WO2000010195A2

WO2000010195A2 - Preparation of metal-precipitates permeable insulator for soi substrate

Info

Publication number: WO2000010195A2
Application number: PCT/US1999/018085
Authority: WO
Inventors: Robert J. Falster; Robert A. Craven
Original assignee: Memc Electronic Materials, Inc.
Priority date: 1998-08-10
Filing date: 1999-08-10
Publication date: 2000-02-24
Also published as: AU5474299A; WO2000010195A3

Abstract

A process for inhibiting the formation of metal-precipitate defects in an SOI wafer is disclosed. The process includes heating the wafer for a time sufficient to reduce metal concentration in the monocrystalline film through diffusion into the bulk of the wafer. The insulator structure is made permeable via holes, channels, or streets spaced at a predetermined distance (L) apart to allow the diffusion of metals to occur.

Description

PROCESS FOR PREPARATION OF SILICON ON INSULATOR SUBSTRATES WITH IMPROVED RESISTANCE TO FORMATION OF METAL PRECIPITATES

BACKGROUND OF THE INVENTION The present invention relates to a process for the preparation of silicon on insulator substrates having a reduced amount of metal precipitate defects. The present invention also relates to a process for preparing silicon on insulator substrates wherein the insulating layer is permeable and metals in the semiconductor film diffuse into the bulk of the structure upon heating.

An SOI bonded wafer has a handle wafer, a device layer, and an insulating film, (typically an oxide layer) between the handle wafer and the device layer. Typically the device layer is between 0.5 and 20 micrometers thick. Various techniques such as SIMOX (See U.S. Patent No. 5,436,175 and Plasma Immersion Ion Implantation For Semiconductor Processing, Materials Chemistry and Physics 46 (1996) 132-139) or BESOI (See U.S. Patent No. 5,189,500) may be used to fabricate SOI substrates.

Metal precipitation defects are a critical technological problem in SOI technology. For example, it is currently believed by some that HF Defects are a form of metal precipitation defects. It is generally believed that the metal precipitation defects are etching artifacts resulting from the precipitation of transition metals

(primarily copper and nickel) in the form of small metal silicide particles on either of both of the surfaces of the thin silicon film following heat treatments of the structure. During heat treatments of SOI wafers, the precipitation of transition metals occurs if sufficiently high concentrations of theses metals are dissolved into the wafer during the heat treatment. The major source of these unintentional metals is residual amounts of surface contamination following cleaning and handling of the samples prior to heat^" treatment. Also, metals can be transported through the gas phase to the sample from the surrounding ambient of the sample in the furnace used to heat treat it. These metals can be dissolved into the sample up to a concentration equal to the solid solubility of the metal at the temperature of the heat treatment. Upon the subsequent cooling of the SOI substrate, the dissolved metals can precipitate in sensitive regions of the substrate and cause a decrease in device performance . The present invention discloses a process for reducing the amount of metal precipitate defects in an SOI substrate. If HF Defects are, in fact, metal precipitation defects, or related to metal precipitation defects, this invention discloses a process for reducing the amount of HF Defects in an SOI substrate. SUMMARY OF THE INVENTION

Among the objects of the present invention, therefore, are the provision of a process for producing SOI structures with reduced metal precipitate defects; the provision of a process for producing SOI structures that are less susceptible to the formation of metal precipitate defects during device fabrication; the provision of a process for producing an SOI structure having a permeable insulating layer; and the provision of a process for producing SOI structures having metals equally dispersed throughout the structure .

Briefly, therefore, the present invention is directed to a process for reducing the amount of metal precipitate defects in an SOI substrate having a permeable insulating layer on top of a semiconductor substrate with a monocrystalline film on top of the insulating layer. The process comprises heating the SOI substrate for a time sufficient to reduce the metal concentration in a monocrystalline semiconductor film by diffusion.

The invention is further directed to an SOI structure having a handle wafer, a monocrystalline film, and an insulating layer between the handle wafer and the monocrystalline film. The insulating layer is permeable and allows for the diffusion of metals.

