GB2269592A - Method for controlling the surface resistivity of plastic articles - Google Patents

Abstract

A process for modifying the surface of plastics comprises incorporating a polypyrrole thereon. The process changes the levels of surface resistivity of plastic articles such as polysulphone which are used in the manufacture of electrostatic dissipative device carriers for the semiconducting industry. A carrier is exposed to a solution of water and pyrrole, and subsequently polymerized and oxidized by exposure to a chloride solution. A method of producing an enhanced solution of pyrrole in water by adding water to an alcohol/pyrrole mixture and evaporating the alcohol is also claimed.

Description

METHOD FOR CONTROLLING THE SURFACE RESISTIVITY OF PLASTIC ARTICLES The present invention relates to methods for achieving control of the surface resistivity of plastic articles and in particular articles manufactured from the polysulphone family of plastics, in order to render the surface sufficiently conductive to enable electrostatic charge dissipation thereby, and in particular relates to such articles for use in the manufacture of devices having a susceptibility to electrostatic damage (ESD).

In the semiconductor manufacturing industry, it is commonly known that certain components or devices have a susceptibility to damage where levels of static charge which build up on objects in close proximity thereto are sufficient to discharge across, for example, thin dielectric films and junctions incorporated within the components. Such discharges can cause breakdown of these films and junctions and result in permanent damage.

ESD is typically a problem throughout the semiconductor manufacturing process, and extensive effort has been made in providing antistatic carriers to convey the devices through the manufacturing processes and during testing and shipment thereafter. Throughout the present specification, the term "carrier" will be used to denote all articles to which it is desirable to render antistatic or static dissipative properties, and can include jigs, cassettes, boxes and the like. At present, the antistatic qualities of such carriers can be unreliable.

The invention described herein is not however, restricted only to such articles used in the manufacture of semiconductor devices, but is equally applicable to any other technical field where resistivity controlled plastic articles may be required.

The current commonly-used technique is to provide a carbon particulate filler material or a graphite stranded filler material incorporated into the plastic of the carrier. In these current techniques the dielectric strength of the plastic is compromised by using the graphite fibres. If the graphite fibres are very closely spaced, the dielectric strength exhibited by the reduced material width therebetween is lowered to a point where breakdown can easily occur, and by this action, any charge is conducted, thus avoiding a build-up of local charge.

However, this is a breakdown effect, and the voltage required to conduct across the dielectric is directly proportional to the thickness of material separating the graphite strands. The separation and distribution of the strands of graphite fibre are not easily controlled, being the result of rheological effects within the moulding process. Areas where fibres acquire a more distant spacing maintain a higher degree of dielectric strength with the consequent effect of supporting a higher local charge.

Such a charge may cause damage to any devices in close proximity.

It has been observed that some areas of "clumping of filler material occur randomly throughout the body of carriers manufactured from such material. These "clumps" form low resistance leakage paths. Areas exhibiting a desirable resistivity occur randomly over the surface of the material: very little has been observed in terms of uniformity. In areas which do exhibit a desired resistivity, a second order effect is observed. This is the degradation of, for example, the polyethersulphone dielectric as a result of the energy associated with its breakdown. This degradation can result in a low resistance path being permanently established.

The present invention provides a material suitable for use in articles such as semiconductor device carriers which need to exhibit reliable and controllable static dissipative properties.

In a further form the present invention provides a process for the modification of plastic materials from which articles such as semiconductor device carriers may be made, which can thereby result in a reliable and controllable static dissipative surface to the plastic material.

The invention also provides a semiconductor device carrier made from plastics material, the surface of the carrier being modified to provide electrostatic damage protection.

The present invention also provides a method of producing an enhanced level of pyrrole in solution.

Embodiments of the present invention will now be described by way of example, and with reference to the accompanying drawings in which: Figure 1 shows diagrammatically a perspective view of a carrier suitable for use for the transportation of ESD susceptible semiconductor devices during manufacturing and shipping thereof; Figure 2 shows schematically a sequence diagram illustrating a possible method for the production of a pyrrole mixture; and Figure 3 shows schematically a sequence diagram illustrating a possible method for the production of a carrier with enhanced ESD properties in accordance with the present invention.

With reference to the drawings, in a specific example the present invention provides a process for the rendering of specific levels of surface conductivity to the polysulphone family of plastics for the purpose of manufacturing electrostatic dissipative device carriers for the semiconductor manufacturing industry.

