The ability to maintain a controlled ambient condition for a prolonged period, such as a low pressure or vacuum, in a microelectronic package is increasingly being sought in such diverse areas as display technologies, micro-electro-mechanical systems (MEMS) and high density storage devices. Computers, displays, and personal digital assistants as well as cellular phones may all incorporate such devices utilizing a controlled ambient condition. Vacuum packaged devices may utilize electrons to traverse some gap, for example, to excite a phosphor in the case of displays, or to modify a medium to create bits in the case of storage devices.
One of the major problems with vacuum packaging of electronic devices is the continuous outgassing of hydrogen, water vapor, carbon monoxide, and other components found in air, and from the internal components of the electronic device. Typically, to minimize the effects of outgassing one uses gas-absorbing materials commonly referred to as getter materials. Generally a separate cartridge, ribbon, or pill incorporates the getter material that is then inserted into the electronic vacuum package. In addition, before the cartridge or cartridges are sealed within the vacuum package, in order to maintain a low pressure over the lifetime of the vacuum device, a sufficient amount of getter material must be contained within the cartridge or cartridges.
Providing an auxiliary compartment situated outside the main compartment is one alternative others have taken. The auxiliary compartment is connected to the main compartment such that the two compartments reach largely the same steady-state pressure. Although this approach provides an alternative to inserting a ribbon or cartridge inside the vacuum package, it still results in the undesired effect of producing either a thicker or a larger package. Such an approach, typically, leads to increased complexity, greater difficulty in assembly as well as generally a larger package size. For small electronic devices with narrow gaps, the bulkier package may be especially undesirable in many applications, such as those used in a mobile environment. In addition, the utilization of a separate compartment increases the cost of manufacturing because it is a separate part that requires accurate positioning, mounting, and securing to another component part to prevent it from coming loose and potentially damaging the device.
Depositing the getter material on a surface other than the actual device surface such as a package surface is another alternative approach taken by others. For example, a uniform vacuum may be produced by creating a uniform distribution of pores through the substrate of the device along with a uniform distribution of getter material deposited on a surface of the package. Although this approach provides an efficient means of obtaining a uniform vacuum within the vacuum package, it will also typically result in the undesired effect of producing a thicker package. The thicker package is required because of the need to maintain a reasonable gap between the bottom surface of the substrate and the top surface of the getter material to allow for reasonable pumping action. In addition, yields typically decrease due to the additional processing steps necessary to produce the uniform distribution of pores.
In all of these approaches, typically, either the entire packaged device is heated to the activation temperature of the getter material used, or electrical connections are provided to heat the getter material. In the former approach all of the components and materials utilized in the packaged device must be able to withstand the activation temperature of the getter material. In the latter approach, the additional electrical connections and electrical traces required to heat the getter material result in even more added complexity.
If these problems persist, the continued growth and advancements in the use electronic devices, in various electronic products, seen over the past several decades, will be reduced. In areas like consumer electronics, the demand for cheaper, smaller, more reliable, higher performance electronics constantly puts pressure on improving and optimizing performance of ever more complex and integrated devices. The ability, to optimize the gettering performance of getters may open up a wide variety of applications that are currently either impractical, or are not cost effective. As the demands for smaller and lower cost electronic devices continues to grow, the demand to minimize both the die size and the package size will continue to increase as well.
a is cross-sectional view of a device according to an embodiment of the present invention;
b is an expanded cross-sectional view of a photomask according to an alternate embodiment of the present invention;
c is an expanded cross-sectional view of a photomask according to an alternate embodiment of the present invention;
d is an expanded cross-sectional view of a photomask according to an alternate embodiment of the present invention;
e is an expanded cross-sectional view of a thermally isolated getter structure according to an alternate embodiment of the present invention;
f is an expanded cross-sectional view of a getter structure according to an alternate embodiment of the present invention;
g is an expanded cross-sectional view of a thermally isolated getter structure according to an alternate embodiment of the present invention;
a is a cross-sectional view of a device according to an alternate embodiment of the present invention;
b is an expanded cross-sectional view of a photomask according to an alternate embodiment of the present invention;
a is a plan view of a photomask misaligned with a substrate having getter structures disposed thereon according to an embodiment of the present invention;
b is a plan view of the photomask shown in
c is a plan view of the photomask shown in
The present embodiments of this invention are directed to devices utilizing a getter structure. For example, getter activation in a vacuum packaged device, typically, involves heating the entire device to a high temperature. Generally, a compromise is made between balancing the desire to heat the getter to a high temperature and the desire to maintain the viability of the semiconductor devices, all while maintaining the integrity of the vacuum seal or bond. Such a compromise is particularly desirable in those devices that include active semiconductor devices and activate the getter by heating the entire device. The present invention utilizes a photomask disposed between a photon source and the getter structure to selectively expose the getter structure to radiation while masking out areas having circuitry, other materials, or devices that are sensitive to high temperatures. In addition, thermal isolation structures such as a cavity formed under the getter structure, a serpentine structure, or a trench isolation structure surrounding the getter structure also may be incorporated into the vacuum device to further reduce the spread of heat out of the getter structure. In this manner a getter structure may be selectively activated to a high temperature while minimizing thermal degradation or damage to devices, materials, and other components that are in close proximity to the getter structure. Typically, the temperatures used to activate a getter such as a zirconium aluminum alloy are upwards of 900 to 1000° C. or for a zirconium vanadium iron alloy temperatures of 300 to 450° C.; these temperatures may be incompatible with circuitry such as various doped structures, or are incompatible with various polymeric materials or may cause delamination or cracking due to thermal expansion mismatches. The selective activation of a getter structure utilizing a photomask allows for increases in integration, improved functionality and lower cost.