The invention is further directed to a process for producing an SOI structure. The process comprises first depositing a silicon nitride layer onto a surface of a handle wafer, and subsequently etching windows into the silicon nitride layer. After etching, the surface of the handle wafer is oxidized to form an oxide layer. Next, the etched silicon nitride windows and oxide layer are removed from the surface of the handle wafer by polishing the surface, and a device wafer is bonded onto the polished handle wafer surface. Finally, the device layer is selectively removed to form a thin monocrystalline film.

The invention is further directed to a process for producing an SOI structure. The process comprises applying a masking layer onto a monocrystalline film of an SOI substrate. Etching windows are subsequently etched into the masking layer, and ions are implanted through the etched windows through the monocrystalline film into the insulating layer. Finally, the masking layer is removed^" from the monocrystalline film.

The invention is further directed to a process for producing an SOI structure. The process comprises applying a masking layer onto an insulating layer positioned on a handle wafer. Windows are etched into the masking layer, and ions are implanted through the etched windows into the insulating layer. The masking layer is removed from the insulating layer, and a device wafer is bonded onto the insulating layer. Finally, the device wafer is selectively removed to form a thin monocrystalline layer.

The invention is further directed to an SOI structure for use in a device manufacturing process wherein the substrate is resistant to the formation of metal precipitates during the device manufacturing process. The structure is comprised of a handle wafer and a monocrystalline film with a permeable insulating layer which allows for the diffusion of metals between the wafer and the film.

Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of the build up of a chemical driving force during cooling in two cases with different metal concentrations.

Fig. 2 is a schematic diagram of a film on a substrate thickness relationship to film and thickness concentration. Figs. 3-5 are schematic diagrams of various methods for producing SOI structures of the present invention.

Fig. 6 is a schematic diagram of the structure of the SOI diffusion simulations. Fig. 7 is a graph of the time development of impurity concentration for varying distances between hole centers.

Fig. 8 is a graph of the time needed for the metal concentration to decrease by a factor of 10^~2 for varying hole spacings. Fig. 9 is a graph of the time development of impurity concentration for varying hole diameters.

Fig. 10 is a graph of the time needed for the metal concentration to decrease by a factor of 10^"2 for varying hole diameters. Fig. 11 is a graph of the time development of impurity concentration for different diffusion coefficients.

Fig. 12 is a graph of the time needed for the metal concentration to decrease by a factor of 10^~2 for varying diffusion coefficients. Fig. 13 is a graph of the time needed for the metal concentration to decrease by a factor of 10^~2 for different surface layer thicknesses.

Fig. 14 is a diagram of a configuration of holes on an SOI substrate. Fig. 15 is a diagram of a configuration of stripes on an SOI substrate.

Fig. 16 is a diagram of a configuration of streets on an SOI substrate.

Figs. 17-19 are graphs of time at temperature (s) vs. metal concentration for various parameters showing the effect of holes, stripes, and streets. Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been discovered that the probability of metal precipitates forming during a heat treatment of an SOI substrate is controlled primarily by the local concentration of the metal in solution present during the cooling of the sample. Surprisingly, it has been shown that the strong driving force for the precipitation which exists in the thin semiconductor layer of an SOI substrate can be reduced and precipitation inhibited if the full dilution potential of the entire substrate is accessed. This result can be achieved if the insulating layer is made to be permeable to metal diffusion. Additionally, SOI structures can be produced which contain a reduced amount of metal precipitates, and have a higher resistance to the formation of metal precipitate defects during device fabrication.

The solubility, C_sol (T) , of metals in silicon is generally an increasing function of temperature. Due to the extremely high diffusivity of transition metals such as copper and nickel in silicon, an equilibration of metal concentration is rapidly reached throughout the thickness of the semiconductor samples in most heat treatments . The equilibrium concentration, C_bulk, in units of cm^"3 (neglecting any metal transport from the gas phase) of metals in a silicon sample of thickness t_s, during a heat treatment at temperature T_ht, will be equal to the surface concentration, C_s (cm^-2) of the metal on the surface prior to the heat treatment divided by the sample thickness multiplied by a factor of 2 , up to a maximum concentration equal to the equilibrium concentration of the metal at T_ht, as shown in (1) and (2) :

C_buiκ = 2 (C_β/t_β) For 2 (C_s/t_s) <C_gol (T_ht) ( 1 )

C_bulk = C_βol (T_ht ) For 2 (C_s/t_s) >C_sol (T_ht) (2 )

