WO2012023899A1

WO2012023899A1 - Hermetic seal and method of manufacture thereof

Info

Publication number: WO2012023899A1
Application number: PCT/SG2010/000300
Authority: WO
Inventors: Daquan Yu; Chengkuo Lee; Cheryl Sharmani Selvanagam; Soon Wee Ho
Original assignee: Agency For Science, Technology And Research
Priority date: 2010-08-16
Filing date: 2010-08-16
Publication date: 2012-02-23

Abstract

According to an embodiment, a hermetic seal is provided. The hermetic seal may include: a first adhesive layer; a second adhesive layer; a first joint structure; a second joint structure; and a third joint structure disposed between the first joint structure and the second joint structure. The material of the third joint structure may have a higher melting temperature than the material of the first joint structure and the second joint structure. The first joint structure, the second joint structure and the third joint structure may be respectively disposed on a common plane which is parallel to both the first adhesive layer and the second adhesive layer. The first joint structure, the second joint structure and the third joint structure may form a joint arrangement that is sandwiched between the first adhesive layer and the second adhesive layer.

Description

HERMETIC SEAL AND METHOD OF MANUFACTURE THEREOF

Technical Field

[001] Various embodiments relate to a hermetic seal and method of manufacture thereof.

Background

[002] Hermetic seals are required to make devices impervious to external influences, such as moisture, and find applications in fields such as optoelectronics.

[003] Hermetic sealing using solder alloy is performed by two methods: low temperature solder seal and low temperature diffusion bonding. The main difference is that for the first method, after reflow seal is achieved by low temperature solder joint; for the latter method, the seal is composed of high temperature intermetallic compounds (IMC) phases and/or high temperature components.

[004] For good hermeti city and mechanical properties, a hermetic seal using low temperature solder joint is preferred. However, the low temperature solder joint is unstable under high temperature conditions.

[005] To have a stable hermetic seal, even if it is exposed under a high temperature process such as reflow for assembly, the IMC seal is preferred as it can withstand high temperature conditions. However, the IMC seal is more susceptible to mechanical faults such as cracks, which propagate fast as soon as they form.

[006] In addition, for an IMC seal to provide good hermeticity and reliability, it has to be formed without voids. It has been found difficult to achieve void free joints.

Known ways of achieving void free joints involve using suitable bonding parameters (e.g. temperature, pressure and time), but these parameter windows are narrow.

[007] There is thus a need to provide a hermetic seal that addresses the above limitations.

Summary

[008] According to an embodiment, a hermetic seal is provided. The hermetic seal may include: a first adhesive layer; a second adhesive layer; a first joint structure; a second joint structure; and a third joint structure disposed between the first joint structure and the second joint structure. The material of the third joint structure may have a higher melting temperature than the material of the first joint structure and the second joint structure. The first joint structure, the second joint structure and the third joint structure may be respectively disposed on a common plane which is parallel to both the first adhesive layer and the second adhesive layer. The first joint structure, the second joint structure and the third joint structure may form a joint arrangement that is sandwiched between the first adhesive layer and the second adhesive layer.

[009] Various embodiments provide a composite seal joint with good hermeticity, whereby the composite seal joint is stable and reliable under high temperature exposure (for example, 260-400°C). Various embodiments provide for composite seal joints that may have better hermeticity and reliability compared to pure intermetallic compounds (IMCs) joints. In addition, low costs materials such as copper (Cu) and tin (Sn) may be used to fabricate the composite seal joint. Various embodiments find applications in low temperature bonding process (for example, the bonding temperature can be 180°C for Snln solder; while the bonding temperature can be 280°C for Sn solder) of semiconductor packaging technology, in particular, to achieve chip to chip, chip to wafer and wafer to wafer hermetic bonding.

[010] In the context of various embodiments, the term "hermetic seal" may mean an airtight seal. A seal may include structures that encapsulate a semiconductor device [such as an optoelectronic device or a Micro-Electro-Mechanical Systems (MEMS)/Nano-Electro-Mechanical Systems (NEMS) structure] to protect the encapsulated device from atmospheric conditions, such as moisture.

[011] In the context of various embodiments, the term "adhesive layer" means a layer wettable by material used to form a seal on or over the layer, allowing the seal to adhere to a respective surface of the layer. To increase the wettability, noble metals such as Au, Pt, Pd or Ag can be deposited on the adhesive layer. The adhesive layer may be needed as a substrate, on which the adhesive layer is formed, may not be wettable by the material used to form the seal. Thus, the adhesive layer may serve to allow the seal to be formed over the substrate. The adhesive layer may also act as a barrier layer that prevents fast diffusion in the material used to form the seal.

[012] In the context of various embodiments, the joint structures may be arbitrarily termed a "first joint structure", a "second joint structure" and a "third joint structure". In the context of various embodiments, the term "joint arrangement" may be used to collectively refer to the "first joint structure", the "second joint structure" and the "third joint structure". In the context of various embodiments, the term "joint" may mean any structure which may facilitate connection between two separate layers. With the joint extending to create a perimeter around a semiconductor device, located between the two separate layers, the joint forms a seal to protect the semiconductor device. In the context of various embodiments, each of the first, second and third joint structures provide a seal, so that the three seals form a composite seal joint.

[013] In various embodiments, material used for the third joint structure may be different from that used for either the first joint structure or the second joint structure. In various embodiments, the same material may be used for both the first joint structure and the second joint structure. In other embodiments, different material may be used for the first joint structure and the second joint structure.

