FIELD OF THE INVENTION

This invention relates to the metallization of light emitting diodes (LEDs) having a wavelength conversion layer, such as phosphor layer, and, in particular, to a technique for metalizing surfaces of such an LED die to improve the upward reflection of light.

BACKGROUND

One type of conventional LED is a blue-light-emitting LED with a phosphor layer deposited over its top light-emitting surface. The LED is usually GaN based. The blue light energizes the phosphor, and the wavelength-converted light emitted by the phosphor is combined with the blue light that leaks through the phosphor. Virtually any color may thus be created, such a white light.

One issue regarding such phosphor-converted LEDs (PCLEDs), discussed in more detail below, is that the light emitted by the phosphor layer is isotropic, where some light is emitted upwards and exits the LED die and some light is emitted in a direction back into the semiconductor portion of the die. Most of this light is then reflected back upwards by the metal contacts on the bottom surface of the LED die. In order to minimize reflectivity losses, the metal contacts should feature very high reflectivity characteristics in the entire visible spectrum, which is often times difficult to achieve.

LED package efficiency is the ability to extract light from the LED after it has been generated/converted. Improving such package efficiency is today considered one of the main obstacles in increasing the luminous efficacy of LEDs.

Increased package efficiency in phosphor converted LEDs can be achieved by increasing the reflectivity of the LED die in architectures such as flip-chips.

In flip-chip (FC) die architectures, light is extracted from the semiconductor N-type layer that typically is the "top" semiconductor surface. The P-type layer is the "bottom" semiconductor layer facing the mounting substrate (e.g., a printed circuit board). Metal contacts (electrodes) are formed on the bottom surface of the die. The N-contact is formed by etching away the P-type layer and active layers (i.e., quantum wells) to expose portions of the N-type layer. A dielectric layer is then patterned over the exposed P-type layer and active layer (in order to avoid short-circuits) within the openings, and a metal layer, such as aluminum, is deposited within the openings to contact the N-type layer. The N-contacts can be arranged in the form of vias across the die and/or grooves around the edge of the die, where current is then spread laterally across the N-type layer. No light is generated over the N-contacts since the active layer has been removed in those areas.

The metal P-contact is usually the largest in surface area, and it is also functionally used as a mirror reflector. The P-contact usually consists of Ag (silver) material. Due to the ability of Ag to migrate, a metal guard sheet layer is generally used to prevent the Ag from migrating into any underlying dielectric layer. Aluminum, and not silver, is typically used for the N-contacts for improved electrical coupling to the N-type layer.

In state-of-the-art technologies, thin-film-flip-chip (TFFC) architectures are achieved by further removing the growth substrate (e.g., the sapphire substrate) followed by a roughening process of the exposed N-layer surface, where the generated (blue) light is extracted from. The roughening improves light extraction by reducing internal reflections.

In phosphor converted TFFC LEDs, a phosphor layer may further be deposited on, or attached to, the roughened N-layer surface, thus converting light from a narrow wavelength range into a well-defined wideband spectrum.

Figs. 1-3 illustrate one type of prior art phosphor converted TFFC LED.

Fig. 1 is a bottom up view of an LED die 10 showing a large metal P-contact layer 12, a narrow N-contact area 14 along the edge where electrical contact is made to the N-type layer, and distributed N-contact areas 16 where additional electrical contacts are made to the N-type layer for good current spreading. The metal layer contacting the N-type layer at areas 14 and 16 is typically Al, which has a reflectivity below 90% for the wavelengths of interest.

Fig. 2 is a cross-sectional view of the edge portion along line 2-2 in Fig. 1. A semiconductor N-type layer 18 is epitaxially grown over a sapphire growth substrate, which has been removed. An active layer 20 and a P-type layer 22 are grown over N-type layer 18. A highly reflectively metal layer (or a stack of metal layers), which may comprise Ag, is then deposited as a P-contact 12 to electrically contact the P-type layer 22. The layers 22 and 20 are etched along the edge to expose the N-type layer 18. A metal guard sheet layer 24 may be deposited on the metal P-contact layer 12 to block the migration of Ag atoms. A dielectric layer 26 is then deposited and etched to expose the N-type layer 18 at area 14. A metal N- contact layer 13 (e.g., Al) is then deposited to electrically contact the N-type layer 18 at area 14 and form a metal ring along the edge of the die 10. In the central area of the LED die 10, the P-contact layer 12 is exposed (see Fig. 1), by etching away the layers 13 and 26, and further metalized to planarize the bottom surface of the LED die 10. If the metal guard sheet layer 24 is used, the electrical contact to the P-contact layer 12 may be made through the metal guard sheet layer 24. The P and N-metal contact layers 12 and 13 are ultimately bonded to corresponding metal anode and cathode pads on a mounting substrate.

