The invention relates to a phosphor material consisting of a metal-borosilicate compound that is activated by Mn doping. The invention further relates to a white-light emitting diode (LED) comprising a light emitting element and a first phosphor material according to the invention, wherein said first phosphor material is further combined with red, yellow, green, and/or blue light emitting phosphor materials. Finally the invention relates to a pro- cess for preparing the inventive phosphor material com ^¬ prising the steps of providing oxide compounds of the al ^¬ kaline earth metal, boron, silicon, and the dopant mate ^¬ rial, mixing said oxide compounds, and sintering said mixed oxide compounds at temperatures below 1000 °C.

The development of white light emitting diodes (LED) has drawn increasing interest in the past few years for its potential applications in solid state lighting, i.e. flat panel displays and illumination systems. The rapid ad- vancement of semiconductor technology together with new concepts in packaging design has led to a significant in ^¬ crease in LED brightness, so that the use of light emit ^¬ ting diodes in backlighting applications has gained in ^¬ creasing importance. LEDs are typically produced from group II-V alloys such as Gallium nitride (GaN) . Light emitted from InGaN-based LEDs is generally in the UV and/or in the blue range of the electromagnetic spectrum. Therefore light emitted from such LEDs is converted to light that is useful for illumination purposes, i.e. cold or warm white light, by coating or covering the LED with a layer of phosphor material. By interposing a phosphor material excited by the radiation generated by the LED, light of a different wavelength, e.g. in the visible range of the spectrum may be generated by the lumines ^¬ cence effect. Nowadays commercially available white LEDs are fabricated from InGaN LED chips which are coated with red/green/blue (RGB) phosphor materials.

Phosphor materials typically consist of a host material that is doped with a transition metal or a rare earth metal such as Europium (Eu) or Cerium (Ce) . Commercially used host materials are inorganic compounds such as gar- nets, ortho silicates, or (oxy) nitrides. For the appli ^¬ cation as phosphor materials with red emission recently fluoride based materials such as K ₂ SiF ₆ or K ₂ T1F ₆ as host materials for transition or rare earth metal doping have been developed [US7497973] . These fluoride based host ma- terials have the disadvantage that they degrade faster in humidity and heat due to the relatively high electronega ^¬ tivity of the fluoride ions (F ^~ ) . This degradation may lead to a shift in brightness or color (C _x C _y shift) which is detectable during testing of reliability and lifetime such as Temperature and Humidity Bias (T&HB) testing or steady state life (SSL) testing.

Disclosure of the Invention It is therefore an object of this invention to provide a phosphor material with red emission that shows an improved degradation behavior in a humid atmosphere at ele ^¬ vated temperatures during reliability testing. It is fur ^¬ thermore an object of this invention to provide a process for manufacturing such a phosphor material.

These objects are accomplished by the characterizing fea ^¬ tures of the independent claims. Advantageous embodiments can be found in the dependent claims.

In a first embodiment of the invention a phosphor material consisting of a metal-borosilicate compound that is activated by Manganese (Mn) doping is described. Borosil- icate compounds also known as borosilicate ceramics or glasses, which are e.g. used as lab ware glasses, are highly inert materials physically as well as chemically. Using a borosilicate compound as a host lattice for a phosphor material has therefore the advantage that the phosphor material will be stable in humid atmosphere at elevated temperatures.

Preferably the metal-borosilicate compound is an alka- line-earth-metal-boron-silicon-oxide. Preferably the met ^¬ al of the alkaline-earth-metal-boron-silicon-oxide is se ^¬ lected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn. By introducing M ²⁺ ions of Be, Mg, Ca, Sr, Ba, Zn into the borosilicate compound the negative excess charge can be compensated and the physical and chemical proper ^¬ ties of the glass or the crystalline material such as melting point, hardness etc. can be adjusted. Further ^¬ more, the choice of the alkaline earth metal can influ ^¬ ence the absorption and emission properties of the phos- phor material.

In a further embodiment the metal-borosilicate compound is activated by Mn doping in the form of Mn ⁴⁺ ion dopant. The use of Mn ⁴⁺ ion dopant has the advantage that the emission of Mn ⁴⁺ is ultra narrow due to a forbidden electric dipole transition.