[014] In various embodiments, the third joint structure may be disposed between the first joint structure and the second joint structure, so that the first joint structure and the second joint structure may be adjacent to opposite sides of the third joint structure, with the three joint structures being located on a common plane. In various embodiments, the common plane may be parallel to a first plane where the first adhesive layer lies on and a second plane where the second adhesive layer lies on.

[015] In the context of various embodiments, the term "parallel" may mean that the common plane, to which the first joint structure, the second joint structure and the third joint structure are disposed, does not intersect with the planes to which the first adhesive layer and the second adhesive layer respective lie on. The common plane and the planes to which the first adhesive layer and the second adhesive layer respective lie on may or may not be flat. In various embodiments, the joint arrangement of the three joint structures may form a layer between the first adhesive layer and the second adhesive layer, the joint arrangement layer being parallel to the first adhesive layer and the second adhesive layer.

[016] In the context of various embodiments, the term "sandwiched" may mean that the joint arrangement may be disposed or located between the first adhesive layer and the second adhesive layer.

[017] In various embodiments, one or more suitable layers may be present between the first adhesive layer and a respective surface of the joint arrangement. Similarly, one or more suitable layers may be present between the second adhesive layer and a respective surface of the joint arrangement. However, in other embodiments, the joint arrangement may be in contact with both the first adhesive layer and the second adhesive layer, i.e. there are no intermediate layers between either of the first and second adhesive layers and a respective surface of the joint arrangement. [018] In various embodiments, only a portion of a respective surface of the joint arrangement, sufficient to form a seal, may be in contact with a facing surface of the respective first adhesive layer or second adhesive layer. Since the joint arrangement includes the first joint structure and the second joint structure, in various embodiments, either a portion of a respective surface of the first joint structure; a portion of a respective surface of the second joint structure; or both may be in contact with a respective facing surface of the first adhesive layer or the second adhesive layer.

[019] In other embodiments, an entire respective surface of the joint arrangement may be in contact with a respective facing surface of the first adhesive layer or the second adhesive layer, so that one surface of the joint arrangement may form an interface with the first adhesive layer, while an opposite surface of the joint arrangement may form an interface with the second adhesive layer. Since the joint arrangement includes the first joint structure and the second joint structure, in various embodiments, an entire respective surface of the first joint structure; an entire respective surface of the second joint structure; or both may be in contact with a respective facing surface of the first adhesive layer or the second adhesive layer. Thus, in various embodiments, one surface of each joint structure may form an interface with the first adhesive layer, while an opposite surface of each joint structure may form an interface with the second adhesive layer.

[020] In various embodiments, the first joint structure and the second joint structure may be in contact with opposite sides of the third joint structure.

[021] In various embodiments, a perimeter of the joint arrangement may be located within the perimeter of the first adhesive layer and the perimeter of the second adhesive layer. In the context of various embodiments, the term "perimeter" may include an outer perimeter and an inner perimeter of either the joint arrangement, the first adhesive layer or the second adhesive layer.

[022] In various embodiments, the first adhesive layer and the second adhesive layer may be any one or more of Ti, Cr, W, Ni, Cr, Pd, Pt and V.

[023] In various embodiments, the third joint structure may have at least one intermetallic compound. The intermetallic compound may be formed from any one or more of Cu, Au, Ag, Ni, Pt, Pd, Ti, Cr and any one or more of Sn, In or alloys InSn, SnBi, SnCu, SnAg alloy. [024] In various embodiments, the first and the second joint arrangements may be solder. The solder may be any one or more of Sn, In, or alloys of InSn SnBi, SnCu or SnAg alloy.

[025] In various embodiments, a thin intermetallic layer may be formed on the first and the second adhesive layers. Thus, there may be an intermetallic compound layer between the first adhesive layer and the first and the second joint structures (i.e. a first intermetallic compound layer acting as an interface between the first adhesive layer and the first joint structure; and a second intermetallic compound layer acting as an interface between the first adhesive layer and the second joint structure). In addition, there may be an intermetallic compound layer between the second adhesive layer and the first and the second joint structures (i.e. a third intermetallic compound layer acting as an interface between the second adhesive layer and the first joint structure; and a fourth intermetallic compound layer acting as an interface between the second adhesive layer and the second joint structure). The first to fourth intermetallic compound layers may have the same or different compositions from each other.

[026] In various embodiments, each of the joint structures may have about the same thickness.

[027] In various embodiments, there may be a first substrate upon which the first adhesive layer is formed; and a second substrate upon which the second adhesive layer is formed. In the context of various embodiments, the term "substrate" may refer to bulk semiconductor material forming a base material for fabricating electronics thereon or therein.

[028] A semiconductor package may be provided, which includes the hermetic seal of any one of the various embodiments. The semiconductor package may further include a MEMS device surrounded by the hermetic seal of any one of the various embodiments.

[029] According to an embodiment, a method of forming a hermetic seal is provided. The method may include: providing a first adhesive layer; providing a second adhesive layer; and forming a joint arrangement between the first adhesive layer and the second adhesive layer. The joint arrangement may include: a first joint structure; a second joint structure; and a third joint structure disposed between the first joint structure and the second joint structure. The material of the third joint structure may have a higher melting temperature than the material of the first joint structure and the second joint structure. The first joint structure, the second joint structure and the third joint structure may be respectively disposed on a common plane which is parallel to both the first adhesive layer and the second adhesive layer.

