> In the grid-biased plasma source ion implantation method, it is possible to process the polymer film in a free standing state with no use of a supporting electrode. In this case, electric fields are concentratedly formed in a plasma sheath which is formed between the polymer film and the grid electrode. If a local pressure resulted from emission of vapor or air adsorbed or absorbed to the surface of the polymer film rise, it is often the case that the spark-on conditions of Pachen curve Curve are satisfied. In case where the moisture or air adsorbed or absorbed to the polymer film is emitted irregularly, a spark discharge may occur within the plasma sheath, consequently causing damage to the surface of the polymer film.

<i2> As mentioned above, the plasma source ion implantation method and the grid- biased plasma source ion implantation method, in which a surface treatment is performed by plasma ion implantation, suffer from a problem in that the surface of a non-conductive object such as a polymer film or the like is damaged by sparks during the surface treatment thereof. [Disclosure] [Technical Problem]

<13> It is an object of the present invention to provide a novel method capable of implanting plasma ions into the surface of a surface treatment object regardless of the kind of the object and without causing damage to the surface of the object.

<14> Another object of the present invention is to provide a novel plasma ion implantation method capable of uniformly implanting plasma ions into a large surface area object and also capable assuring mass production in a cost- effective manner.

<i5> A further object of the present invention is to provide a plasma ion implantation apparatus for realizing the novel methods noted above. [Technical Solution]

<i6> In one aspect of the present invention, there is provided a plasma ion implantation method for implanting ions into a surface of an object, including the steps of: positioning the object and a conductive grid electrode within a vacuum chamber, the object having an ion implantation surface spaced apart a predetermined distance from the grid electrode! forming plasma around the object and the grid electrode received within the vacuum chamber! applying a negative voltage to the grid electrode so that plasma ions can make reciprocating movement within an ion matrix sheath formed around the grid electrode! and restoring the negative voltage applied to the grid electrode to an original state in such a way that some of the ions are recti linearly moved to impinge against the surface of the object for implantation into the surface of the object.

<i7> In another aspect of the present invention, there is provided a plasma ion implantation apparatus including: a chamber for receiving an object with a surface! a conductive grid electrode spaced apart a predetermined distance from the surface of the object! and a voltage application means for applying a negative voltage to the grid electrode, wherein the grid electrode is arranged to ensure that the surface of the object lies outside an ion matrix sheath formed by the negative voltage applied to the grid electrode, and wherein the voltage application means is designed to apply the negative voltage to the grid electrode in such a way that ions are trapped and reciprocat ingly moved within the ion matrix sheath and to restore the negative voltage applied to the grid electrode in such a way that the ions trapped are released and recti linearly moved to impinge against the surface of the object for implantation into the surface of the object.

<i8> In the present invention, the applying step and the restoring step may be repeatedly performed at a predetermined time interval. Referring to Fig. 7, the applying step and the restoring step can be repeatedly carried out by applying the pulse as illustrated to the grid electrode. In this case, the period of the pulse applied to the grid electrode is composed of a negative voltage application time (Tr), a negative voltage restoration time (Tf) and a restored voltage maintaining time (Tm). In the subject specification, the negative voltage restoration time will be sometimes referred to as a voltage falling time or a pulse falling time in terms of the pulse. It is preferred that the ratio of a negative voltage applying time period to a negative voltage restoring time period is in a range of from 1:10 to 1:10,000. It is also preferred that the ratio of the time period from the beginning of application to the beginning of restoration of the negative voltage to the time period from the beginning to the end of restoration of the negative voltage is in a range of from 5:1 to 10,000:1. With the present method, it is possible to implant ions into a metal body, an alloy body, a semiconductor, a ceramic body and a polymer body without causing damage to the surface of an object regardless of whether the object is a conductor or a non-conductor.

[Advantageous Effects]

<20> The plasma ion implantation method of the present invention differs from the conventional grid-biased plasma source ion implantation method in that the surface of the ion implantation object lies outside the plasma sheath region defined by the grid electrode and that the voltage applied to the grid electrode is restored to the original state.

<2i> Since the surface of the ion implantation object lies outside the plasma sheath region in the present invention, the electric fields resulting from the high voltage applied to the grid electrode have little influence on the object surface, thereby preventing generation of sparks on the object surface. In the conventional grid-biased plasma source ion implantation method, ion implantation occurs while the high voltage pulses are being applied to the grid electrode, and the ion implantation quantity is largely affected by the intensity of the voltage applied to the grid electrode. In contrast, the ion implantation quantity in the plasma ion implantation method of the present invention is primarily affected by the time period of the step of restoring the high voltage applied to the grid electrode to the original state, namely the time period from the initiation of restoration to the completion of restoration of the negative voltage applied (hereinafter referred to as "pulse falling time"). [Description of Drawings]

<22> Fig. 1 is a view for explaining the ion implantation principle in a conventional plasma source ion implantation method.