Preferably the Mn ⁴⁺ ions are located in tetrahedral sites and the Mn ⁴⁺ ions preferably replace the Si ⁴⁺ ions in the tetrahedral sites of the alkaline-earth-metal-boron- silicon-oxide lattice. Similar to the Si ⁴⁺ octahedral sites in K ₂ SiF6 the Si ⁴⁺ tetrahedral sites in metal- borosilicate compounds such as Sr ₂ B ₂ Si208 and Ba2B ₂ Si208 are suitable locations for the Mn ⁴⁺ dopant. Therefore the Mn ⁴⁺ ions will be stable in the tetrahedral Si ⁴⁺ sites of Sr ₂ B ₂ Si208 and Ba2B ₂ Si ₂ 08. In a further embodiment the phosphor material is repre ^¬ sented by the formula M ₂ B ₂ Si ₍₂ - _X ) Os : (x)Mn ⁴⁺ and

0, 001<x<0, 05. Preferably M is selected from the group of Sr and Ba. A dopant concentration between 0,1% and 5% has the advantage that the lattice of the host materials Sr ₂ B ₂ Si208 and Ba2B ₂ Si208 is not yet disturbed by a too high dopant concentration.

In a further embodiment one or more co-dopants are added to the phosphor material. Preferably the co-dopants are selected from the group of transition metals or rare earth metals. A co-doping has the advantage that the lu ^¬ minescence can be enhanced or adjusted to a certain wave ^¬ length .

In a further embodiment the phosphor material when excit ^¬ ed by light sources is emitting light in the red spectral region. This has the advantage that the phosphor material can be used as red component in white light emitting LED.

In a further embodiment a white-light emitting LED is claimed comprising a light emitting element and a first phosphor comprising the inventive phosphor material wherein said first phosphor is further combined with red, yellow, green, and/or blue light emitting phosphors.

In a further embodiment a backlight unit module for liq ^¬ uid crystal display (LCD) technology comprising at least one white-light emitting LED comprising the inventive phosphor material is claimed. The use of LED instead of a cold cathode fluorescence lamp (CCFL) for backlight ap ^¬ plications in LCD technology is advantageous since e.g. a higher contrast can be achieved. Finally a process is disclosed for preparing a phosphor material consisting of a metal-borosilicate compound that is activated by Mn doping, said process comprising (1) providing oxide compounds of the alkaline earth metal, boron, silicon, and dopant material, (2) mixing said ox ^¬ ide compounds, and (3) sintering said mixed oxide com ^¬ pounds at temperatures below 1000°C. This manufacturing process has the advantage that low cost educts such as SrO, B ₂ O ₃ or S 1 O2 are used and that the phosphor material can be manufactured by an inexpensive low temperature sintering process with process temperatures below 1000°C.

Brief Explanation of Drawings

Fig. 1: 2-dimensional simulation image of BaB2 S i 20s or S rB2 S i 2<08 crystal structure showing the dopant sites for Mn ⁴⁺ . Fig. 2: Schematic cross section of an SMT (surface mount technology) LED.

Fig. 3: Schematic cross section of the construction of direct backlighting with LEDs .

Detailed Description

In Fig. 1 a 2 dimensional (simplified) simulation image of the 3 dimensional crystal structure of Barium borosil- icate (BaB ₂ Si208) or Strontium borosilicate (SrB ₂ Si208) is shown which can both be used as host lattices for phos ^¬ phor materials. The crystal structure of those chemical compounds consists of linked S 1 O ₄ tetrahedrons. The S 1 O ₄ tetrahedrons are linked via the oxygen corner atoms. In larger sites formed by the 3d network of corner linked Si0 ₄ -tetrahedrons Sr and/or Ba in form of Sr ²⁺ or Ba ²⁺ ions are located. The boron ions B ³⁺ replace a part of the Si ⁴⁺ ions. The resulting excess negative charge due to the replacement of Si ⁴⁺ ions by B ³⁺ ions of is compen- sated by the alkali earth metal ions Sr ²⁺ or Ba ²⁺ located in the larger sites. Apart from Sr or Ba also other alka ^¬ li earth metals such as Be, Ca or Zn can be used. The al ^¬ kali earth metals can also be mixed e.g. a compound Bao, 5Sr ₀ , 5B2S12O8 can be used. Furthermore, Boron can be re ^¬ placed by Aluminum (Al) and silicon (Si) can be replaced by Germanium (Ge) . The manganese (Mn) dopant shown in Fig. 1 is located in the S1O ₄ tetrahedral sites replacing Silicon. Mn can ex ^¬ ist in various oxidation states from 0 (Mn-metal) to +7 in the MnO ^4- ion. Mn can be used as activator in phosphor materials in various oxidation states. In the current in- vention Mn ⁴⁺ ion doping is used as activator. In general Mn ⁴⁺ can be easily compressed into small lattice sites such as octahedral sites of the SiF ₆ octahedrons of K ₂ SiF ₆ or tetrahedral site of Si0 ₄ tetrahedrons in BaB ₂ Si ₂ 0s or SrB2Si2<08 as shown in Fig. 1.