A typical carrier 10 is shown diagrammatically in Figure 1. The carrier is subjected to a process using 1 AZA-24-cyclopentadiene (pyrrole) whereby the low surface conductivity 10.16 mho normally encountered in the plastics polyether-polyaryl-sulphone is permanently increased to a level suitable for ESD purposes, for example 10-9 to 10.10 mho as required within the semiconductor assembly and test operations, by alteration of the conductivity of the surface of the polysulphone plastic.

An exemplary manufacturing process will now be described for rendering the surface of polysulphone family plastics static dissipative including a practical method of diffusing pyrrole into the surface of polysulphone plastic without the solvent effect degrading the mechanical integrity or altering the dimensions of the host material.

The process is based of the ability to form conductive polymers from 1-AZA-Z4-cyclopentadiene pyrrole. This process renders surfaces conductive to between 10-9 and 10-1 mho for the specific purpose of static charge dissipation and is particularly applicable to CMOS "open lead" package carriers as shown in Figure 1. However, the technique can be extended to allow any carrier or production handling tool made from the polysulphone family plastics to be static dissipative.

The method herein described is also applicable to solvents other than pyrrole which exhibit suitable plastic solvent properties in combination with the requisite mechanical properties and which can subsequently be polymerized, having free electrons when in the polymerized state, for example, polyacetylene, polythiophene or polyaniline.

A preferred example of the basic process will now be described with reference to Figures 2 and 3.

Polysulphone carriers 10 are firstly cleaned and dried. Newly moulded carriers can be considered thus since the presence of release agents does not appear to cause a detrimental effect to the process. In terms of dryness the normal condition of a typical carrier represents an allowable moisture uptake of between 0.22% to 0.57%.

A pyrrole water solution is prepared by heating water to 20C (12), adding methanol (14) and Pyrrole (16) to give a mixture of 90% water, 5% methanol and 5% Mono Pyrrole (18). The mixture is stirred and is heated to between 35 to 60C (20) to evaporate the methanol, the time for this stage being dependent on the temperature, which will determine the evaporation rate of the methanol.

When the methanol has evaporated, a hot solution of water and 5% pyrrole is obtained (22). The clean and dry carrier 10 is then immersed in the solution for approximately 5 minutes at a temperature of 55C (24).

The carrier 10 is then removed from the pyrrole water solution and immersed in a bath of methanol (26). For intricate parts the bath will be of the ultrasonic type.

The cleaning time (26) to remove surface pyrrole (pyrrole having been absorbed below the surface) is not critical.

The carrier 10 is then subjected to a water wash (28) the time and temperature of which stage is not critical.

Polymerisation of the pyrrole and its oxidation is achieved immediately after the wash process by immersing the carrier 10 in a bath containing a 50% solution of copper chloride in water at 40C for 15 minutes (30). The time of this stage is not critical.

A further washing stage (32) follows in which the carrier is washed in water and the now treated carrier 100 is then dried in air (34) and stored for future use.

In the exemplary described process, the pyrrole diffuses into the surface of the polysulphone carrier 10 to a depth of 3 to 5 microns and is polymerised and oxidised by exposure to the copper chloride solution, the depth having been verified by micrograph sections and measurement.

The exemplary process is the preferred process for CMOS chip carriers giving the required surface conductivity in a process which is both cost and time efficient. The basic process of diffusing pyrrole into the surface of a polysulphone plastics article can however be carried out using a similar process but with different parameters as described hereinafter.

For the preferred process a concentration of 5% pyrrole is required, and this is obtained as described using a mixture of methanol but other alcohols could be used. The importance in respect of an efficient process having a higher concentration of pyrrole is now explained.

The concentration at which pyrrole forms a soluble solution in water at room temperature (+20C) is low -approximately 1.5%. This concentration can be increased if the kinetics of the solution can be increased -- this means that if the temperature of both the water and the pyrrole were increased the higher levels of movement of both molecules would allow them to exist together at much lower ratios. This increased solution "strength" is required for the process to work in an efficient manner. However, the kinetics of the solution also plays an important part in the process. The degree to which the pyrrole molecules diffuse into the surface of the polysulphone (up to a limit of about 3 to 5 microns) is a function of concentration and kinetics.If the concentration of the pyrrole is too high, its solvent action will result in a very heavily pyrrole enriched layer of polysulphone, whose mechanical attachment to the main body of the plastic would be questionable and whose electrical conductivity would be too high. A lower concentration will result in the correct enrichment but the layer thickness for a given level of final conductivity will be very small. This very thin layer will also have an abrupt interface with the body plastic and its mechanical adhesions would be poor. To get the correct levels of surface enrichment and depth of diffusion without the surface of the plastic suffering any dimensional degradation the maximum level of concentration of pyrrole was experimentally established at approximately 7.0%.