[030] In various embodiments, forming the joint arrangement may include: providing a layer arrangement on the first adhesive layer. The layer arrangement may include: a first layer formed on the first adhesive layer; and a second layer formed over the first layer. The material of the first layer may have a higher melting temperature than the material of the second layer. The layer arrangement on the first adhesive layer may be bonded to the second adhesive layer to create the third joint structure, the third joint structure being a compound including a portion of the first layer and a portion of the second layer. A remaining portion of the second layer may form the first and second joint structures. In various embodiments, the third joint structure may include the entire material of the first layer. In various embodiments, the third joint structure may further include a portion of the second adhesive layer.

[031] In various embodiments, forming the joint arrangement may include: providing a layer arrangement on the first adhesive layer. The layer arrangement may include: a first layer formed on the first adhesive layer; and a second layer formed over the first layer. The material of the first layer may have a higher melting temperature than the material of the second layer. A layer arrangement may be provided on the second adhesive layer. The layer arrangement may include: a first layer formed on the second adhesive layer; and a second layer formed over the first layer. The material of the first layer may have a higher melting temperature than the material of the second layer. The layer arrangement on the second adhesive layer may be bonded to the layer arrangement on the first adhesive layer to create the third joint structure, the third joint structure being a compound comprising a portion of the first layer formed on the first adhesive layer, a portion of the first layer formed on the second adhesive layer and a portion of the second layer of each of the layer arrangements. A remaining portion of the second layer of each of the layer arrangements may form the first and second joint structures. In various embodiments, the third joint structure may include the entire material of the first layer.

[032] In the context of various embodiments, the layers formed on the first adhesive layer or the second adhesive layer may be arbitrarily termed a "first layer" and a "second layer". The "first layer" and the "second layer" may be materials that are suitable for forming the first joint structure, the second joint structure and the third joint structure of various embodiments. In the context of various embodiments, the term "layer arrangement" may be used to collectively refer to the "first layer" and the "second layer". In the context of various embodiments, the term "bonding" may refer to applying heat, pressure or both so that any one of wetting, diffusion, soldering occurs between the first and the second layers formed on the respective first or second adhesive layers. When the first layer and the second layer are different materials, IMCs may result from the bonding.

[033] In various embodiments, bonding of the layer arrangement may further include displacing the portion of the second layer, that forms the first and second joint structures, to opposite sides of the created third joint structure, to laterally surround the created third joint structure. In the context of various embodiments, the term "laterally" may mean a direction that is along the common plane, to which the first joint structure, the second joint structure and the third joint structure are disposed and does not intersect with the planes to which the first adhesive layer and the second adhesive layer respective lie on.

[034] In various embodiments, the second layer may cover only the first layer to which the second layer is formed on. In various embodiments, the second layer may extend to cover the adhesive layer to which the second layer is formed on.

[035] In the context of various embodiments, the term "cover" may mean that the second layer may either be formed over a portion of or an entire surface of the first layer. Further, one or more suitable layers may be present between the first layer and the second layer. Accordingly, in various embodiments, a buffer layer may be formed between the first layer and the second layer. In various embodiments, an oxidation protection layer may be formed over the first layer.

[036] In various embodiments, there may be a wetting layer formed on both the first adhesive layer and the second adhesive layer. The wetting layer increases the wettability of the first adhesive layer and the second adhesive layer. The wetting layer may include any one or more of Au, Pt, Pd or Ag.

[037] In various embodiments, compounds for the first layer and the second layer are chosen such that the created third joint structure is an intermetallic compound.

[038] In various embodiments, the first layer may include any one of Cu, Au, Ag, Ni, Pt, Pd, Ti or Cr. The second layer may include solder, wherein the solder may include any one of Sn, In, or alloys of InSn, SnBi, SnAg, SnCu. The buffer layer may include any one of Ni, Ti, Cr or Pt. The oxidation protection layer may include Au, Pt or Pd. The first adhesive layer and the second adhesive layer may include any one or more of Ti, Cr, W, Ni, Cr, Pd, Pt or V.

[039] In various embodiments, forming the joint arrangement may be performed under pressure of l-20MPa. Forming the joint arrangement may be performed at a temperature of about 20-80°C higher than the melting temperature of solder. Forming the joint arrangement may be performed for a duration of around 1 to 30 minutes.

[040] In various embodiments, the first adhesive layer may be provided on a first substrate and the second adhesive layer may be provided on a second substrate. Brief Description of the Drawings

[041] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[042] Figure 1 shows a top view of a semiconductor package.

[043] Figure 2 shows a cross-sectional view of a hermetic seal according to one embodiment.

[044] Figure 3 shows a flow chart illustrating a method of forming a hermetic seal, according to various embodiments.

[045] Figures 4 and 5 show flow charts illustrating formation of joint arrangements of a hermetic seal, according to various embodiments.

[046] Figures 6A to 6B show cross-sectional views in the manufacture of a hermetic seal in accordance to one embodiment.

[047] Figures 7A to 7B show cross-sectional views in the manufacture of a hermetic seal in accordance to one embodiment.

[048] Figures 8A to 8B show cross-sectional views in the manufacture of a hermetic seal in accordance to one embodiment.

[049] Figure 9 shows a quarter model of a hermetic seal, according to one embodiment.

[050] Figure 10 illustrates a metallization stage in the manufacture; of a hermetic seal in accordance to one embodiment.

[051] Figures 1 1A to 11C show SEM and TEM pictures of a hermetic seal according to an embodiment. [052] Figure 1 ID shows a TiSN binary phase diagram.