<23> Fig. 2 shows a voltage distribution when ions are implanted according to the conventional plasma source ion implantation method.

<24> Fig. 3 is a view for explaining the ion implantation principle in a conventional grid-biased plasma source ion implantation method.

<25> Fig. 4 is a schematic view an ion implantation apparatus for use in the conventional grid-biased plasma source ion implantation method.

<26> Fig. 5 is a partially enlarged view of the ion implantation apparatus shown in Fig. 4. <27> Figs. 6(a) to 6(d) are schematic views for explaining an ion implantation method in accordance with the present invention. <28> Fig. 7 is a view showing the time periods of a negative voltage applying step, a negative voltage restoring step and a restored voltage maintaining step in the present invention. <29> Figs. 8(a) and 8(b) are views for explaining the plasma ion implantation principle in the present method. <30> Fig. 9 is a view for explaining the accelerated ion generation principle in the present method. <3i> Figs. 10(a), 10(b) and 10(c) are views illustrating the results of computer simulation for the movement of ions performed according to the present ion implantation method. <32> Fig. 11 is a graph representing the results of calculation of the energy of ions reaching the surface of a surface treatment object depending on the restoration time of pulses in the computer simulation illustrated in Fig. 10: <33> Fig. 12 is a schematic view showing a plasma ion implantation apparatus in accordance with one embodiment of the present invention. <34> Fig. 13 is a schematic view showing a plasma ion implantation apparatus in accordance with another embodiment of the present invention. <35> Fig. 14 is a plan view showing one example of a grid electrode employed in the plasma ion implantation apparatus of the present invention.

[Best Mode] <36> A plasma ion implantation method in accordance with the present invention will now be described in detail with reference to Figs. 6 through 11. <37> Figs. 6(a) to 6(d) are schematic views for explaining an ion implantation method in accordance with the present invention. Referring to Fig. 6(a), an ion implantation object 20 is first placed on a table 16 housed within a chamber 11. A conductive grid electrode 21 is arranged in a position spaced apart a specified distance from the ion implantation surface of the object

20. At this time, the conductive grid electrode 21 is installed to ensure that the surface of the object 20 lies outside a plasma sheath region formed by a high voltage applied to the grid electrode 21. Referring next to Fig. 6(b), a vacuum pump 13 is operated to develop vacuum within the chamber 11, and a gas for formation of plasma is supplied into the chamber 11 from a gas supply device 14. Depending on the ion implantation purpose and the kind of the object, the gas thus supplied may be one or more selected from the group consisting of helium, argon, nitrogen, neon, krypton and xenon. If the conditions within the chamber 11 are rendered suitable for generating plasma, a plasma generator 15 is operated to generate plasma within the chamber 11. Since the method of generating plasma is apparent to those skilled in the art, no description will be made in that regard. Referring next to Fig. 6(c), a pulse generator 15 connected to the grid electrode 21 is operated to apply a negative high voltage to the grid electrode 21. As the negative high voltage is applied to the grid electrode 21 that remains dipped into the plasma, a plasma sheath 21a of specified thickness composed of reciprocatingly moving ions is formed around the grid electrode 21 as illustrated in Fig. 6(c).

<38> The phenomenon that the ions are reciprocatingly moved within the plasma sheath 21a will be described in more detail. If a negative high voltage is applied to the grid electrode 21 lying within the plasma, the electrons present around the grid electrode 21 are rapidly repelled away from the grid electrode 21 (at the reaction time of 1/plasma electron vibration frequency). The ions that are not reacted due to a relatively high weight are uniformly distributed in a space around the grid electrode 21. This is referred to as an ion-matrix sheath, the thickness (S) of which is given by equation 1:

χ

<39> ,where ^De is the Debye length of electrons,

^Va is the voltage applied, and ^Te is the temperature of electrons. Immediately upon application of the high voltage, the electrons around the grid electrode 21 are initially diffused at a very high speed (at the time equivalent to the vibration period of plasma electrons, i.e., l/^ωV) so that they can reach the vicinity of an S-position. Once the electrons are diffused to the vicinity of the S-position, the ions are slowly moved toward an electrode. As a consequence, the diffusion speed of electrons is slowed down and the sheath region is expanded at the speed of an ion acoustic wave (Vs) given by equation 2: y_^ 1/2

<40> , where ^mι is the mass of ions.