An embodiment of device 100 of the present invention, in a cross-sectional view, is shown in
It should be noted that the drawings are not true to scale. Further, various elements have not been drawn to scale. Certain dimensions have been exaggerated in relation to other dimensions in order to provide a clearer illustration and understanding of the present invention.
In addition, although some of the embodiments illustrated herein are shown in two dimensional views with various regions having depth and width, it should be clearly understood that these regions are illustrations of only a portion of a device that is actually a three dimensional structure. Accordingly, these regions will have three dimensions, including length, width, and depth, when fabricated on an actual device. Moreover, while the present invention is illustrated by various embodiments, it is not intended that these illustrations be a limitation on the scope or applicability of the present invention. Further it is not intended that the embodiments of the present invention be limited to the physical structures illustrated. These structures are included to demonstrate the utility and application of the present invention in presently preferred embodiments.
In the embodiment shown in
Photons emitted from a photon source (not shown) such as a laser impinge upon photomask 150. Any of a number of different photon sources may be utilized in the present invention. For example, various lasers such as semiconductor diode lasers, carbon dioxide lasers, ultraviolet lasers or neodymium YAG lasers all may be utilized. In addition, non-laser sources such as infrared lamps also may be utilized. Those photons (i.e. transmitted photons 110) incident on transmissive region 152 pass through photo mask 150 and impinge upon, and are absorbed by, getter structure 140. Those photons incident on non-transmissive region 153 are either reflected for those embodiments utilizing a reflective photomask, absorbed for those embodiments utilizing an absorbing photomask, or are destroyed by destructive interference or canceled for those embodiments utilizing a quarter wavelength dichroic filter or grating. In alternate embodiments, combinations of the various types of masks also may be combined and utilized in a single mask.
An example where photomask 150 includes reflective region 154 is illustrated in an expanded cross-sectional view in
An example where photomask 150 includes regions of destructive-interference 155 is illustrated, in an expanded cross-sectional view, in
An example where photomask 150 includes absorptive regions 156 is illustrated in an expanded cross-sectional view in
Substrate 120, in this embodiment, is an aluminum oxide substrate; however, substrate 120 may be formed from a wide range of materials including ceramics, metals, various semiconductor materials such as silicon, gallium arsenide, indium phosphide, germanium; various glasses such as any of the borosilicate, soda lime or quartz glasses (including crystalline and amorphous glasses) as well as silicon oxides, nitrides, and silica mixed with oxides of, for example, potassium, calcium, barium or lead; other various ceramics such as boron nitride, silicon carbide, and sapphire. In this embodiment substrate 120 may be any suitable material including polymeric materials having the desired thermal properties to withstand the activation temperature of getter structure 140 without suffering substantial degradation or damage.
Sealed package 102, in this embodiment includes a vacuum seal formed between sealing plate 160 and substrate 120. In alternate embodiments, sealed package 102 may be formed, for example, between a package cover and a chip carrier on which a chip or chips are mounted, or between two chip carriers each having a chip or chips mounted thereto. Any packaging arrangement providing a controlled environment for device 100 to operate in may be utilized in the present invention. For example, a vacuum package forming a vacuum environment in which electron emitters may be utilized for displays and storage devices. Another example is the use of a vacuum environment to reduce gas viscosity damping of a mechanical resonator. In still other embodiments, sealed package 102 also may be an enclosure providing fluid flow for other applications such as, for example, micro turbines, fuel cells, chemical reactors, and catalytic fuel crackers. Further, sealed package may provide an enclosure to hold a particular gas or liquid such as a micro-mirror display or other micro-mover device.