This level of approximately 7.0% is not, however, obtainable in a solution with water in a straightforward manner as now explained.

If pyrrole is simply introduced into water with the water temperature raised to +55C, the pyrrole molecules will be in very close proximity to one another on entry, and as they become more energetic will react and dimerise leaving only a few mono-pyrrole molecules available. If the pyrrole is added a few percent at a time as the water temperature is increased, more or less the same thing will happen with the final quantities of pyrrole as they are added. Again, the real concentration of reactive monopyrrole will be lower than needed. One could add more than the required levels to compensate but this would be haphazard.

In the present invention, a solvent that has physical properties close to that of both water and hydrocarbons is used by which the ability to dissolve a higher concentration of pyrrole can be achieved.

-A simple alcohol was selected. This forms a solution with water or pyrrole at any ratio required and allows the adjustment of the solubility of one with the other. This allows the mixture of pyrrole in water at the concentration of approximately 7.0% at 20C, while keeping the pyrrole molecules well distributed in the water alcohol mix until the solution temperature is increased to a level where the kinetics will hold the two main components evenly dispersed thus avoiding a situation similar to above.

The preferred alcohol is methanol because its boiling point of +65C means that at the +35 to +60C range of the preferred process the methanol evaporates rapidly and leaves the water pyrrole mix. Other alcohols could be used but this point of the rate of evaporation must be taken into account. If, for example ethanol was used, the boiling point would be raised slightly to +78C and this would have the effect of slowing down the evaporation rate.

This would not make a great difference to a practical process, so methanol could technically be replaced with ethanol. However, if one were to go up the scale a little more and use say, 1-propanol (+97C) the evaporation would take place almost at the same rate as the water/pyrrole mix at the process temperature. Thus in practical terms methanol or ethanol could be selected and so could 2propanol or isopropyl alcohol (just) at +82C but the border line would be at 1-propanol or propyl alcohol.

With reference to the required concentration of pyrrole, the effective working range of concentration to achieve a change in the surface conductivity of polysulphone has been found to be from 0.78 to 7.0% above which concentration the action of the pyrrole becomes detrimental to the surface of the plastic. In the lower levels of concentration below 2.5t however, the change in surface conductivity is not sufficient to achieve the required conductivity for ESD protection. Thus for ESD protection of carriers a solution of between 2.5 to 7.08 is a preferred range within which the required surface conductivity can be achieved without any substantial damage to the surface of the polysulphone. The preferred process in stage 24 uses 5% at a temperature of +55C and a time of 5 minutes, the time and temperature being selected for a best cost process. Temperatures from +20C to +75C may be used in stage 24, the time of the process being generally inversely proportional to the temperature.

It will be recognized that although the present invention has been described using total immersion of the carriers in the requisite solutions, it is possible to treat only those portions of the carriers or surfaces thereof which specifically require static dissipative properties. This may be done by, for example, masking techniques.

A particular advantage of the present invention is that it allows control of the surface conductivity of the carriers. This has particular utility in that by arranging for the resulting resistivity to be, for example, in the range of 108 to 10ill, it is possible to facilitate the insitu testing of circuits contained within such carriers without triggering leakage detection circuits in the testing equipment.

Although the present invention has been described specifically in the context of carriers used in integrated circuit packaging, it will be recognized that the invention has applicability to any environment in which protection of articles from a build-up of static charge is required.

With reference to stage 30 the strength of the copper chloride solution should preferably not be lower than 50%.

Above this is acceptable, however, a stronger solution has been found to make little or no difference in the process time since the reaction is rapid. The temperature range of stage 30 may be from 35C to 60C: increasing the temperature has been found not to significantly accelerate the process.

In stage 30, ferric chloride can be used but the results are found not as stable over the long term. The process for ferric chloride is the same as copper chloride.