<4i> Expansion of the sheath is maximized when reached an equilibrium state, at which time the thickness (d) of the sheath is given by equation 3:

<43> The time for which expansion of the sheath reaches the equilibrium state differs slightly depending on the status of plasma and is approximately several milliseconds in case of an ion implantation process. In view of the expansion of the plasma sheath occurring around the grid electrode when the negative high voltage is applied to the grid electrode, the interval between individual parallel wires of the grid electrode arranged in a circular cross- section is set twice as great as the S value. This will ensure that the space between two parallel wires is kept substantially equipotential with the wires to which the high voltage is applied. In the present invention, it is preferred that the grid electrode includes a frame and a plurality of parallel rods arranged at an equal interval within the frame and further that the interval between the rods is set no greater than twice the thickness of the ion matrix sheath formed by application of the high voltage to the grid electrode. If the interval is too great, there exists a region where no ion is trapped, thereby reducing the number of ions for use in ion implantation. If the interval is too small, the trapped ions impinge against the grid electrode, thereby reducing the number of ions for use in ion implantation.

<44> Fig. 8(a) illustrates a voltage distribution in the space around the grid electrode when a negative high voltage is applied to the grid electrode 21. Referring to Fig. 8(a), if the negative high voltage is applied to the grid electrode formed of parallel wires according to the theoretical consideration mentioned above, the sheath is expanded. As a consequence, an extremely small number of the ions accelerated toward the grid electrode impinge against the grid electrode and are absorbed by the grid electrode, while the remaining ions are moved through the spaces between the respective wires of the grid electrode. The ions passed through the grid electrode 21 are moved toward the boundary of the sheath and decelerated by the negative high voltage applied to the grid electrode. If the moving speed of the ions becomes zero, the ions are turned 180 degrees and accelerated toward the grid electrode 21 once again. Such reiterative reciprocating movement of the ions around the grid electrode continues to occur while the negative high voltage is applied to the grid electrode 21. This state is referred to as ion trapping. Fig. 6(c) illustrates a state that the ions are trapped around the grid electrode to make reciprocating movement. Reference numerals 21b and 21c designate the boundaries of the plasma sheath.

<45> In the final step of the plasma ion implantation method according to the present invention, the pulse generator 15 is operated to remove (restore to the original state) the negative high voltage applied to the grid electrode 21, as illustrated in Fig. 6(d). If the negative high voltage applied to the grid electrode 21 is removed, the reciprocatingly moving ions make rectilinear movement away from the grid electrode 21. Fig. 8(b) illustrates a voltage distribution in the space around the grid electrode when the negative high voltage applied to the grid electrode is removed (restored to the pre-appl icat ion state). Referring to Fig. 8(b), the environment around the grid electrode 21 is restored to the original plasma voltage state simultaneously with removal of the negative high voltage, thereby extinguishing the electric fields formed around the grid electrode. At the moment of extinguishment of the electric fields used in keeping the ions trapped, the ions trapped and reciprocatingly moved around the grid electrode 21 under the negative high voltage make uniform rectilinear movement while maintaining their kinetic energy as it is. The state in which the ions are released from the trapped state to make uniform rectilinear movement is referred to as a ballistic mode. As viewed from the standpoint of the grid electrode 21, the ions are moved away from the grid electrode 21 in the opposite directions, one half in one direction. If a neutral gas

_3 pressure (less than 10 Torr) with little impact is maintained in the plasma generation state, the ions released from the trapped state to make uniform movement are not decelerated during movement through the plasma and are moved to the surface of the ion implantation object 20 or the wall surface of the chamber 11.

<46> According to the present invention, when an attempt is made to implant ions into the surface of a polymer film or other non-conductive objects, the surface is positioned perpendicularly to the moving path through which the ions released from the trapped state are moved as ballistic particles at a high speed. This allows ions to be implanted into the surface. Furthermore, by keeping the surface of the polymer film sufficiently spaced apart from the grid electrode to ensure that no high voltage is induced on the surface of the polymer film, it is possible to essentially prevent generation of sparks on the surface of the polymer film.

<47> Fig. 9 is a view for explaining how to generate accelerated ions in the present method. In order to increase, as far as possible, the speed of the ions (ballistic particles) released from the trapped state and moved at a uniform speed, it is necessary to shorten, as far as possible, the voltage fall time (Tf in Fig. 6(d)) during which the high voltage applied to the grid electrode is removed (or restored to the original state). The number of the ions trapped grows high in proportion to the time period (pulse width) for which the high voltage applied to the grid electrode is maintained, but gets saturated if the pulse width becomes equal to or greater than a specified value. In case where a pulse voltage is applied to the grid electrode, ions are trapped within the sheath region of the grid electrode and are reciprocatingly moved during the flat-top period for which the pulse voltage applied is maintained.