The factor of 2 in equations (1) and (2) arises from the fact that a sample has two surface, and it is assumed that the amount of residual metal on both surfaces is equal . Upon cooling of the sample from the heat treatment temperature T_ht, the solubility concentration of the metal in silicon decreases according to the functional dependence of C_gol with temperature, C_sol (T) for the metal in question. At some temperature, T_ss, between the heat treatment temperature and room temperature, the metal solution will become supersaturated. This supersaturation occurs at the temperature T_ss at which C_aol(T) exceeds C_bulk. As the temperature is further reduced from T_gs, a positive chemical driving force for metal precipitation is built up continuously according to (3) :

ΔG = kTln{C_bulk/C_sol(T)} (3)

There exists a critical energy for the production of metal suicide precipitates as silicon surfaces, ΔG_crit. If a critical amount of supersaturation in the metal-silicon system is reached during the cooling of the sample, a surface precipitation event will occur at a temperature, T_crit, which satisfies the relation

ΔG_crit=kTln{ C_bulk/C_sol (T_crlt) } (4 )

Whether this criterion is met in a given sample in a given heat treatment is determined solely by the value C_bulk. The is shown schematically in Fig. 1. Also, there is a critical concentration C_bulk ^crit below which no metal precipitation will occur in a given heat treatment and subsequent cooling to room temperature due to the lack of sufficient driving force for the reaction. The critical concentration, C_bulk ^crit, for both copper and nickel, for example, lie in the region of about 10¹² atoms/cm³..

Present silicon wafer cleaning technologies are generally capable of preparing surfaces with residual metal amounts of about 10⁸ atoms/cm². Considerably higher amounts are, however, still common. Therefore, in a typical conventional silicon wafer having a thickness of 675 micrometers, the best cleaning technologies presently available are capable of a resulting bulk metal concentration, C_bulk, as low as about 3xl0⁹ cm^"3 (following equation (1)) following a heat treatment. This is well below the precipitation threshold. Conventional silicon wafers are thus processed with relative immunity from metal precipitation problems. Relatively thick conventional silicon wafers are, therefore, generally capable of sufficiently diluting unintentional metal contamination to concentrations where precipitation is effectively suppressed. In thinner samples, however, metal dilution is less efficient. The dilution effect is directly proportional to the sample thickness. From equation (1), in a sample only 1 micrometer thick, the dilution of 10⁸ cm^"2 surface metal contamination achieved is 2xl0¹²cm^"3. Therefore, even with the best current cleaning systems, such thin SOI films come very near, and may exceed, the critical concentration for precipitation during heat treatments. For a layer with a thickness of 0.1 micrometers, the very best cleaning technology results in about 10 times the critical concentration and unavoidable metal precipitation.

It has, however, been found that the extreme thinness of SOI films limits the total amount of metal available and hence limits the ultimate potential size of the precipitate or precipitate clusters. Generally, small precipitates are less harmful than large ones and SOI structures of reasonable quality are generally produced. The precipitates size and low density makes direct analysis extremely difficult. In the thin SOI films, the solubility concentration may easily be reached in normal processing conditions. The maximum amount of metal possible in a thin film is determined in equation (5) :

Total metal in thin film(cm^"2) =C_sol (T_ht) t_film (5)

As previously discussed, SOI structures are a composite of a thick layer and a thin layer separated by an insulating layer. It is believed that the oxide layer between the thin monocrystalline film of silicon and the thick substrate handle wafer is a partial or complete barrier to the diffusion of metal. If the barrier is complete and an equal amount of metal is on both surfaces ' prior to the heat treatment, a large difference will exist in the bulk concentration of unintentional metal contamination which is dissolved into the two sides of the structure separated by the barrier oxide layer. The thin film, which is the technologically important part of the SOI structure, will contain a concentration of metal equal to (T_sub3trate/T_film) times that of the handle wafer, as illustrated in Fig. 2. It has now been discovered that the full dilution potential of the entire structure can be accessed if the oxide layer is made to be permeable to metal diffusion.