[053] Figures 12A to 12B show SEM pictures of a known hermetic seal.

[054] Figures 13A and 13B show results of hermeticity measurements and mechanical tests conducted on different hermetic seals, including a hermetic seal in accordance to one embodiment and known hermetic seals.

[055] Figure 14 shows simulated Mises stress profiles of joints.

Detailed Description

[056] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[057] Various embodiments provide for a hermetic seal that has both a solder seal and an IMC seal and therefore provides advantages associated with each of the solder seal and the IMC seal. Accordingly, various embodiments provide for a seal that is stable when exposed to high temperature, due to the IMC seal; and has good hermeticity and mechanical properties, due to the low temperature solder joint.

[058] Various embodiments provide a composite seal which contains a high temperature joint and a low temperature joint, wherein the high temperature joint contains intermetallic compounds formed by solder and thick metal with a rapid diffusion process, and wherein the low temperature joint part is formed by solder and a thin barrier metal with a slow inter-diffusion rate.

[059] Figure 1 shows a top view of a semiconductor package 100.

[060] The semiconductor package 100 includes a hermetic seal 200 (shown in more detail in Figure 2) according to various embodiments. The hermetic seal 200 may be provided as a seal ring, as shown in Figure 1.

[061] The semiconductor package 100 may further include a semiconductor device 102 surrounded by the hermetic seal 200. The semiconductor device 102 may include an optoelectronic device or a Micro-Electro-Mechanical Systems (MEMS)/Nano- Electro-Mechanical Systems (NEMS) structure. [062] In creating a perimeter around the semiconductor device 102, the hermetic seal 200 may thus provide an airtight seal that protects the semiconductor device 102 from atmospheric conditions, such as moisture. From Figure 1, it can be seen that the hermetic seal 200 has an outer perimeter 200o and an inner perimeter 200i. A cross- sectional view of the hermetic seal 200, taken along section X-X of Figure 1, is shown in Figure 2.

[063] From Figure 2, the hermetic seal 200 includes: a first adhesive layer 202; a second adhesive layer 204; a first joint structure 206; a second joint structure 208; and a third joint structure 210 disposed between the first joint structure 206 and the second joint structure 208. The hermetic seal 200 further includes a first substrate 216 and a second substrate 218. The first adhesive layer 202 is formed on the first substrate 216, while the second adhesive layer 204 is formed on the second substrate 218.

[064] The material of the third joint structure 210 has a higher melting temperature than the material of the first joint structure 206 and the second joint structure 208. The first joint structure 206, the second joint structure 208 and the third joint structure 210 are respectively disposed on a common plane 212 which is parallel to both the first adhesive layer 202 and the second adhesive layer 204. The first joint structure 206, the second joint structure 208 and the third joint structure 210 form a joint arrangement 214 that is sandwiched between the first adhesive layer 202 and the second adhesive layer 204. In the embodiment shown in Figure 2, each of the joint structures 206, 208 and 210 have about the same thickness. In other embodiments (not shown), each of the joint structures may have different thickness.

[065] In the embodiment shown in Figure 2, the common plane 212 to which the joint arrangement 214 is disposed, does not intersect with the planes 220 and 222 to which the first adhesive layer 202 and the second adhesive layer 204 respective lie on. The common plane 212 and the planes 220 and 222 to which the first adhesive layer 202 and the second adhesive layer 204 respective lie on may or may not be flat.

[066] As the common plane 212 does not intersect with the planes 220 and 222, the joint arrangement 214 forms a layer that is parallel to the first adhesive layer 202 and the second adhesive layer 204, and also disposed or located between the first adhesive layer 202 and the second adhesive layer 204.

[067] Each of the first, second and third joint structures (206, 208 and 210) provides an individual seal. In the embodiment shown in Figure 2, the third joint structure 210 is an IMC seal formed of one or more high melting temperature components (such as Cu or Au) and one or more low melting temperature components (such as Sn or In), while the first and the second joint structures 206 and 208 are low melting temperature solder joint seal composed of solder material (such as Sn or In) and IMC (such as Ti-Sn, Ni-Sn) formed between the solder material and the adhesive layers (202, 204). A high melting temperature may be around 400-600°C, while a low melting temperature may be around 120-230 °C.

[068] The third joint structure 210 (being an IMC) renders thermal stability to the hermetic seal 200 under high temperature conditions, while the first and the second joint structures 206 and 208 (being solder material) renders good mechanical stability. Thus, the joint arrangement 214 provides a composite seal joint with good hermeticity, stability and reliability, even under high temperature exposure of around 260-350°C, depending on the materials used to form the IMC.

[069] In the embodiment shown in Figure 2, one entire surface of the joint arrangement 214 is in contact with a facing surface of the first adhesive layer 202. Similarly, an entire opposite surface of the joint arrangement 214 is in contact with a facing surface of the second adhesive layer 204. Thus, one surface of the joint arrangement 214 forms an interface with the first adhesive layer 202, while an opposite surface of the joint arrangement 214 forms an interface with the second adhesive layer 204. In more detail, between the first adhesive layer 202 and the joint arrangement 214: a facing surface of the first adhesive layer 202 is in contact with a respective entire surface of the first joint structure 206; a facing surface of the first adhesive layer 202 is in contact with a respective entire surface of the second joint structure 208; and a facing surface of the first adhesive layer 202 is in contact with a respective entire surface of the third joint structure 210. Between the second adhesive layer 204 and the joint arrangement 214: a facing surface of the second adhesive layer 204 is in contact with a respective entire surface of the first joint structure 206; a facing surface of the second adhesive layer 204 is in contact with a respective entire surface of the second joint structure 208; and a facing surface of the second adhesive layer 204 is in contact with a respective entire surface of the third joint structure 210. In summary, one surface of each joint structure (206, 208 and 210) may form an interface with the first adhesive layer 202, while an opposite surface of each joint structure (206, 208 and 210) may form an interface with the second adhesive layer 204. [070] In another embodiment (not shown), one or more suitable layers may be present between the first adhesive layer and a respective surface of the joint arrangement. Similarly, one or more suitable layers may be present between the second adhesive layer and a respective surface of the joint arrangement.