<48> Referring to Fig. 9, description will be made as to why the energy emitted from the trapped ions when the pulse voltage applied is removed and restored to the original state differs between the fast fall time and the slow fall time. The curve p indicated by a dot line in Fig. 9 shows a voltage distribution around the grid electrode immediately before the pulse voltage applied to the grid electrode is restored to the original state. The uppermost solid line curve r represents a voltage distribution around the grid electrode in case of the fast fall time during which the pulse voltage is restored fast. The intermediate solid line curve q indicates a voltage distribution around the grid electrode in case of the slow fall time during which the pulse voltage is restored slower than the restoration speed in the uppermost solid line curve r. In this regard, the X-axis denotes the positions of ions with respect to the grid electrode, and the Y-axis stands for the voltage. The minus values in the Y-axis are equal to the kinetic energy of ions, meaning that the speed of ions becomes faster in the positions where the minus values are kept greater.

<49> If an ion is moved from position A to position B within a time of ^Δt, the energy thereof is reduced in proportion to the potential rise. Due to the pulse voltage drop during this time, the spatial voltage V₀ in position B is increased to a voltage V2 in case of the fast fall time (the voltage distribution curve r) and a voltage V₁ in case of the slow fall time (the voltage distribution curve q) . The ions having the kinetic energy equivalent to the spatial voltage V₀ gain additional kinetic energy proportionate to the voltage rise and escape from the trapped state, consequently flying toward the surface of the ion implantation object. <50> Under the environment discussed herein, the conditions given by the following inequity are generally satisfied:

<51> v_a{x_λy v_a( x_x+AX)€ v_a{ t_λy v_a( t_x+u)

<52> Also establ i shed are the fol lowing equat ions :

AX

I _{Z= Z 1} - V ^ f ₁ )

<53> Δ/ ; and

<54> ,where ' is the moving speed of an ion. In other words, an ion has the kinetic energy equivalent to a potential difference [Vo-Vi] in case of the slow fall time but has the kinetic energy equivalent to a potential difference [V0-V2] in case of the fast fall time. It can be seen in Fig. 8 that the potential difference [V0-V2] is greater than the potential difference [V₀-V₁]. Consequently, it can be noted that the number of ions flying with high kinetic energy when escaped from the trapped state is increased if the time for which the pulse voltage is restored to the original state becomes shorter.

<55> In case where ions are trapped by application of the negative high voltage to the grid electrode, the kinetic energy (or the moving speed) of the ions released from the trapped state is decided by the pulse drop speed. If the pulses are dropped with a time constant far longer than the plasma vibration period, the ions conform to the change in the electric fields of the plasma sheath and lose their energy for the most part. In other words, the pulse falling time within which ion implantation is hard to occur may be given by equation 4:

7» — — ω ^■

<56> ^ ; and equat i on 5 : 2 ^1/2

<57> , where / is the pulse falling time, ^ω P> is

the ion plasma vibration frequency, ⁿ is the plasma density, ^⁰ is the

dielectric constant under vacuum, and ' is the mass of an ion. As can be

seen in equation 5, ^p' is the frequency that varies with the plasma density when a plasma gas is decided. In view of equations 4 and 5, it is preferred that the pulse falling time within which ion implantation can occur is equal to or smaller than ten times of the ion plasma vibration period (T_Pj=l/ω_pi), namely Tf ≤ 10T_pi, or equal to or smaller than two microseconds. In case where ions are to be implanted into a polymer film, the voltage for accelerating the ions needs to be at least 1OkV.

<58> Figs. 10(a), 10(b) and 10(c) are views illustrating the results of computer simulation for the movement of ions performed according to the present ion implantation method. Fig. 11 is a graph representing the results of calculation of the energy of ions reaching the surface of a surface treatment object depending on the pulse restoration time (pulse falling time) in the computer simulation illustrated in Fig. 10.

<59> In order to confirm the phenomenological principle of the energy and quantity of the ions released from the trapped state depending on the voltage applied to the grid electrode, computer simulation was performed using a plasma particle computer simulation program (OOPIOCode). The results of computer simulation are shown in Figs. 10 and 11. Data of the plasma conditions used in the computer simulation are as follows: pulse rising time T_r=100ns; pulse width

pulse falling time T_f=300ns; pulse application

15 -3 voltage V_p=-5kV; plasma density n_o=2.4xlθ #/m ; and vacuum degree P=2xlO

Torr . <60> Fig. 10(a) is a diagram illustrating the range in which the movement of ions is simulated by the computer simulation. The range is 100mm in length and 60mm in height. Fig. 10(b) illustrates a pulse restoration model according to the restoration time of a pulse used in the computer simulation. The encircled numerals represent the timings for the capture of moving images in the computer simulation and correspond respectively to the pictures illustrated in Fig. 10(c).