An SOI structure can be made in such a way that the insulating layer between the thin monocrystalline film and the substrate is not continuous and drains, or regions which are permeable to metals are made between the film and substrate at several points distributed over the area of the structure so that diffusion of metals can occur and the full dilution potential of the structure realized. Such drains could be, for example, made by direct silicon to silicon contact. The diffusion of transition metals such as copper and nickel in silicon is so rapid that these contact regions may be relatively far apart and still achieve complete concentration equilibration between the film and substrate during a heat treatment. Qualitative evidence exists indicating surface diffusion enhancements over normal volume diffusion coefficients for copper and nickel in silicon. This further relaxes the need for close spacing of the holes. In most cases, spacings on the order of about 1 centimeter are sufficient. However, one skilled in the art would realize that spacings could be on the order of between about 0.3 and about 3 centimeters or continuous hole placement could also be utilized. The diameter of the holes could be between about 0.1 and about 3 millimeters.

Individual contact areas, or holes, could be of arbitrary shapes. A wide variety of possible contact areas would be available. Even quite small contact areas would achieve the desired result . The contact areas could be isolated from each other or form a periodic or aperiodic grid. A large degree of freedom in the design of contact pint distributions should exist and thus contact point designs should be highly compatible with other, primarily device design, considerations. Three geometries for hole type permeable oxide layers in SOI substrates are possible. Contacts for metal diffusion are made to the substrate wafer at points located outside of the square circuit die. Three configurations are considered: 1) edge hole type; 2) stripes; and 3) streets. These configurations are shown in Figs. 14, 15, and 16 respectively.

It is desirable that equilibrium throughout the SOI substrate occur as quickly as possible. If equilibrium occurs at a quick enough rate, the formation of metal precipitates during device fabrication is minimized. Estimates for the rates of loss of metal concentration from the film to the substrate can be made. These estimates are valid for times not too short (film concentration = roughly 50% of initial concentration or more) or too long (film concentration--: roughly 10^"3 x initial concentrations or less.) The calculations refer to the concentrations in the center of the die, and thus refer to a worst case.

For the case of edge holes the following equation (6) can be derived:

C(t) = C_x exp (- [2πDNt]/[(ln (R/r₀) ) + (d/r₀) ] ) (6)

For the case of stripes the following equation (7) can be derived:

C(t) = (4/π) C, exp (- [π² DT] / [L²] ) (7)

For the case of streets, the following equation (8) can be derived:

C(t) = (4/π)² C, exp (- [2π²/L²] DT) (8)

Wherein d = film thickness, r₀ = hole radius, C_x = initial metal concentration in film, D = metal diffusion coefficient at process temperature, N = l/L² and R = 1/square root of πN. The streets approach is the most preferred. Figs. 17, 18, and 19 are graphs of time at temperature (s) vs. metal concentration at the center of the die for varying die sizes, hole sizes, and metal diffusion coefficients. These Figures indicate that at die sizes of 0.5 centimeters and 1.0 centimeters and diffusion coefficients of 10^"4 and 10^"3 cm²/s that the streets method is the most preferred.

One method in which an SOI structure can be produced in accordance with the present invention is shown in Fig. 3. In this method, a silicon nitride layer 4 is first deposited onto a handle wafer substrate 2, and windows 6 are subsequently etched into the silicon nitride layer 4. The handle wafer 2, including the windows 6, is thereafter oxidized to form an oxide layer 8 on the substrate. Subsequently, the silicon nitride layer 4 and oxide layer 8 are removed by chemical -mechanical polishing down to the original handle wafer 2, which now contains oxide layer 8. A device wafer 10 is then bound on the polished surface 14, and subsequently selectively removed to form a thin monocrystalline film 12 separated from the handle wafer 2 by insulating film 8 with open channels 16.

Another method of producing an SOI structure in accordance with the present invention is shown in Fig. 4. With this method, an SOI structure 24 comprising a handle wafer 2, an insulating layer 16 and a monocrystalline film 18 has an oxide or photoresist masking layer 20 applied on top of the film 18. Windows 22 are subsequently etched into layer 20. Thereafter, ions, such as silicon ions, are implanted through windows 22 into insulating layer 16 to form channels 26. After the formation of channels 26, the layer 20 is removed to form an SOI structure of the present invention comprising a handle wafer 2, insulating layer 16, channels 26, and film 18.