The emission spectrum of Mn ⁴⁺ is dominated by the spin- forbidden ² E- ⁴ A ₂ transition which is very narrow (ultra narrow) with approximately 2 nm at full width at half maximum (FWHM) . For Mn ⁴⁺ doped K ₂ SiF ₆ (K ₂ SiF ₆ :Mn) the emis- sion peak wavelength is approximately 630 nm at 450 nm excitation while the excitation bands are spin allowed and relatively broad. The emission peak wavelength of Mn ⁴⁺ in tetrahedral sites had been reported to be in the red region of the visible electromagnetic spectrum for Mn doped Mg ₂ Ti0 ₄ (Mg ₂ Ti0 ₄ :Mn) [Lit. from Invention disclo ^¬ sure] , where Mn ⁴⁺ dopant is replacing Ti ⁴⁺ in tetrahedral sites. Therefore, BaB ₂ Si ₂ 0 ₈ or SrB ₂ Si ₂ 0 ₈ doped with Mn ⁴⁺ . (BaB ₂ Si ₂ 0 ₈ : Mn or SrB ₂ Si ₂ 0 ₈ : Mn) are expected to have the emission peak also in the red region of the visible elec- tromagnetic spectrum and are therefore well suited as phosphor materials emitting red light.

The concentration of the Mn activator covers the range from largely zero up to 0,5 atomic portions in other words up to 50 atomic percent of the Si atoms in BaB2Si20s or SrB ₂ Si ₂ <0 ₈ can be replaced by Mn . This results in the stoichiometry range BaB ₂ Si ₍₂ - _X ) Os : (x) Mn or SrB ₂ Si ₍₂ - _Χ )θ ₈ : (x)Mn wherein 0<x<0,5. The preferred range of Mn ⁴⁺ dopant in BaB2Si20s or SrB2Si20s is between 0,1 and 5 atom ^¬ ic percent which leads to a stoichiometry range of

BaB ₂ Si (2-x)0 ₈ : (x)Mn ⁴⁺ or SrB ₂ Si ₍₂ -x) 0 ₈ : (x)Mn ⁴⁺ wherein

0, 001<x<0, 05. It is expected that doping of Mn ⁴⁺ ions with such a small concentration does not cause changes in crystal structure of the host lattice which could lead to instability and an easier degradation of the borosilicate compounds . Further transition metal or rare earth metal ions like Ce, Yb, Tb, Dy, etc. may be co-doped into the phosphor material and act as co-activators. Sensitizer ions like Ce ³⁺ , Pb ²⁺ , Sb ³⁺ , Sn ²⁺ , Eu ²⁺ etc. may be included in the phosphor material for energy transfer to Mn ⁴⁺ . The con- centration of these co-activators and/or sensitizers can be adjusted in the range from zero up to 50 atomic per ^¬ cent .

Phosphor materials comprising SrB2Si20s or BaB2Si20s host lattices and Mn dopant can be prepared via a solid state reaction method. Strontium carbonate or Barium carbonate (SrC0 ₃ or BaC0 ₃ , 98%), boric acid (H ₃ B0 ₃ , 99, 8%); Silicon dioxide (Si0 ₂ , 99%), Manganese dioxide (Mn0 ₂ , 99%) are employed as precursor materials [Lit. from Invention dis- closure] . The raw materials are weighted in stoichio ^¬ metric balance to achieve the desired dopant concentra ^¬ tion. To obtain a homogeneous mixture the raw materials were mixed in a mortar and ball milled and the resulting powder was dry pressed to ensure intimate contact. The mixtures were calcinated at 350°C for 45 min to remove the moisture and then sintered at 900°C (Barium borosili ^¬ cate) or 950°C (Strontium borosilicate) for 5 hours in air atmosphere in an electric oven. Alternatively the re ^¬ spective oxides (SrO, BaO, B ₂ 0 ₃ , S1O2, and Mn02) can be used as precursor materials. Instead of Mn0 ₂ other manga ^¬ nese oxides such as Mn ₂ <3 ₃ can be also used as precursor for the Mn doping. Alternatively the sintering can be performed in a reducing atmosphere such as carbon monox ^¬ ide (CO) gas.

The raw materials for the synthesis of Mn doped SrB ₂ Si ₂ 0s or BaE> ₂ Si ₂ 08 are cheap and readily available. In addition sintering of mixed oxides is a low cost process compared to K ₂ SiF6 which is synthesized by a co-precipitation method. Furthermore the possibility of low temperature (<1000°C) sintering also results in a cost advantage. Therefore the phosphor materials of this invention are cost competitive and well suited for production on an in ^¬ dustrial scale.