<6i> In the pictures continuously illustrated in Fig. 10(c), the X-axis denotes one-dimensional coordinates with respect to the grid electrode, and the Y-axis stands for the speed of ions. The small dots appearing in the respective pictures are individual ions used in the computer simulation, each of the ions having the energy corresponding to its position. The results of simulation clearly show that the ions are trapped around the grid electrode (line a in Fig. 10(c)) by application of pulses and further that the ions released from the trapped state at the end of pulse application are moved toward the left and right ion implantation objects (the surfaces lying

outside line b in Fig. 10(c)). The first picture shows a state that the ions are trapped around the grid electrode before the end of a flat section of the pulse applied. Since the first picture has a spatial voltage distribution as indicated by a dot line p in Fig. 9, the ions make reciprocating movement in such a pattern that the energy of the ions becomes greatest around the grid electrode and gets smaller as they are moved away from the grid electrode. The second through sixth pictures are taken during the time when the pulse applied is restored to the pre-application state. These pictures show that the high energy ions escaped from the trapped state move fast to the left and right and the low energy ions move slow to the left and right. As can be seen in these pictures, the ions positioned above and below the middle point of the Y-axis move faster but the ions positioned at the middle point move slower. Until the pulse applied is restored to the original state, the ions are trapped in the grid electrode and reciprocatingly moved to the left and right in the shape of a rhombus. Immediately upon starting restoration of the voltage applied, the ions are escaped from the trapped state and moved farther to the left and right, as a result of which the distribution of the ions is progressively changed from a rhombus shape to a parallelogram shape.

<62> Fig. 11 is a graph representing the results of computer simulation performed with many different restoration time periods (pulse falling time) under the above-noted plasma simulation conditions, in which graph the number of ions reaching the ion implantation object (the Y-axis) during the voltage restoration time of 200ns to 2,000ns is plotted against the energy (the X- axis) thereof. It can be confirmed in this graph that, as the pulse falling time becomes shorter, the greater number of ions with higher energy reach the ion implantation object (target). Quantitative analysis of the maximum energy under the given plasma simulation conditions reveals that the maximum ion energy at the pulse falling time of about 300ns amounts to about 40% of the voltage applied, while the maximum ion energy in case of applying a high speed pulse with a pulse falling time of 100ns amounts to about 65% of the voltage applied. In case of applying the high speed pulse with a pulse falling time of 100ns, the ions impinge against the target with the energy of about 35keV under an operation condition that the pulse voltage is set equal to -5OkV, thereby performing ion implantation. If the pulse falling time is equal to 100ns, the maximum energy of ions reaching the target during application of a pulse voltage of 5kV is evenly distributed up to about 3keV. In case of a slow pulse having a pulse falling time of 2,000ns (the uppermost graph in Fig. 11), the ions trapped in the grid electrode by application of a pulse voltage of 5kV has the maximum energy of no more than 70OeV in conformity with the slow falling of the pulse voltage, which means that it is substantially impossible to perform ion implantation. In view of the results of computer simulation noted above, it is preferred that the pulse falling time required in restoring the negative voltage applied to the grid electrode to the original state is set equal to or smaller than 2 microseconds {^^JS) in order for the trapped ions to have a speed high enough to perform ion implantation.

<63> In another aspect, the present invention provides a plasma ion implantation apparatus free from generation of sparks which would otherwise cause damage to the surface of an ion implantation object.

<64> The present ion implantation apparatus includes a chamber for receiving an object, a conductive grid electrode spaced apart a specified distance from the surface of the object and a voltage application means for applying a negative voltage to the grid electrode. The grid electrode is arranged to ensure that the object surface lies outside the ion matrix sheath formed around the grid electrode by application of the negative voltage thereto. The voltage application means is designed to apply the negative voltage to the grid electrode so that ions can be trapped and reciprocatingly moved within the ion matrix sheath formed around the grid electrode. Furthermore, the voltage application means serves to restore the negative voltage applied to the grid electrode to its original state so that the ions can be released from the trapped state to make rectilinear movement. Thus, the ions impinge against the object surface for implantation into the same. In order to assure effective ion implantation, it is preferable for the voltage application means to make the pulse falling time as short as possible. It is also preferred that the pulse falling time is equal to or smaller than ten times of the ion plasma vibration period or equal to or smaller than two microseconds (^).

<65> In the present plasma ion implantation apparatus, it is preferred that the grid electrode includes a frame and a plurality of parallel rods arranged at an equal interval within the frame and further that the interval between the rods is set no greater than twice the thickness of the ion matrix sheath formed by application of the negative voltage to the grid electrode.