Fig. 3 is a cross-sectional view along line 3-3 in Fig. 1 showing a portion of a distributed N-contact area 16, where the N-type layer 18 is contacted by the metal N-contact layer 13. The metallization and etching to create the N-type layer 18 contact at area 16 are performed at the same time the contact is made to the edge area 14.

The sapphire growth substrate may be removed by laser lift-off or other process. The exposed top N-type layer surface 28 is then roughened (e.g., by etching or grinding) to improve light extraction. A phosphor layer 30 is then deposited or otherwise affixed (as a tile) to the top surface.

It will be assumed the phosphor layer 30 is a YAG phosphor that generates a yellow- green light, which, when combined with blue light, results in white light. When a photon generated by the active layer 20 energizes a phosphor particle 32 (Fig. 2), the resulting wavelength-converted light is usually scattered isotropically, so a significant portion of the energy is direct back into the LED die. Fig. 2 illustrates some energized phosphor particles 32 emitting light rays 34 upward and downward. The downward light is ideally reflected upward by the metal N-contact layer 13 at areas 14 and 16 and the P contact layer 12.

However, the N-contact layer 13 is typically aluminum, which is not a good reflector.

Accordingly, light that impinges on the N-contact layer 13 at areas 14 and 16 is significantly attenuated. Good package efficiency relies upon a high metal contact reflectivity to avoid light absorption in the die.

Besides the limited reflectivity of the N-contact layer 13 at areas 14 and 16, the package efficiency of LED dies like shown above is also limited by the capability of the textured N-type layer surface 28 to extract light from the GaN semiconductor material (a high index material, e.g., n= 2.5) to the lower index phosphor layer (e.g., n= 1.6).

Thus, what is needed is an LED die structure that mitigates such limitations, resulting in superior package efficiency.

SUMMARY One purpose of the proposed invention is to increase the effective reflectivity of the die area exposed to the phosphor layer light, where the wavelength-converted light is emitted isotropically. To achieve this, the following techniques are used in one embodiment of the present invention:

Highly reflective regions are added to an otherwise conventional LED die that contribute to an overall higher die reflectivity. These highly reflective regions should be located at areas on the die for efficiently reflecting light generated by the phosphor layer. In one embodiment, the highly reflective regions are in areas that do not generate light. The percentage of the highly reflective area relative to the total die area should be significant (e.g., up to 50%). In order to keep the same area of quantum wells (where electrons are converted into photons) as in standard LED die sizes, the active layer area (and consequently the phosphor area) is increased generally in proportion to the added highly reflective area.

The highly reflective regions can be formed around the edge of the die as well as distributed around the central portion of the die. The reflective regions may be used as electrical contact regions to the N-type layer, or even the P-type layer. In one embodiment, the transparent growth substrate (e.g., sapphire) is not removed, and the phosphor layer is ultimately provided over the top surface of the substrate. The highly reflective regions are within trenches etched through the semiconductor layers that expose the substrate. The exposed surfaces are then coated with a highly reflective material, such as Ag. If the reflective material is a metal, proper electrical isolation may be needed. The reflective metal in the trenches may or may not carry current to the N-type layer. In one embodiment, the electrical contacting of the P-type layer is not affected by the invention, since the P-contacts are already highly reflective.

In another embodiment, a dielectric layer, having a relatively low index of refraction, is formed between the substrate and the highly reflective metal layer, or between the GaN and the metal layer, to create an index of refraction mismatch at the dielectric layer surface. Therefore, light incident on the interface at greater than the critical angle will reflect by total internal reflection without losses, and light that enters the dielectric layer will be reflected by the metal layer.

Instead of, or in addition to, a reflective metal creating the highly reflective regions, the reflective layer may be a distributed Bragg reflector using stacked dielectric layers having thicknesses and indices of refraction selected so as to reflect 100% of the wavelengths of interest.