[071] In the embodiment shown in Figure 2, the first joint structure 206 and the second joint structure 208 may be in contact with opposite sides (2101 and 21 Or respectively) of the third joint structure 210. However, in another embodiment (not shown) the first joint structure and the second joint structure may be adjacent to opposite sides of the third joint structure. In such an embodiment, a gap or an additional joint may be present either between the first joint structure and a respective side of the third joint structure; between the second joint structure and a respective side of the third joint structure; or both.

[072] In the embodiment shown in Figure 2, the first joint structure 206 and the second joint structure 208 may be solder joints. Solder may be any one or more of Sn, In, or an InSn alloy. The third joint structure 210 may be an intermetallic compound. The intermetallic compound (IMC) may be any one or more of Cu, Au, Ag, Ni and any one or more of Sn, In or an InSn alloy. Thus in the embodiment shown in Figure 2, a solder/IMC/solder composite joint is formed. Since the third joint structure 210 is IMC joint and the low temperature solder is converted into IMC, the third joint structure 210 has a higher melting temperature than the first joint structure 206 and the second joint structure 208. The first joint structure 206 and the second joint structure 208 may have a melting temperature of around 232°C if pure Sn solder is used, while the third joint structure 210 may have a melting temperature of at least 400°C for various IMC such as Cu-Sn, Ni-Sn. The high melting temperature of the third joint structure 210 may ensure that the hermetic seal 200 does not fail when exposed to high temperature operation, such as multi reflow at 260 °C for assembly.

[073] Eutectic diffusion bonding may, for example, be used to produce the IMC of the third joint structure 210. For eutectic diffusion bonding, a low melting point (LMP) component (such as materials used in solder combinations: In or Sn) may be bonded with a high melting point (HMP) component such as Au, Ni, Ag, Cu, Pt and Pd. However, when compared with other noble metals such as Au, Ag, and Cu are preferred as they quickly dissolve into molten Sn, In and their alloys and rapidly react to form the IMC. Thus, when a molten layer of Sn or In is sandwiched between layers of Au or Cu, the molten layer may be consumed to form a high melting IMC layer. For instance, the IMC may be formed using Cu and either In, Sn or an In-Sn alloy, since Cu is widely used in modem packaging technology and cheaper compared with Au; while In, Sn or an In-Sn alloy is attractive because of their low eutectic temperature and good wettability with various common substrates.

[074] Via diffusion, eutectic diffusion bonding may bond the LMP component and the HMP component to form the IMC at a temperature (such as below around 180°C when using InSn solder) which is lower than the melting temperature of the EVIC. Forming the third joint structure 210 at a low temperature is advantageous due to applications of the hermetic seal according to various embodiments. For instance, fabrication of image sensor modules and radio frequency devices may occur at a temperature below 200°C. Since eutectic diffusion bonding may occur at such a temperature, the third joint structure 210 may be fabricated together with image sensor modules and radio frequency devices, thereby achieving system integration.

[075] To protect the surface of the LMP component from oxidation, a thin noble metal (usually Au) layer may be deposited on its top. Further, to control diffusion between the LMP component and the HMP component, a buffer layer may be placed between the LMP component and the HMP component.

[076] However, after eutectic diffusion bonding, further IMCs may form due to the noble metal and the buffer layers. In the long term, an IMC seal may fail due to poor adhesion of the further IMCs. Due to the small fracture roughness of IMC materials, cracks that form propagate fast and cause the IMC seal to collapse.

[077] To improve adhesion, each of the first adhesive layer 202 and the second adhesive layer 204 may be wettable by the joint arrangement 214, so that the first joint structure 206, the second joint structure 208 and the third joint structure 210 adhere to both the first adhesive layer 202 and the second adhesive layer 204. Metals, such as Ti, Ti(W), Ni, Ni(p), Ni(v) and Cr, which have a slow reaction rate or diffusion rate when soldering with Sn, In, and their alloys may be used for the first adhesive layer 202 and the second adhesive layer 204.

[078] The first adhesive layer 202 and the second adhesive layer 204 may further be needed as the first and the second substrates 216 and 218, upon which the first adhesive and the second adhesive layers 202 and 204 are respectively formed, may not be wettable by the material used to form the joint arrangement 214. Thus, the adhesive layers 202 and 204 may serve to allow the joint arrangement 214 to be formed on the substrates 216 and 218. The adhesive layers 202 and 204 may also act as a barrier layer that prevents fast diffusion in the material used to form the joint arrangement 214.

[079] From Figure 2, it can be observed that the width of the joint arrangement 214 is contained within the widths of the first adhesive layer 202 and the second adhesive layer 204. It will thus be appreciated that the perimeter of the joint arrangement 214 is located within the perimeters of the first adhesive layer 202 and the second adhesive layer 204.