An alternative method of preparing an SOI structure in accordance with the present invention is shown in Fig. 5. With this method, a handle wafer 2 having an insulating layer 16 has an oxide or photoresist masking layer 20 applied on top of the insulating layer 16. Windows 22 are etched into layer 20, and ions, such as silicon ions, are implanted into layer 16 to form channels 26. The layer 20 is removed to form a substrate 30 having a handle wafer 2 having an insulating layer 16 with channels 26. To the insulating layer of this substrate 30 is bonded a device wafer 10, which is subsequently selectively removed to form an SOI structure of the present invention having a handle wafer 2, an insulating layer 16 containing channels 26, and a thin monocrystalline layer 18.

A further amount of metal reduction can be achieved in the active parts of the SOI film (those parts not connected directly with the substrate) through metal gettering at dislocations arising from unavoidable low angle grain boundaries at the contact point between the film and the substrate. The gettering sites would lie in non-device active areas. Such boundaries and associated dislocations occur as a result of the practical impossibility of achieving perfect alignment of crystal orientations during the bonding of two wafers .

The dilution effect is the stronger and more important. The dilution effect, as opposed to the gettering effect, can take advantage of the full time and temperature of the heat treatment for lateral diffusion to the contact areas. Lateral transport to spatially distributed gettering sites on the other hand is limited since the gettering effect requires undercooling. The requirement for undercooling means that range in which useful lateral transport can occur is limited to a small time transient during the cooling of the sample between the onset temperatures of the gettering precipitation events and the precipitation events which must be suppressed. Since neither of these temperatures can be know a priori, an engineering of the sample cooling to maximize the gettering effect is thought to be impractical. In an alternative approach, the gaps in the oxide layer could be filled with polycrystalline silicon prior to bonding. In this case, a similar additional gettering effect, analogous that due to the grain boundary dislocations described above would be achieved in the polycrystalline silicon contact areas.

The present invention is illustrated by the following example which is merely for the purpose of illustration and is not to be regarded as limiting the scope of the invention or manner in which it may be practiced.

EXAMPLE 3D diffusion simulations of the surface contamination of an SOI wafer were performed, and are shown in Figs. 6- 13. The simulated problem consisted of bulk silicon with a thickness of 600 micrometers covered by a 1 micrometer thick silicon oxide diffusion barrier and a silicon surface layer, also 1 micrometer thick, when no other values are specified (Fig. 6) . The diffusion barrier has a hole with varying diameters which is filled with silicon. The influence of hole spacing, the influence of surface layer thickness, and the influence of diffusion coefficient on the diffusion process were investigated. It was assumed that the problem was symmetric to an axis of rotation in the center of the hole. Initially, surface contamination of about 10⁹ - 10¹⁰ cm^" ² distributes homogeneously in the surface layer resulting in a volume concentration of about 10¹³ - 10¹⁴ cm^"3. The contaminant has a diffusion coefficient of D = 10^"4 cm²s^"-^* if no other values are specified. To obtain a volume concentration below lxlO¹² cm^"3 in the surface layer, a decrease of the concentration by a factor of 10^"2 is necessary. All simulations show the concentration in relation to an initial concentration of 1 in the surface layer and 0 in the bulk layer. Fig. 7 shows the time development of the maximum contaminant concentration in the surface layer for different distances between the hole centers for a hole size of 1 millimeter. The concentration decreases exponentially until the concentration differences are small, then the concentration does not change anymore and stays constant. Fig. 8 shows the diffusion time needed until the maximum concentration in the surface layer has fallen by a factor of 100. The time increases proportional to the square of the hole spacing. Fig. 9 shows the time development of the maximum contaminant concentration in the surface layer for hole sizes from 0.1 to 2 millimeters. For these simulations the distance between the hole centers was 6 millimeters. Fig. 10 shows the time for a decrease by a factor of 100 versus the hole size. It was found that the hole diameter was proportional to (diffusion - time)^"1. The simulations for different diffusion coefficients is shown in Fig. 11. Diffusion coefficients of lxlO^"4 cur's^"1, 5xl0^"4 cm²s^"x, and lxl0^"3cm²s^"1. The hole size was 1 millimeter and the hole spacing was 6 millimeters. The time necessary for a decrease by a factor of 100 is shown in Fig. 12. As expected the diffusion coefficient is proportional to (diffusion - time)^"1.