With reference to stage 32, no particular washing techniques are needed after the copper chloride step. A water spray or immersion is acceptable but if immersion is used the bath must be monitored for copper chloride build up. About 3 to 5 minutes is an acceptable time for stage 32.

The process can be applied to other plastics but extreme care needs to be taken if degradation of the surface of the plastic is to be avoided. The process has been applied to polyimide and polyamide plastics but in both cases substantial surface damage was observed to achieve the required level of surface conductivity for ESD protection. However, should surface degradation be acceptable, then this process could be applied to other plastics but these would not be acceptable as carriers if surface degradation substantially distorted the dimensions of the carriers.

Claims (20)

1. A method of surface modifying plastic materials, said method rendering the plastic material sufficiently conductive to effect electrostatic charge dissipation, said method including the steps of: a) immersion of said plastic material in a solution of pyrrole; b) subjecting said plastic material to a first washing process; c) polymerization of said plastic material by immersion in a chloride solution; d) subjecting said plastic material to a second washing process and e) drying of said plastic material.

2. A method as claimed in claim 1 wherein said solution of pyrrole is between 0.78 to 7.0% pyrrole in water.

3. A method as claimed in claim 1 wherein said solution of pyrrole is between 2.5 and 7.0t pyrrole in water.

4. A method as claimed in claim 1 wherein said solution of pyrrole is 5% in water.

5. A method as claimed in claim 1 wherein the solution of pyrrole is at a temperature between 20 to 75C.

6. A method as claimed in claim 1 wherein the solution of pyrrole is at a temperature of 55 to 600.

7. A method as claimed in claim 1 wherein said chloride solution is greater than 50% copper chloride in water.

8. A method as claimed in claim 1 wherein said chloride solution is at a temperature between 35 to 60C.

9. A method as claimed in claim 1 wherein the plastic material is polysulphone.

10. A method as claimed in claim 1 wherein said first washing process comprises a two stage process a first stage comprising cleaning said plastic material in methanol and a second stage comprising a water wash stage.

11. A method as claimed in claim 9 wherein said first stage comprises an ultrasonic cleaning stage.

12. A method as claimed in claim 1 wherein said solution of pyrrole is produced by the steps of: a) adding to water at a first low temperature a first percentage of pyrrole and a first percentage of alcohol to form a mixture; b) heating said mixture to a second higher temperature to evaporate said alcohol and to thereby produce said solution comprising pyrrole and water.

13. A method as claimed in claim 11 wherein said alcohol comprises methanol.

14. A method as claimed in claim 11 wherein said second higher temperature is between 35 to 60C.

15. A method as claimed in claim 11 wherein said first and second percentages are 5%.

16. A method as described in claim 1 wherein said plastics material comprises a semiconductor device carrier and in which the surface resistivity of the material of the carrier following said process is between 10-9 and 100 mho.

17. An electrostatic dissipative device carrier comprising a shaped carrier formed of a polysulphone plastic material and having at least a portion of the surface of said shaped carrier modified by polymerized pyrrole incorporated thereon.

18. A method of modifying the surface conductivity of a semiconductor device carrier comprising the steps of: a) producing a solution of pyrrole and water by i) mixing together a mixture of water, pyrrole and alcohol to form a first solution, ii) heating said first solution at a temperature to evaporate the alcohol to form a second solution comprising pyrrole and water, b) immersing said semiconductor device carrier in said second solution; c) subjecting said semiconductor device carrier to a first washing process to remove any excess pyrrole solution; d) immersing said semiconductor device carrier in a second solution comprising a chloride solution; e) subjecting said semiconductor device carrier to a second washing process to remove excess chloride solution and f) drying said semiconductor device carrier.

19. A method of producing an enhanced solution of pyrrole in water and method comprising the steps of a) adding to water at a first low temperature a first percentage of pyrrole and a first percentage of alcohol to form a mixture; b) heating said mixture to a second higher temperature to evaporate said alcohol and to thereby produce said solution comprising pyrrole and water.

20. A method of surface modifying plastic materials, said method rendering the plastic material sufficiently conductive to effect electrostatic charge dissipation, said method including the steps of: a) immersion of said plastic material in a solution of polymerizable solvent; b) subjecting said plastic material to a first washing process; c) polymerization of said plastic material to form a conducting polymer on the surface thereof; d) subjecting said plastic material to a second washing process and e) drying of said plastic material.