By not removing the growth substrate (e.g., sapphire), the substrate helps to scatter the downward light from the phosphor layer to reduce internal reflections, the substrate provides good mechanical support, and the substrate (having an index of about n=1.8) reduces internal reflections by providing an index between that of the GaN (n=2.5) and the phosphor (n=1.6).

The substrate may undergo texture patterning on its growth side prior to growing the epitaxial layers to improve light extraction at the epitaxial layer-substrate interface.

Other embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a bottom up view of a prior art flip-chip LED die, showing the metal contact areas.

Fig. 2 is a cross-sectional view of the LED die edge along line 2-2 in Fig. 1. Fig. 3 is a cross-sectional view of a portion of a distributed contact area along line 3-3 in Fig. 1.

Fig. 4 is a bottom up view of an LED die in accordance with one embodiment of the invention. Fig. 5 is a cross-sectional view of a portion of the highly reflective region along line

5-5 in Fig. 4.

Fig. 6 is a cross-sectional view of a portion of the highly reflective region along line 5-5 in Fig. 4 in accordance with another embodiment of the invention where the highly reflective metal electrically contacts the N-type layer. Fig. 7 is a cross-sectional view of an edge portion of the highly reflective region along line 7-7 in Fig. 4.

Fig. 8 is a cross-sectional view of an edge portion of the highly reflective region along line 7-7 in Fig. 4, where the edge of the substrate is coated with a reflector rather than phosphor. Fig. 9 is a cross-sectional view of a portion of a distributed N-contact along line 9-9 in Fig. 4 showing how the highly reflective metal electrically contacts the N-type layer.

Fig. 10 is an alternative cross-sectional view along line 5-5 of Fig. 4, illustrating how the dielectric layer may be between the substrate and the highly reflective metal for enhancing reflectivity. Fig. 11 is an alternative cross-sectional view along line 5-5 of Fig. 4, illustrating how the dielectric layer of Fig. 10 may be opened so the reflective metal may contact the N-type layer.

Fig. 12 is an alternative cross-sectional view along line 7-7 of Fig. 4, illustrating how a first metal layer may contact the N-type layer, and a higher reflectivity metal layer may be formed over a dielectric layer. The phosphor layer extends over the sides of the substrate. Fig. 13 is an alternative cross-sectional view along line 7-7 of Fig. 4, illustrating how a first metal layer may contact the N-type layer, and a higher reflectivity metal layer may be formed over a dielectric layer. A reflector is formed on the sidewalls of the substrate.

Fig. 14 is an alternative cross-sectional view along line 7-7 of Fig. 4, illustrating how the metal reflective layer, formed over a dielectric layer, may contact the N-type layer, similar to Fig. 7.

Fig. 15 is an alternative cross-sectional view along line 9-9 of Fig. 4, illustrating how the metal reflective layer, formed over a dielectric layer, may contact the N-type layer, similar to Fig. 9. Fig. 16 is a magnified view of a highly reflective area illustrating how dielectric layers may be stacked to form a distributed Bragg reflector (DBR) instead of, or in addition to, using a highly reflective metal layer.

Elements that are the same or similar are labeled with the same numeral.

DETAILED DESCRIPTION Fig. 4 is a bottom up view of an LED die 40 in accordance with one embodiment of the invention. The LED die 40 includes an added highly reflective region 42 that may or may not serve as an electrical contact. Also, the perimeter of the LED die 40 includes a relatively wide highly reflective edge region 44, in comparison to Fig. 1. In one embodiment, the area of the active layer 20 is the same as the prior art so that similar electrical specifications apply to both. However, the LED die 40 is made larger due to the added area for the regions 42 and 44, and the light output is increased due to the increased package efficiency.

In the embodiments shown, the prior art P-contact layer 12 is not significantly changed since the P-contact layer 12 (comprising of Ag) is already a good reflector.

In one embodiment, the LED die 40 has sides on the order of 1mm x 1mm. In the example of Fig. 4, the region 42 is formed as a cross; however, it may be any shape and preferably designed to provide a fairly uniform light output across the top surface of the LED die 40. Region 42 may take up from 10%-50% of the die surface area. Since the region 42 removes a portion of the active layer 20, the die may be made larger to compensate for the loss of light generation area.