Fig. 13 shows the time development of the maximum contaminant concentration for different surface layer thicknesses of 1, 2, 3, and 10 micrometers. The hole spacing was 6 millimeters and the hole size was 1 millimeter. The final value is different for each surface layer thickness due to the initial situation where it was assumed a homogeneous concentration of 1 in the whole surface layer. For different surface layer thicknesses this results in different final concentrations.

In view of the above, it will be seen that the several objects of the invention are achieved.

As various changes could be made in the above- described processes without departing from the scope of the invention, it is intended that all matter contained in the above description be interpreted as illustrative and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A process for inhibiting the formation of metal precipitate defects in an SOI wafer, the SOI wafer having an insulating layer permeable to metals on top of a semiconductor substrate and a monocrystalline semiconductor film on top of the insulating film, the process comprising heating the SOI wafer to reduce metal concentration in the monocrystalline semiconductor film by diffusion.

2. A process as set forth in claim 1 wherein the concentration of metal is reduced through the diffusion of the metal through holes in the insulating layer.

3. A process as set forth in claim 2 wherein the holes are spaced apart by between about 0.1 and about 3 centimeters .

4. A process as set forth in claim 2 wherein the holes are spaced apart by about 1 centimeter.

5. A process as set forth in claim 2 wherein the holes have a diameter of between about 0.1 and about 3 millimeters .

6. A process as set forth in claim 2 wherein the holes have a diameter of between about 0.1 and about 3 millimeters and are spaced between about 0.1 and about 3 centimeters apart.

7. An SOI structure, the structure comprising: a handle wafer; a monocrystalline film; and an insulating layer between the handle wafer and the monocrystalline film, the insulating layer being permeable to metals and allowing for the diffusion of metals.

8. The structure as set forth in claim 7 wherein the diffusion of metals occurs through holes in the insulating layer.

9. The structure as set forth in claim 8 wherein the holes are spaced between about 0.1 and about 3 centimeters apart .

10. The structure as set forth in claim 8 wherein the holes are spaced about 1 centimeter apart.

11. The structure as set forth in claim 8 wherein the holes have a diameter of between about 0.1 and about 3 millimeters .

12. The structure as set forth in claim 8 wherein the holes have a diameter of between about 0.1 and about 3 millimeters and are spaced between about 0.1 and about 3 centimeters apart.

13. A process for producing an SOI structure, the process comprising: depositing a silicon nitride layer onto a surface of a handle wafer; etching windows into the silicon nitride layer; oxidizing the surface of the handle wafer containing the etched silicon nitride windows to form an oxide layer; removing the etched silicon nitride windows and oxide layer from the surface of the handle wafer by polishing the surface; bonding a device wafer onto the polished handle wafer surface; and selectively removing the device layer to form a thin monocrystalline film.

14. A process for producing an SOI structure, the process comprising: applying a masking layer onto a monocrystalline film of an SOI substrate, the SOI substrate having a handle wafer, an insulating layer on top of the handle wafer, and the monocrystalline film on top of the insulating film; etching windows into the masking layer; implanting ions through the etched windows through the monocrystalline film into the insulating layer; and removing the masking layer from the monocrystalline film.

15. A process for producing an SOI structure, the process comprising: applying a masking layer onto an insulating layer positioned on a handle wafer; etching windows into the masking layer; implanting ions through the etched windows into the insulating layer; removing the masking layer from the insulating layer; bonding a device wafer onto the insulating layer; and selectively removing the device wafer to form a thin monocrystalline layer.

16. As SOI structure for use in a device manufacturing process, the substrate being resistant to the formation of metal precipitates during the device manufacturing process and comprising: a handle wafer; a monocrystalline film; and an insulating layer between the handle wafer and the monocrystalline film, the insulating layer being permeable to metals and allowing for the diffusion of metals.

17. The structure as set forth in claim 16 wherein the diffusion of metals occurs through holes in the insulating layer.

18. The structure as set forth in claim 17 wherein the holes are spaced between about 0.1 and about 3 centimeters apart .

19. The structure as set forth in claim 17 wherein the holes are spaced about 1 centimeter apart.

20. The structure as set forth in claim 20 wherein the holes have a diameter of between about 0.1 and about 3 millimeters .

21. The structure as set forth in claim 20 wherein the holes have a diameter of between about 0.1 and about 3 millimeters and are spaced between about 0.1 and about 3 centimeters apart.