Substrate bond structure 137 is disposed on substrate 120. Depending on the particular sealing technology utilized, substrate bond structure 137 may be formed on substrate 120 directly or it may be formed on a compatible layer or film that is formed on substrate 120. Sealing plate bond structure 161 is disposed on sealing plate 160. Again depending on the particular sealing technology utilized, Sealing plate bond structure 161 may be formed on sealing plate 160 directly or it may be formed on a compatible layer or film that is formed on sealing plate 161. Sealing plate bond structure 161 and substrate bond structure 137 form package seal 103 forming interspace region 114. In this embodiment, substrate bond structures 137 and sealing plate bond structures 161 may utilize a wide variety of materials depending on the particular sealing technology utilized. For example, a gold-silicon eutectic for bonding may be utilized to bond substrate 120 to sealing plate 160 if substrate 120 is a silicon substrate. A softer lower melting-point solder also may be utilized if substrate 120 is, for example, a silicon, glass, or other inorganic material. In alternate embodiments, a frit glass seal may be utilized to form sealed package 102. In still other embodiments, package seal 103 may be made by a variety of techniques such as, for example, thermal compression bonding or brazing, as well as other techniques.
The material utilized for the bond structures will depend on the particular materials utilized for substrate 120, and sealing plate 160. In those embodiments utilizing a chip carrier, various ceramic materials including various glasses as well as metals may be utilized to form one or both of the carriers, however, at least one carrier should either be transmissive to the photon energy utilized to activate the getter structure or structures, or have transmissive regions aligned with the getter structure or structures formed in at least one carrier. The particular material utilized to form both a carrier as well as the bond structures will depend on, for example, the desired pressure to be maintained; the temperature and humidity and other environmental factors to which the device will be exposed; and the amount of stress that may be imparted to the device as a result of the packaging process; as well as, the particular sealing technology to be utilized.
Anodic bonding may be utilized to attach device 100 made on a silicon substrate to the sealing plate either made out of glass or having a glass surface to bond to the silicon. The silicon surface of the substrate and, for example, the glass surface of the sealing plate are placed between two electrodes applying an appropriate polarity voltage across the interface of the two materials. The particular bonding process will depend on various parameters such as the magnitude and duration of the applied voltage, the temperature of the two surfaces during the bonding process, and the area to be bonded. Getter material may also be applied or deposited on various portions of sealing plate 160 (as shown in
An alternate embodiment of a getter structure utilized in a device of the present invention is shown, in a cross-sectional view, in
Getter structure 140 includes first major getter surface 145 facing away from first major substrate surface 123 and second major getter surface 146 facing toward first major substrate surface 123. Cavity 128 provides a path for molecules or atoms to impinge upon both first and second major getter surfaces 145 and 146 of getter structure 140, thus, increasing the exposed surface area available for pumping residual molecules or atoms. The increased surface area provides an increase in the effective pumping speed of getter structure 140. In addition, cavity 128 also provides thermal isolation of getter structure 140 from substrate 120. Heat generated within getter structure 140, typically, may be lost through radiation, convection, or through thermal conduction along the length of suspended mass portion 142 to the thermally coupled portion. In this embodiment, getter structure 140 is illustrated in
An alternate embodiment of a getter structure utilized in a device of the present invention is shown, in a cross-sectional view, in
An alternate embodiment of a getter structure utilized in a device of the present invention is shown, in a cross-sectional view, in
An alternate embodiment of the present invention is shown in
Substrate 220, in this embodiment, is a mono-crystalline silicon substrate; however, any substrate suitable for forming electronic devices, such as germanium, zinc selenide, silicon carbide, gallium arsenide, indium phosphide, glass, and sapphire are a just a few examples that also may be utilized. In addition materials such as magnesium fluoride, and cryolite, and various glasses such as any of the borosilicate, soda lime or quartz glasses (including crystalline and amorphous glasses) as well as silicon oxides, and silica mixed with oxides of, for example, potassium, calcium, barium or lead also may be utilized. For those embodiments where the photons used to activate the getter structure are transmitted through the substrate, the substrate may include any suitable material having sufficient transmittance in the wavelength region of photons utilized to provide sufficient heat to activate the getter structure as well as having thermal properties sufficient to withstand the activation of the getter structure without suffering substantial degradation or damage. The present invention is not intended to be limited to those devices fabricated in silicon semiconductor materials, but will include those devices fabricated in one or more of the available semiconductor materials and technologies known in the art, such as thin-film-transistor (TFT) technology using polysilicon on glass substrates. Further, the substrate is not restricted to typical wafer sizes, and may include processing a sheet or film, for example, a single crystal sheet or a substrate handled in a different form and size than that of conventional wafers or substrates. The actual substrate material utilized will depend on various system components such as the particular environment to which the device will be subjected, the presence or absence of active devices, the pressure to be maintained within the device, as well as the expected lifetime of the device.