<66> The present plasma ion implantation apparatus may further include an unwinder for continuously supplying a rolled polymer film into the chamber so that ions can be implanted into the surface of the polymer film as an object and a winder for continuously recovering the polymer film discharged from the chamber after ions have been implanted into the surface of the polymer film. The voltage application means is designed to apply the negative voltage to the grid electrode at a predetermined time interval and to rapidly restore (reduce) the negative voltage applied to the grid electrode. Preferably, the present plasma ion implantation apparatus may further include a pair of vacuum-keeping means respectively arranged between the unwinder and the chamber and between the winder and the chamber for keeping the chamber in a vacuum state while allowing the polymer film to be supplied into and discharged out of the chamber. The pair of vacuum-keeping means is preferably formed of a low-vacuum leaf seal and a high-vacuum leaf seal.

<67> Preferably, the present plasma ion implantation apparatus may further include a support means for keeping the object spaced apart a predetermined distance from the grid electrode. The support means may be installed either inside or outside the chamber. In case of implanting ions into a polymer film, the support means may preferably include a cooling bed that makes contact with the polymer film to cool the same. Preferably, the cooling bed may be in the shape of a rotatably installed cylinder (or drum or roller) for cooling and guiding the polymer film continuously supplied. In this case, the grid electrode is bend into a cylindrical shape so that it can be spaced apart a predetermined distance from the outer circumferential surface of the cylindrical cooling bed.

<68> Fig. 12 is a schematic view showing a plasma ion implantation apparatus in accordance with one embodiment of the present invention. The plasma ion implantation apparatus 100 of the present invention is an apparatus for continuously implanting ions into the surface of a conductive film. The plasma ion implantation apparatus 100 includes a chamber 110 that provides a vacuum atmosphere needed in a plasma ion implantation process, a vacuum pump 120 for creating vacuum within the chamber 110 by exhausting a gas from the latter, a gas supply device 130 for supplying a plasma generation gas into the chamber 110, a plasma generation means 150 for creating a plasma atmosphere within the chamber 110, a table 200 for making contact with the continuously supplied polymer film to guide and support the same and for cooling the polymer film heated by ion implantation, and a grid electrode 140 for forming electric fields to accelerate ions. The plasma generation means 150 is composed of a plasma electrode 152 for forming plasma and a plasma power source 155 for supplying an electric current to the plasma electrode 152. The plasma power source 155 has output power of 0 to 1OkW and serves to supply an electric current to the plasma electrode 152 within the chamber 110 through a feed-through 153. If the electric current is applied to the plasma electrode 152, the gas molecules existing around the plasma electrode 152 are ionized so that plasma in which positive ions and electrons coexist in an ionized state can be formed within the chamber UO. The gas supply device 130 is provided with a gas injection valve 131 through which a gas is supplied into the chamber 110. The gas supplied from the gas supply device 130 include, e.g., helium, argon, nitrogen, neon, krypton and xenon, one or more of which may be used independently or in combination. The plasma ion implantation apparatus of the present embodiment further includes a voltage application means 160 for applying a negative pulse voltage to the grid electrode 140.

<69> Referring to Fig. 14, the grid electrode 140 includes a rectangular frame 141 and a plurality of parallel electrode rods 142 arranged within the frame 141 at a regular interval. As mentioned earlier, the interval L between the respective electrode rods 142 is set equal to or less than twice of the thickness of the ion matrix sheath formed by application of the high voltage to the grid electrode 140. In the present embodiment, the polymer film 210 moving in contact with the table 200 is arranged to lie outside the ion matrix sheath formed by application of the high voltage to the grid electrode 140.

<70> The ion implantation apparatus 100 of the present embodiment further includes an unwinder 170 for continuously supplying a rolled polymer film 210 into the chamber 110 and a winder 180 for continuously recovering the polymer film 210 discharged from the chamber 110 after ions have been implanted into the surface of the polymer film 210. The ion implantation apparatus 100 further include a pair of vacuum-keeping means 190 respectively arranged between the unwinder 170 and the chamber 110 and between the winder 180 and the chamber 110 for keeping the chamber 110 in a vacuum state while allowing the polymer film 210 to be supplied into and discharged out of the chamber 110. The pair of vacuum-keeping means 190 is preferably formed of a low- vacuum leaf seal 191 and a high-vacuum leaf seal 192. Although not shown in the drawings, a cooling means for cooling the polymer film 210 heated by ion implantation is installed inside the table 200.

<7i> The ion implantation apparatus 100 of the present embodiment may further include a cleansing device for cleansing the polymer film 210 before it is supplied into the chamber 110 from the unwinder 170. Foreign materials such as dust, fingerprints, lipids and the like may adhere to the surface of the polymer film 210. These foreign materials may contaminate the internal space of the chamber 110 with foreign gases and consequently may generate sparks within the chamber 110, which may be a major culprit in impairing the stability of plasma. Therefore, there is a need to remove the foreign materials in advance with the cleansing device, thereby maintaining a stable plasma atmosphere within the chamber 110.