Fig. 5 is a cross-sectional view of a portion of the highly reflective region 42 along line 5-5 in Fig. 4.

The transparent sapphire growth substrate 46 is not removed. The substrate 46 is optionally thinned prior to depositing the phosphor layer 30. The phosphor layer 30 may be coated on the substrate 46 surface using any number of well-known techniques or may be affixed as a pre-formed tile to the substrate 46 surface.

A trench 48 (formed as a cross in Fig. 4) is then etched through the various layers to expose the transparent substrate 46 surface.

The P-contact layer 12 metal (e.g., Ag) is deposited over the P-type layer 22 (which may be done prior to or after forming the trench 48. The guard sheet layer 24 and dielectric layer 26 are then deposited and patterned to expose the substrate 46 but cover the P-contact layer 12.

On the exposed surface of the substrate 46 and over any portion of the dielectric layer 26, a highly reflective layer 50, such as Ag or an alloy, is then deposited and patterned. The reflectivity of Ag is about 95% for the wavelengths of interest, while the reflectivity of Al is less than 90% at the wavelengths of interest.

A guard sheet layer 52 may then be deposited over the reflective layer 50 if Ag migration is a concern.

The reflective layer 50, guard sheet layer 24, and dielectric layer 26 are patterned to expose the P-contact layer 12 at areas out of the view of Fig. 5 so the exposed P-contact layer 12 can be used as an anode electrode when mounting to a submount or printed circuit board. Any reflective layer 50 under the P-contact layer 12 is not exposed to light and would only be used for electrically contacting the N-type layer 18.

Fig. 5 illustrates various phosphor particles 32 emitting light rays 34 in different directions. Light is shown being reflected off the Ag P-contact layer 12 as well as the Ag layer forming the reflective layer 50. Elsewhere, light may also be reflected off the reflective layer 50 located in the distributed contact areas 54 (Fig. 4) and along the edges of the die.

In the example of Fig. 5, there is no electrical contact made between the reflective layer 50 and the N-type layer 18.

Fig. 6 is an alternative embodiment along line 5-5 in Fig. 4 but where electrical contact is made to the N-type layer 18 by the reflective layer 50 at area 56, where the dielectric layer 26 has been etched away. The narrow contact area 56 extends all the way around the edge of the cross-shaped pattern in Fig. 4 for good current spreading.

Accordingly, the guard sheet layer 52 and the reflective layer 50 may form part of the bottom cathode electrode that is bonded to a submount or printed circuit board. Fig. 7 is a cross-sectional view of an edge portion of the die 40 showing the highly reflective region 44 along line 7-7 in Fig. 4, with the addition of the phosphor layer 30 extending around the side walls of the substrate 46. The manufacture of the various layers may be the same as described above. An edge of the die 40 is etched to expose the substrate 46, and the various layers, including the reflective layer 50 (e.g., Ag), are deposited as shown. Fig. 7 also shows the reflective layer 50 making electrical contact to the N-type layer 18 using a metal ring 58 that circumscribes the central portion of the die 40. The metal used to form the ring 58 may comprise aluminum and may be a conventional metal stack conventionally used to make ohmic contact with N-type GaN. The ring 58 is deposited and patterned simultaneously with the metal used to make contact with the N-type layer 18 in the distributed contact areas 54 shown in Fig. 4, discussed later with respect to Fig. 9. Although electrical contact to the N-type layer by the reflective layer 50 along the edge may be made simply by opening up the dielectric layer 26, as shown in Fig. 6, the interface metals forming the ring 58 and guard sheet layer portion 60 provide an interface for a better electrical connection. Such an interface may also be used in Fig. 6. To block migration of the Ag atoms from the reflective layer 50, a guard sheet layer portion 60 is formed as a barrier between the metal ring 58 and the reflective layer 50. The guard sheet layer portion 60 may be formed simultaneously with the guard sheet layer 24.

The dielectric layer 26 isolates the reflective layer 50 and metal ring 58 from the metal P-contact layer 12 (which may also comprise Ag for high reflectivity).