Examples of getter materials that may be utilized in the present invention include zirconium, thorium, hafnium, vanadium, yttrium, niobium, tantalum, molybdenum, terbium, and mixtures thereof. In the embodiments shown in
In this embodiment, any of the photon sources described above that emit in the infrared region of the electromagnetic spectrum from about 1.2 micrometer in wavelength to about 10 micrometers in wavelength such as a carbon dioxide laser or solid state lasers may be utilized. Photomask 250, in this embodiment, may be any of the photomasks described above. For example, photomask 250 may be a reflective type mask such as gold or aluminum deposited on second major substrate surface 224 of substrate 220; however, in alternate embodiments photomask 250 may be disposed on any other layer or layers, formed on the substrate. In still other embodiments, as illustrated in the expanded cross-sectional view shown in
Sealed package 202, in this embodiment includes a seal formed between package component 260 and substrate 220. Substrate bond structure 237 is disposed on substrate 220 and package bond structure 261 is disposed on package component 260. Package bond structure 261 and substrate bond structure 237 form package seal 203 forming interspace region 214. This embodiment may utilized any of the package sealing structures, materials and techniques described above in
A flow diagram of a method of activating a getter structure enclosed in a device package, according to an embodiment of the present invention, is shown in
For those embodiments utilizing a photomask disposed away from the device package photomask illuminating process 390 also may utilize a positioning process as illustrated, in a plan view, in
The rotation about axis 451 is for illustrative purposes only; any suitable axis of rotation of photomask 450 may be utilized. In addition, any suitable axis of rotation of device 400 also may be utilized where device 400 is rotated with respect to photomask 450. Movement in some combination of X and Y may be utilized to reduce or limit any misalignment in the plane formed by the two mutually orthogonal directions as illustrated in going from
Photon transmission process 392 is utilized to provide selective illumination of a getter structure. Illumination of a photomask utilizing both transmissive and non-transmissive regions provides for selectively exposing the getter structure to radiation via transmission of photons through the transmissive region while masking out or preventing transmission of photons in areas having circuitry, other materials, or devices that are sensitive to high temperatures. In this manner, by reducing or limiting the temperature excursion experienced in more sensitive regions, a getter structure may be selectively heated to a high temperature while minimizing thermal degradation or damage to devices, materials, and other components that are in close proximity to the getter structure. Generally, the temperatures used to activate a getter such as a zirconium aluminum alloy are upwards of 900 to 1000° C. For a zirconium vanadium iron alloy temperatures of 300 to 450° C. may be used. Such temperatures may be incompatible with circuitry such as various doped structures, or may be incompatible with various polymeric materials or may cause delamination or cracking due to thermal expansion mismatches. The localized heating of the getter structure in the present invention reduces or may eliminate these problems.
Those photons incident on a transmissive region pass through the photo mask and impinge upon a getter structure. Photons incident on a non-transmissive region are not transmitted through the photomask and do not contribute to heating either the substrate or the getter structure as illustrated in
For those embodiments utilizing a grating mask or a mask having absorption regions with similar partially transmissive regions may be formed in the mask to provide selective heating of various portions of the substrate. An example of such a grating mask is illustrated in
Photon absorbing process 394 is utilized to selectively provide heat to a getter structure. After transmission through the transmissive regions of the photomask the incident photons will either pass through the substrate, or the sealing plate, or other structure depending on the particular arrangement utilized, and impinge on the getter structure where they are absorbed. Generally the getter material utilized to form the getter structure will have a high absorption coefficient, however in those embodiments where additional absorption is desired an absorption layer also may be utilized as illustrated in
Getter structure heating process 396 is utilized to selectively heat a getter structure to its activation temperature and will depend on the particular getter material utilized. For example, a zirconium aluminum alloy may be activated at temperatures in the range from about 900° C. to about 1000° C. For a zirconium vanadium iron alloy temperatures from about 300° C. to about 450° C. may be used. Zeolites may be activated by heating to temperatures from about 100° C. to about 300° C. In addition, the activation of a getter material is generally temperature time dependent. The lower the temperature used to activate the getter the longer the getter material is held at that temperature. The particular activation temperature utilized will depend not only on the getter material used, but also on other factors such as the substrate material (in particular the thermal conductivity of the substrate) used, the presence or absence of temperature sensitive circuitry, materials, or devices in the vicinity of the getter structure, and the presence or absence of thermal isolation structures utilized with the getter structure. The longer the getter structure is heated the greater will be the heat load to other parts of the device due to thermal conduction, radiation, and convection. In addition, getter structure heating process 396 may also include reactivating the getter structure at a specified pressure or after or at a specified time from the previous activation.