[080] Figure 3 shows a flow chart 300 illustrating a method of forming a hermetic seal, according to various embodiments.

[081] At 302, a first adhesive layer may be provided.

[082] At 304, a second adhesive layer may be provided.

[083] At 306, a joint arrangement may be formed between the first adhesive layer and the second adhesive layer. The joint arrangement may include: a first joint structure; a second joint structure; and a third joint structure disposed between the first joint structure and the second joint structure. The material of the third joint structure may have a higher melting temperature than the material of the first joint structure and the second joint structure. The first joint structure, the second joint structure and the third joint structure may be respectively disposed on a common plane which is parallel to both the first adhesive layer and the second adhesive layer.

[084] Formation of the joint arrangement at 306 may include the further following processes as shown in the flow charts of Figures 4 and 5.

[085] Turning to Figure 4, at 402, a layer arrangement may be provided on the first adhesive layer. The layer arrangement may include: a first layer formed on the first adhesive layer; and a second layer formed over the first layer. The material of the first layer may have a higher melting temperature than the material of the second layer.

[086] At 404, the layer arrangement on the first adhesive layer may be bonded to the second adhesive layer to create the third joint structure, being a compound including the first layer and a portion of the second layer. A remaining portion of the second layer may form the first and second joint structures.

[087] Turning to Figure 5, at 502, a layer arrangement may be provided on the first adhesive layer. The layer arrangement may include: a first layer formed on the first adhesive layer; and a second layer formed over the first layer. The material of the first layer may have a higher melting temperature than the material of the second layer.

[088] At 504, a layer arrangement may be provided on the second adhesive layer. The layer arrangement may include: a first layer formed on the second adhesive layer; and a second layer formed over the first layer. The material of the first layer may have a higher melting temperature than the material of the second layer.

[089] At 506, the layer arrangement on the second adhesive layer may be bonded to the layer arrangement on the first adhesive layer to create the third joint structure, being a compound of the first layer formed on the first adhesive layer, the first layer formed on the second adhesive layer and a portion of the second layer of each of the layer arrangements. A remaining portion of the second layer of each of the layer arrangements may form the first and second joint structures.

[090] The method shown in Figures 3 to 5 allow for a diffusion soldering process being a combination of a soldering method and a diffusion bonding method. A low process temperature may be used, yet the hermetic seal fabricated may have a high service temperature, may offer good mechanical properties and may be fluxless. The bonding process to form an intermetallic compound (EMC) in the third joint structure may use a diffusion soldering process. Diffusion soldering may produce a high melting temperature joint at a lower soldering temperature, to reduce stresses and avoid damage of electronic devices fabricated under a high temperature environment.

[091] In various embodiments, formation of a hermetic seal may be as follows. Fabricate a first structure: a thick metal layer (at least one of Au, Cu, Ag) and a thin barrier metal layer (at least one of Ti, W, Ni, Ni(v), Cr) at the sides of the thick metal layer; a uniform solder layer deposited on the thick metal layer or on both the thick metal layer and thin barrier metal layer. Bond the first structure to a second structure (being identical to the first structure) or to a wettable substrate with a barrier layer. At a suitable bonding temperature and bonding pressure, part of the solder on thick metal is laterally squeezed out to flow over the thin barrier metal layers. The following is noted between the thin barrier metal layer and the solder, and the thick metal layer and the solder. Diffusion between the thin barrier metal layer and solder is slower, so after bonding, a solder joint with lower melting temperature is formed. Diffusion between the thick metal layer and the solder layer is faster, so after bonding, an IMC joint with high melting temperature is formed. After bonding, a solder/IMC/solder composite joint is formed. Fabrication of a hermetic seal is explained in further detail with reference to Figures 6A to 8B.

[092] Figures 6A to 6B show cross-sectional views in the manufacture of a hermetic seal in accordance to one embodiment.

[093] In Figure 6A, a first substrate 616 and a second substrate 618 are provided.

[094] A first adhesive layer 602 is provided on the first substrate 616. A layer arrangement 640 is provided on the first adhesive layer 602. The layer arrangement 640 includes: a first layer 642 formed on the first adhesive layer 602 and a second layer 644 formed over the first layer 642. The material of the first layer 642 has a higher melting temperature than the material of the second layer 644. In the embodiment shown in Figure 6A, the first layer 642 covers only a portion of the first adhesive layer 602, while the second layer 644 covers an entire surface of the first layer 642. Although not shown, the second layer 644 may extend to cover the adhesive layer 602. Optionally, a further barrier layer 646 may be formed on a portion of the first adhesive layer 602, adjacent to opposite sides of the first layer 642.

[095] A second adhesive layer 604 is provided on the second substrate 618. A layer arrangement 660 is provided on the second adhesive layer 604. The layer arrangement 660 includes: a first layer 662 formed on the second adhesive layer 604 and a second layer 664 formed over the first layer 662. The material of the first layer 662 has a higher melting temperature than the material of the second layer 664. In the embodiment shown in Figure 6A, the first layer 662 covers only a portion of the second adhesive layer 604, while the second layer 664 covers an entire surface of the first layer 662. Although not shown, the second layer 664 may extend to cover the adhesive layer 604. Optionally, a further barrier layer 666 may be formed on a portion of the second adhesive layer 604, adjacent to opposite sides of the first layer 662.

[096] Before applying a bonding process, 0₂ plasma descum is conducted to remove any oxide layers and organic contaminants to obtain clean surfaces for the second layers 644 and 664.