Fig. 7 illustrates phosphor particles 32 emitting light rays 34 in various directions. Note how one particle 32 emits a light ray 62 that is reflected off the reflective layer 50 along the edge. If the reflective layer 50 is used for an N-contact, the reflective layer will typically extend to a bottom surface of the die to serve as a cathode electrode. Alternatively, the reflective layer 50 may be electrically connected to another type of less-reflective metal that extends along the bottom surface of the die 40, since any metal that is below the metal P- contact layer 12 does not receive any light. For a cathode electrode, other well-known metals may be deposited over the reflective layer 50, such as Ni and Au, to facilitate bonding to metal pads of a submount or printed circuit board.

As seen by a comparison of Figs. 1 and 4, the edge that is etched is much wider, and no light is generated along the edge. However, the die 40 may be made larger to compensate for the loss of the light generating area. The package efficiency will, however, be greater than that of the die 10 in Fig. 1 since there is increased reflectance of the light generated by not only the phosphor layer 30 but by the active layer 20. Therefore, the LED die 40 will have the same electrical specifications as the prior art LED die 10 of Fig. 1 but will be brighter.

In one embodiment, the area of the trench 48 around the edge of the die 40 that is covered by the reflective layer 50 is 10%-50% of the die 40 surface area.

Fig. 8 is similar to Fig. 7 but the edge of the substrate 46 is coated with a reflector 66 rather than phosphor. The substrate 46 may be many times thicker than the LED

semiconductor layers and thus the light emitted from the sides is significant. If such side light is not desired, then forming the reflector 66 is recommended. The reflector 66 may be Ag or other suitable material. Fig. 8 shows a light ray 68 from a phosphor particle 32 being reflected off both the reflective layer 50 and the reflector 66.

Fig. 9 is a cross-sectional view of a portion of a distributed N-contact along line 9-9 in Fig. 4 showing how the reflective layer 50 electrically contacts the N-type layer 18 via a metal contact 70 forming a narrow ring within the circular etched opening in the LED layers. The metal contact 70 is the same metal forming the metal ring 58 in Fig. 7 and formed at the same time. Although Fig. 4 shows four identical distributed contact areas 54, there may be many more for improved current uniformity. The distributed contact areas 54 may be circular or generally frustum-shaped, as shown, or may be rectangular or other shapes. The metal contact 70 would therefore take the shape of the edge of the contact area 54. A guard sheet layer portion 72 is also shown, which is formed simultaneously with the guard sheet layer portion 60 in Fig. 7. Electrical contact to the N-type layer 12 is made by the various electrical contacts shown in Figs. 6, 7, and 9 to evenly spread current.

Therefore, since the distributed contact areas 54 and the reflective edge region 44 will reflect about 95% of the impinging light from the phosphor layer 30, and the P-contact layer 12 is also highly reflective, very little phosphor light is absorbed by the die 40, in contrast to the die 10 of Fig. 1 where there is significant absorption by the metal N-contact layer 13 at the areas 14 and 16. Accordingly, the overall efficiency of the LED is improved.

In another embodiment, instead of adding the trench 48 to form the cross-shaped reflective layer 50, the distributed contact areas 54 are made larger than the distributed areas 16 in Fig. 1, where the electrical contact to the N-type layer 18 is made along the edges of the contact areas 54 (shown in Fig. 9) and the central areas of the contact areas 54 are solely for adding the highly reflective areas. Note that, in the prior art Fig. 3 and in contrast to Fig. 9, the distributed areas 16, for contacting the N-type layer 18, are solely for making electrical contact with the N-type layer 18, and the contact metal used significantly absorbs the phosphor light.

The areas of the highly reflective regions, using Ag, are preferably much larger than the areas where the N-contact metal, typically Al, contacts the N-type layer 18, and the Al should only be used for the electrical interface between the reflective layer 50 and the N-type layer 18. Preferably the Al should only occupy no more than the strictly necessary for good electrical contact to the N-type layer 18, such as providing a contact width not larger than 2*Lt, where Lt is the transfer length of the metal-semiconductor contact, typically about lum. The remaining exposed regions are preferably covered by the highly reflective metal (e.g., Ag). The highly reflective layer 50 may or may not be used as a current carrier while still achieving the goals of the present invention.