A flow diagram of a method of manufacturing a getter structure enclosed in a device package, according to an embodiment of the present invention, is shown in
For those embodiments in which the getter structure is formed on a thermally isolated structure various wet or dry etchants may be utilized along with conventional photolithography techniques. For example, a dry etch may be used when vertical or orthogonal sidewalls are desired. Sacrificial materials also may be deposited on the top and bottom surfaces of the getter structure. The sacrificial materials such as amorphous silicon may be removed selectively by utilizing a xenon difluoride or sulfur hexafluoride plasma etch to create free-standing thermally isolated getter structures. Alternatively an anistropic wet etch such as potassium hyrdoxide (KOH) may be used to etch a (110) oriented silicon wafer to also produce vertical sidewalls. Further, the use of an anisotropic wet etch such as KOH or tetra methyl ammonium hydroxide (TMAH), may be utilized to etch a (100) oriented silicon wafer to produce various structures with sloped side walls generated by the slower etch rate of the (111) crystallographic planes. In still other embodiments, combinations of wet and dry etch may also be utilized when more complex structures are desired. Further, other processes such as laser ablation, reactive ion etching, ion milling including focused ion beam patterning may also be utilized to form a thermally isolating structure.
Photomask creation process 793 is utilized to create or form and mount a photomask to a device package surface. Any of a wide range of techniques may be utilized to create the photomask. For example, the photomask may be desposited on a device package surface such as the back side or second major substrate surface utilizing various deposition techniques such as sputter deposition, chemical vapor deposition, plasma enhanced chemical vapor deposition, evaporation, or other vapor deposition techniques. In addition, other deposition techniques such as electrodeposition, or laser activated deposition also may be utilized. Generally for both metal reflective and quarter wave dielectric reflective masks, as well as antireflective layers any of the deposition techniques utilized to deposit metals or dielectrics may be used. Both additive and subtractive processes may be utilized to form the desired pattern of the mask. For those embodiments utilizing a grating mask the grating material may be deposited on the device package surface and then the grating etched into the surface utilizing conventional patterning and etching techniques. In still other embodiments the grating may be etched into the device package surface directly without an additional deposition step. Besides the more conventional techniques of depositing an inorganic dielectric, noted above, with subsequent patterning; grating masks also may be fabricated by coating or solution casting a polymer such a polycarbonate or polymethylmethacrylate on the device package surface followed by various patterning techniques including micromolding. Another example is where the mask is formed separate as a free-standing mask and then is attached to a device package surface utilizing an adhesive or a mask release layer. In still other embodiments where the photomask is a specified distance away from the device package surface any of the wide variety of techniques used to form and pattern photomasks may be utilized.
Device package sealing process 795 is utilized to seal and enclose the getter structure in a device package. Any of a wide range of techniques may be utilized to seal a device package. For example to bond a silicon die to a ceramic package or metal can a gold-silicon etutectic or a softer lower melting point solder, may be utilized. The particular sealing or bonding material as well as the sealing technique will depend on various factors such as, on the desired pressure to be maintained in the enclose region of the getter, on the temperature and humidity and other environmental factors to which the micro-fabricated device will be exposed, and on the amount of stress that may be imparted to device as a result of the sealing process. Thermal compression bonding, brazing, and anodic bonding, are just a few of the many techniques that may be utilized. A low melting-point inorganic oxide glass such as, lead oxide or boric oxide also may be used to form the bond structures used to generate a sealed package. In still other embodiments, anodic bonding may be utilized to attach a silicon substrate to the sealing plate either made out of glass or having a glass surface to bond to the silicon. The silicon surface of the substrate and, for example, the glass surface of the sealing plate are placed between two electrodes applying an appropriate polarity voltage across the interface of the two materials. A frit glass seal may be utilized to form a sealed package. In addition, various adhesives and adhesive structures also may be utilized.
Getter structure activating process 797 is utilized to activate the getter material and may be any of the processes described above for heating the getter material to its activation temperature.