[097] To form the hermetic seal 600 of Figure 6B, the layer arrangement 660 on the second adhesive layer 604 is bonded to the layer arrangement 640 on the first adhesive layer 602 to create a third joint structure 610.

[098] In one embodiment, the third joint structure 610 is a compound of a portion of the first layer 642 formed on the first adhesive layer 602, a portion of the first layer 662 formed on the second adhesive layer 604 and a portion of the second layer (644 and 664) of each of the layer arrangements (640 and 660). Thus, in this embodiment, after formation of the third joint structure 610, there is residual material for both the first layer 642 of the layer arrangement 640 and the first layer 662 of the layer arrangement 660. A remaining portion of the second layer (644 and 664) of each of the layer arrangements (640 and 660) forms a first joint structure 606 and a second joint structure 608.

[099] In another embodiment, the third joint structure 610 is a compound of the first layer 642 formed on the first adhesive layer 602, the first layer 662 formed on the second adhesive layer 604 and a portion of the second layer (644 and 664) of each of the layer arrangements (640 and 660). Thus, in contrast with the earlier embodiment, after formation of the third joint structure 610, there is no residual material for both the first layer 642 of the layer arrangement 640 and the first layer 662 of the layer arrangement 660. A remaining portion of the second layer (644 and 664) of each of the layer arrangements (640 and 660) forms a first joint structure 606 and a second joint structure 608.

[0100] Figures 7 A to 7B show cross-sectional views in the manufacture of a hermetic seal in accordance to one embodiment.

[0101] In Figure 7A, a first substrate 716 and a second substrate 718 are provided.

[0102] A first adhesive layer 702 is provided on the first substrate 716. A layer arrangement 740 is provided on the first adhesive layer 702. The layer arrangement 740 includes: a first layer 742 formed on the first adhesive layer 702 and a second layer 744 formed over the first layer 742. The material of the first layer 742 has a higher melting temperature than the material of the second layer 744. In the embodiment shown in Figure 7A, the first layer 742 covers only a portion of the first adhesive layer 702, while the second layer 744 covers an entire surface of the first layer 742. Although not shown, the second layer 744 may extend to cover the adhesive layer 702.

[0103] A second adhesive layer 704 is provided on the second substrate 718.

[0104] Before applying a bonding process, 0₂ plasma descum is conducted to remove any oxide layers and organic contaminants to obtain a clean surface for the second layer 744. [0105] To form the hermetic seal 700 of Figure 7B, the layer arrangement 740 on the first adhesive layer 702 is bonded to the second adhesive layer 704 to create a third joint structure 710.

[0106] In one embodiment, the third joint structure 710 is composed of a compound formed between a portion of the first layer 742 and a portion of the second layer 744, and a compound formed between a portion of the second layer 744 and a portion of the second adhesive layer 704. Further, compounds may be formed between a portion of the first layer 742 and material from the second adhesive layer 704. Thus, in this embodiment, after formation of the third joint structure 710, there may be residual material for the first layer 742 of the layer arrangement 740. A remaining portion of the second layer 744 forms a first joint structure 706 and a second joint structure 708.

[0107] In another embodiment, the third joint structure 710 is a compound of the first layer 742 and a portion of the second layer 744. Thus, in contrast with the earlier embodiment, after formation of the third joint structure 710, there is no residual material for the first layer 742 of the layer arrangement 740. A remaining portion of the second layer 744 forms a first joint structure 706 and a second joint structure 708.

[0108] Figures 8 A to 8B show cross-sectional views in the manufacture of a hermetic seal in accordance to one embodiment.

[0109] In Figure 8 A, a first substrate 816 and a second substrate 818 are provided.

[0110] A first adhesive layer 802 is provided on the first substrate 816. A layer arrangement 840 is provided on the first adhesive layer 802. The layer arrangement 840 includes: a first layer 842 formed on the first adhesive layer 802 and a second layer 844 formed over the first layer 842. The material of the first layer 842 has a higher melting temperature than the material of the second layer 844. A buffer layer 850 is formed between the first layer 842 and the second layer 844. Further, an oxidation protection layer 852 is formed over the first layer 842 and, in the embodiment shown in Figure 8A, also the buffer layer 850. In the embodiment shown in Figure 8A, the first layer 842 covers only a portion of the first adhesive layer 802, while the second layer 844 covers an entire surface of the first layer 842 and extends to cover the adhesive layer 802 to which the second layer 844 is formed on.

[0111] A second adhesive layer 804 is provided on the second substrate 818. A layer arrangement 860 is provided on the second adhesive layer 804. The layer arrangement 860 includes: a first layer 862 formed on the second adhesive layer 804 and a second layer 864 formed over the first layer 862. The material of the first layer 862 has a higher melting temperature than the material of the second layer 864. A buffer layer 856 is formed between the first layer 862 and the second layer 864. Further, an oxidation protection layer 858 is formed over the first layer 862 and, in the embodiment shown in Figure 8A, also the buffer layer 856. In the embodiment shown in Figure 8 A, the first layer 862 covers only a portion of the second adhesive layer 804, while the second layer 864 covers an entire surface of the first layer 862 and extends to cover the adhesive layer 804 to which the second layer 864 is formed on.

[0112] Before applying a bonding process, 0₂ plasma descum is conducted to remove any oxide layers and organic contaminants to obtain clean surfaces for the second layers 844 and 864.