<72> In the present embodiment, the voltage application means 160 is designed to apply a negative voltage to the grid electrode 140 so that the ions can be trapped and reciprocatingly moved within the ion matrix sheath formed around the grid electrode 140. Furthermore, the voltage application means 160 serves to rapidly restore (or reduce) the negative voltage applied to the grid electrode 140 to its original state so that the ions can be released from the trapped state to make rectilinear movement. Thus, the ions impinge against the surface of the polymer film 210 for implantation into the same. In order for the ions to be trapped around the grid electrode 140 and to have a great enough speed for ion implantation, it is preferred that the voltage application means 160 keeps the pulse falling time as short as possible. Preferably, the pulse falling time is equal to or smaller than ten times of the ion plasma vibration period or equal to or smaller than two microseconds (^⁵O. The high voltage pulse applied to the grid electrode 140 is in a range of from 1OkV to 10OkV. It is preferred that the on-section pulse width of the high voltage pulse applied to the grid electrode 140 is in a range of from 1 to 1,000 microseconds, with the off-section pulse width being in a range of from 100 microseconds to 100 milliseconds.

<73> The molecules and atoms of the polymer film are mainly composed of carbon-bonds. Since the polymer film are formed of perfect bonds such as a single bond, a double bond and a triple bond, no movement of electrons occur even if electric fields are generated by the application of an electric current to the polymer film. In other words, the polymer film itself essentially has no electric conductivity. If the ions accelerated with a high level of energy are infiltrated into the surface of the polymer film and impinge against the molecules of the polymer film, the single bonds and the double bonds of the polymer film are broken. Therefore, a multiplicity of holes and free electrons are newly formed in the molecular bonds existing on the shallow surface of the polymer film. The free electrons generated in the surface of the polymer film are able to move with ease as the electric fields are formed by the application of an electric current. Thus, the surface of the polymer film is modified to have electric conductivity.

<74> Fig. 13 is a schematic view showing a plasma ion implantation apparatus in accordance with another embodiment of the present invention. The plasma ion implantation apparatus shown in Fig. 13 differs from the apparatus shown in Fig. 12 in that, in place of the planar table 200, a cylinder (drum or roller) 230 rotatably mounted to the chamber 110 is used as a support means and further that the grid electrode 240 is bent along the outer circumferential surface of the cylinder 230 and is spaced apart a predetermined distance from the outer circumferential surface of the cylinder 230. If the rotatably mounted cylinder 230 is used as a support means, it becomes possible reduce physical friction between the cylinder 230 and the polymer film 210, consequently reducing damage which would otherwise be caused to the polymer film 210. This also helps improve adhesion between the polymer film 210 and the cylinder 230, thereby making it possible to enhance the cooling effect .

<75> The embodiments set forth hereinabove have been presented for illustrative purpose only and, therefore, the present invention is not limited to these embodiments. It will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention defined in the claims. [Industrial Applicability]

<76> With the present invention, there is provided a novel method by which ions are trapped around a grid electrode and are burst so that they can be implanted into the surface of an ion implantation object. This makes it possible to implant plasma ions with no generation of sparks that may cause damage to the surface of the ion implantation object. Furthermore, the present invention provides an ion implantation apparatus that can realize the novel ion implantation method. The present ion implantation apparatus is capable of assuring mass production with no likelihood of causing damage to an object surface and, therefore, is useful in a surface treatment for imparting electric conductivity to a polymer film. In addition, the present ion implantation apparatus is not limited to a particular treatment object and is advantageous in that it has an ability to selectively treat a desired portion (a surface) of a treatment object while keeping the chemical and physical properties of the object intact.

<77> Moreover, the present ion implantation method is readily controllable because it employs high voltage pulse technology. The present ion implantation method imparts electric conductivity to a treatment object by combining or breaking the molecular structures thereof. Therefore, the present ion implantation method does not produce particles or other contaminants that may impair semiconductors, integrated electronic devices and equipments. Additionally, there is provided an advantageous effect that an operator can enjoy a clean environment.

Claims

[CLAIMS] [Claim 1] <79> A plasma ion implantation method for implanting ions into a surface of an object, comprising the steps of: <80> positioning the object and a conductive grid electrode within a vacuum chamber, the object having an ion implantation surface spaced apart a predetermined distance from the grid electrode! <8i> forming plasma around the object and the grid electrode received within the vacuum chamber; <82> applying a negative voltage to the grid electrode so that plasma ions can make reciprocating movement within an ion matrix sheath formed around the grid electrode; and <83> restoring the negative voltage applied to the grid electrode to an original state in such a way that some of the ions are recti linearly moved to impinge against- the surface of the object for implantation into the surface of the object .