The inventive phosphor material can be applied to a white light emitting LED comprising a semiconductor chip as a light source and various red green blue (RGB) light emit ^¬ ting phosphor materials. A schematic cross section of an SMT (surface mount technology) LED 10 is shown in Fig. 2. An SMT LED 10 consists of one or more semiconductor chips (or dies) 4 as a light source, a housing 3 as a means for mechanical stress protection and for light reflection, a lead frame 2 as a means for electrical connection, ther ^¬ mal dissipation and light reflection, and a transparent encapsulation 5 for stress buffering, protection against mechanical impact, and corrosion in ambient atmosphere. In addition to its protection function the encapsulant 5 reduces light coupling losses and directs the LED light towards a specific viewing angle while acting as a prima ^¬ ry lens. Materials for encapsulation 5 may be polymers such as epoxy resins, epoxy/silicone/hybrids , silicone resins or glasses. The housing 3 can be formed by a mold material and partially surrounds the lead frame 2, which is attached to a printed circuit board (PCB) 1. The light emitting part of semiconductor chip 4 depending of the desired wavelength of the emitted light can be based on any suitable III-V, II-VI or IV-IV semiconductor layer and usually has an emission wavelength of about 250 nm to 550 nm. In particular the semiconductor chip may contain at least one semiconductor layer comprising GaN, ZnSe, or SiC. Alternatively any appropriate radiation source can be used including but not limited to organic light emitting diodes (OLED) .

The die attachment 8 of semiconductor chip 4 to the lead frame 2 can be done by glueing or soldering. Metal parti ^¬ cles (e.g. silver or gold) may be added to the die at- tachment glue for higher electrical conductivity. A bond wire 7 for electrical connection is attached to the semi ^¬ conductor chip 4 and to an electrically separated posi ^¬ tion 9 of the lead frame 2.

Generally one or more phosphor material types are incor ^¬ porated within the encapsulant 5 and/or alternatively are located as a conversion layer 6 at the light emitting surface of the semiconductor chip. Typically the phosphor materials absorb the ultraviolet (UV) or blue light emit ^¬ ted from the light source and convert it by luminescence into light of the visible spectrum e.g. red, green or blue. The phosphor material of this invention which is emitting in the red part of the visible light spectrum combined with other phosphor materials emitting e.g. in the green, blue or red part of the visible light spectrum may result in a white light emitting LED. Alternatively several semiconductor chips 4 containing different light emitting layers or different conversion layers 6 can be mounted as multiple light sources on the lead frame 2 in one housing 3. These multiple light sources emit light of different colors, e.g. red, green, blue. As a consequence by mixing the individual light colors such multicolor LEDs also can emit white light. Recently LEDs are frequently used in backlighting appli ^¬ cations for liquid crystal display (LCD) technology in ^¬ stead of cold cathode fluorescent (CCFL) backlighting. A schematic cross section of direct backlighting with LEDs is shown in Fig. 3. A LED based back light unit (BLU) module 12 in principle consists of one or more LED light sources 10 that are attached to a printed circuit board (PCB) 1, which is located at the bottom of a reflector box 13. The LEDs are evenly distributed on the PCB 1 within the reflector box 13. The reflector box 13 with reflecting bottom and side walls 14 is attached to an LCD screen 11. The reflector box 13 serves to mix and homoge ^¬ nize the light. In the reflector box 13 between the LEDs 10 and the LCD screen 11 a layer diffusor 15 and an optical film 16 are located. The diffusor 15 is a means to diffuse the light evenly across the surface of the LCD screen 11, while the optical film 16 serves to increase and homogenize the brightness. As an LED light source 10 white light emitting LEDs or alternatively single color

LEDs can be used for backlighting applications. If single color LEDs are used the individual colors are mixing into white light in the reflector box. While only certain features have been illustrated and de ^¬ scribed herein, many modifications and changes will occur to those skilled in the art. It is therefore to be under ^¬ stood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of this invention.

REFERENCE SYMBOLS

1 Printed circuit board (PCB)

2 Lead frame

3 Housing

4 Semiconductor chip (light source)

5 Encapsulation

6 Conversion layer

7 Bond wire

8 Die attachment

9 Wire bonding position on lead frame

10 Light emitting diode (LED)

11 Liquid crystal display (LCD)

12 Backlight unit (BLU) module

13 Reflector box

14 Reflecting bottom and side walls

15 Diffusor

16 Optical film