Fig. 10 illustrates how the dielectric layer 26 may be between the substrate 46 and the metal highly reflective layer 50 for enhancing reflectivity. The index of refraction of the dielectric layer 26 (e.g., 1-4-1.5) is selected to be lower than that of the substrate 46. Fig. 10 may illustrate any of the areas of high reflectivity, such as across lines 5-5, 7-7, or 9-9 of Fig. 4. Therefore, light incident the interface at greater than the critical angle, such as light ray 74, will reflect by total internal reflection without losses, and light that enters the dielectric layer 26, such as light ray 76, will be reflected by the reflective layer 50.

Further, in one example, a thinned N-type layer 18, including the N-type layer surface 28, may extend to the left edge of Fig. 10. If the dielectric layer 26 and reflective layer 50 are formed over the thinned N-type layer 18, the relatively low index of the dielectric layer 26 will cause light incident at larger than the critical angle to reflect off the GaN/dielectric interface without losses. Light that enters the dielectric layer 26 will be reflected by the reflective layer 50. The reflective layer 50 may or may not carry current for the N-type layer 18.

The lower the refractive index of the dielectric layer 26, the lower the critical angle (in accordance with Snell's law) and hence the larger the range of the light rays that will be fully reflected at the interface by total internal reflection.

Fig. 11 is an alternative cross-sectional view along line 5-5 of Fig. 4 (or other edges of a reflective area), illustrating how the dielectric layer 26 of Fig. 10 may be opened at area 80 so the metal reflective layer 50 may electrically contact the N-type layer 18 to carry N- type layer 18 current. Fig. 12 is an alternative cross-sectional view along line 7-7 of Fig. 4, illustrating how a first metal layer 84 (e.g., aluminum) may contact the N-type layer 18 at area 86 through an opening in the dielectric layer 26. The reflective layer 50, formed of a higher reflectivity metal such as Ag, may be formed the first metal layer 84 and over the dielectric layer 26. As in Figs. 10 and 11, the dielectric layer 26 contacting the substrate 46 reflects some light by total internal reflection. The phosphor layer 30 extends over the sides of the substrate.

Fig. 13 is an alternative cross-sectional view along line 7-7 of Fig. 4, illustrating how the first metal layer 84 may contact the N-type layer 18 near the edges of the die. Fig. 13 differs from Fig. 12 in that a reflector 66 is formed over the sidewalls of the substrate 46.

Fig. 14 is an alternative cross-sectional view along line 7-7 of Fig. 4, and similar to Fig. 7, illustrating how the metal reflective layer 50, formed over the dielectric layer 26, may contact the N-type layer 18 via a metal ring 58 and a guard sheet layer portion 60..

Fig. 15 is an alternative cross-sectional view along line 9-9 of Fig. 4, illustrating how the metal reflective layer 50, formed over the dielectric layer 26, may contact the N-type layer 18 using a metal contact 70 and guard sheet layer portion 72, similar to Fig. 9.

Instead of, or in addition to, a reflective metal creating the highly reflective regions, the reflective layer may be a distributed Bragg reflector (DBR), as shown in Fig. 16, using stacked dielectric layers 90 A, 90B, and 90C, having thicknesses and indices of refraction selected so as to reflect 100% of the wavelengths of interest. In an actual embodiment, there may be many more stacked layers. Forming DBRs is well known for other applications. Light (e.g., light ray 94) that fully penetrates the DBR will be reflected by the metal layer forming the reflective layer 50. The metal layer may be optional. The DBR may be formed below the P-type layer 22 for use as a dielectric layer and may be an extension of the dielectric layer 26.

Note that the DBR could also be extended over the mesa sidewalls to obtain mesa sidewall reflectance.

By not removing the growth substrate 46, the substrate helps to scatter the downward light from the phosphor layer to reduce internal reflections, the substrate 46 provides good mechanical support, and the substrate 46 (having an index of about n=1.8) reduces intemal reflections by providing an index between that of the GaN (n=2.5) and the phosphor layer 30 (n=1.6). The growth surface of the substrate 46 may be roughed to further improve light extraction by reducing internal reflections.

Additionally, since the phosphor layer 30 is separated from the semiconductor layers, there is less heat transferred to the phosphor layer 30, allowing the use of phosphors that have lower temperature requirements.

Instead of a phosphor layer, any other wavelength conversion layer may be located over the substrate 46, such as a quantum dot layer. The wavelength conversion layer does not have to be in direct contact with the substrate 46.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.