[Claim 2] <84> The method as recited in claim 1, wherein the time period from the beginning to the end of restoration of the negative voltage in the restoring step is equal to or smaller than ten times of an ion plasma vibration period

[Claim 3]

<85> The method as recited in claim 2, wherein the applying step and the restoring step are repeatedly performed at a predetermined time interval.

[Claim 4]

<86> The method as recited in claim 3, wherein the time period from application to restoration of the negative voltage is shorter than the time period from restoration to application of the negative voltage.

[Claim 5] <87> The method as recited in claim 4, wherein the ratio of a negative voltage applying time period to a negative voltage restoring time period is in a range of from 1:10 to 1:10,000.

[Claim 6] <88> The method as recited in claim 5, wherein the ratio of the time period from the beginning of application to the beginning of restoration of the negative voltage to the time period from the beginning to the end of restoration of the negative voltage is in a range of from 5:1 to 10,000:1.

[Claim 7] <89> The method as recited in any one of claims 1 to 6, wherein the object is selected from the group consisting of a metal body, an alloy body, a semiconductor, a ceramic body and a polymer body.

[Claim 8] <90> The method as recited in any one of claims 3 to 6, wherein the object comprises a polymer film provided to continuously move through the vacuum chamber, the grid electrode being arranged parallel to one major surface of the polymer fi Im.

[Claim 9] <9i> The method as recited in any one of claims 1 to 6, wherein the grid electrode comprises a frame and a plurality of parallel rods arranged within the frame at a regular interval, the interval between the rods being equal to or smaller than two times of the thickness of the ion matrix sheath.

[Claim 10] <92> The method as recited in claim 1, wherein the time period from the beginning to the end of restoration of the negative voltage in the restoring step is equal to or smaller than two microseconds.

[Claim 11]

<93> A plasma ion implantation apparatus comprising: <94> a chamber for receiving an object with a surface; <95> a conductive grid electrode spaced apart a predetermined distance from the surface of the object; and <96> a voltage application means for applying a negative voltage to the grid electrode,

<97> wherein the grid electrode is arranged to ensure that the surface of the object lies outside an ion matrix sheath formed by the negative voltage applied to the grid electrode,

<98> and wherein the voltage application means is designed to apply the negative voltage to the grid electrode in such a way that ions are trapped and reciprocatingly moved within the ion matrix sheath and to restore the negative voltage applied to the grid electrode in such a way that the ions trapped are released and recti linearIy moved to impinge against the surface of the object for implantation into the surface of the object.

[Claim 12]

<99> The apparatus as recited in claim 11, wherein the voltage application means is designed to restore the negative voltage so that the time period from the beginning to the end of restoration of the negative voltage can be equal to

or smaller than ten times of an ion plasma vibration period (T_pi=l/ ^p').

[Claim 13]

<ioo> The apparatus as recited in claim 12, wherein the voltage application means is designed to apply and restore the negative voltage at a predetermined time interval. [Claim 14]

<ioi> The apparatus as recited in claim 13, wherein the object comprises a polymer film and further comprising an unwinder for continuously supplying the polymer film into the chamber and a winder for continuously recovering the polymer film discharged from the chamber after the ions are implanted into the polymer fi Im. [Claim 15]

<iO2> The apparatus as recited in claim 14, further comprising a pair of vacuum- keeping means respectively arranged between the unwinder and the chamber and between the winder and the chamber for keeping the chamber in a vacuum state while allowing the polymer film to be supplied into and discharged out of the chamber . [Claim 16]

<iO3> The apparatus as recited in claim 15, wherein the pair of vacuum-keeping means comprises a low-vacuum leaf seal and a high-vacuum leaf seal. [Claim 17]

<iO4> The apparatus as recited in any one of claims 14 to 16, further comprising a support means for supporting the polymer film, the support means having a contact surface for making contact with one major surface of the polymer film to support and guide the polymer film, the support means comprising a cooling means for cooling the contact surface. [Claim 18]

<!05> The apparatus as recited in any one of claims 14 to 16, further comprising a support means for supporting the polymer film, the support means comprising a rotatably installed cylinder with an outer circumferential surface with which the polymer film makes contact, the grid electrode being bent along the outer circumferential surface of the cylinder and spaced apart a predetermined distance from the outer circumferential surface of the cylinder. [Claim 19]

<iO6> The apparatus as recited in any one of claims 11 to 16, wherein the grid electrode comprises a frame and a plurality of parallel rods arranged within the frame at a regular interval, the interval between the rods being equal to or smaller than two times of the thickness of the ion matrix sheath. [Claim 20]

<1O7> The method as recited in claim 11, wherein the time period from the beginning to the end of restoration of the negative voltage is equal to or smaller than two microseconds.