[0113] To form the hermetic seal 800 of Figure 8B, the layer arrangement 860 on the second adhesive layer 804 is bonded to the layer arrangement 840 on the first adhesive layer 802 to create a third joint structure 810.

[0114] In one embodiment, the third joint structure 810 is a compound formed by a portion of the first layer 842, the buffer layer 850 and the oxidation protection layer 852 (i.e. the layers 842, 850 and 852 formed on the first adhesive layer 802); a portion of the first layer 862, the buffer layer 856 and the oxidation protection layer 858 (i.e. the layers of 862, 856 and 858 formed on the second adhesive layer 804); and a portion of the second layer (844 and 864) of each of the layer arrangements (840 and 860). Thus, in this embodiment, after formation of the third joint structure 810, there is residual material for both the first layer 842 of the layer arrangement 840 and the first layer 862 of the layer arrangement 860. A remaining portion of the second layer (844 and 864) of each of the layer arrangements (840 and 860) forms a first joint structure 806 and a second joint structure 808.

[0115] In another embodiment, the third joint structure 810 is a compound of the first layer 842, the buffer layer 850 and the oxidation protection layer 852 formed on the first adhesive layer 802; the first layer 862, the buffer layer 856 and the oxidation protection layer 858 formed on the second adhesive layer 804; and a portion of the second layer (844 and 864) of each of the layer arrangements (840 and 860). Thus, in contrast with the earlier embodiment, after formation of the third joint structure 810, there is no residual material for both the first layer 842 of the layer arrangement 840 and the first layer 862 of the layer arrangement 860. A remaining portion of the second layer (844 and 864) of each of the layer arrangements (840 and 860) forms a first joint structure 806 and a second joint structure 808.

[0116] In another embodiment (not shown), the second substrate may only be provided with an adhesive layer. To form the hermetic seal 800 of Figure 8B, the layer arrangement 840 on the first adhesive layer 802 is bonded to the adhesive layer of the second substrate. The third joint structure 810 is compounds formed between a portion of the first layer 842 and a portion of the second layer 844, and material from the adhesive layer of the second substrate. Thus, in this embodiment, after formation of the third joint structure 810, there is residual material for the first layer 842 of the layer arrangement 840. It is also possible, in another embodiment, that there is no residual material left from the first layer 842, after formation of the third joint structure 810. A remaining portion of the second layer 844 forms the first joint structure 806 and the second joint structure 808.

[0117] Accordingly in the embodiments shown in Figures 6B, 7B and 8B, a hermetic seal (600, 700 and 800) includes a first adhesive layer (602, 702 and 802) and a second adhesive layer (604, 704 and 804). A joint arrangement (614, 714 and 814) is formed between the first adhesive layer (602, 702 and 802) and the second adhesive layer (604, 704 and 804). The joint arrangement (614, 714 and 814) includes: a first joint structure (606, 706 and 806); a second joint structure (608, 708 and 808); and a third joint structure (610, 710 and 810) disposed between the first joint structure (606, 706 and 806) and the second joint structure (608, 708 and 808). The material of the third joint structure (610, 710 and 810) has a higher melting temperature than the material of the first joint structure (606, 706 and 806) and the second joint structure (608, 708 and 808). The first joint structure (606, 706 and 806), the second joint structure (608, 708 and 808) and the third joint structure (610, 710 and 810) are respectively disposed on a common plane (612, 712 and 812) which is parallel to both the first adhesive layer (602, 702 and 802) and the second adhesive layer (604, 704 and 804).

[0118] Referring to the Figures 6A to 8B, bonding of the layer arrangement (640, 660; 740; and 840, 860) displaces the portion of the second layer (644, 664; 744; and

844, 864) that forms the first joint structure (606, 706 and 806) and the second joint structure (608, 708 and 808) to opposite sides of the created third joint structure (610,

710 and 810), to laterally surround the created third joint structure (610, 710 and 810).

Displacement of the respective portion of the second layer (644, 664; 744; and 844, 864) is along the common plane (612, 712 and 812). Bonding of the layer arrangement (640, 660; 740; and 840, 860) to form the joint arrangement (614, 714 and 814) may be performed under the following conditions: at a pressure of around 1- 20MPa, at a temperature of around 20-80°G higher than the melting temperature of the second layer (644, 664; 744; and 844, 864), which may be solder materials such as Sn and Snln, and for a duration of around 1 to 30 minutes.

[0119] Referring to the Figures 6 A, 7A and 8 A, one or more suitable layers may be present between the first layer (642, 662; 742; and 842, 862) and the second layer (644, 664; 744; and 844, 864). Accordingly, in various embodiments . (see for example, Figure 8A), a buffer layer may be formed between the first layer (642, 662; 742; and 842, 862) and the second layer (644, 664; 744; and 844, 864). In various embodiments, an oxidation protection layer may be formed over the first layer (642, 662; 742; and 842, 862).

[0120] In the embodiments shown in Figures 6A to 8B, compounds for the first layer (642, 662; 742; and 842, 862) and the second layer (644, 664; 744; and 844, 864) are chosen such that the created third joint structure (610, 710 and 810) is an intermetallic compound. The first layer (642, 662; 742; and 842, 862) may include any one of Cu, Au, Ag, Ni, Pt, Pd, Ti or Cr. The second layer (644, 664; 744; and 844, 864) may include solder, wherein the solder may include any one of Sn, In, or alloys of InSn, SnAg, SnCu or SnBi. The buffer layer (850 and 856) may include any one of Ni, Ti, Cr or Pt. The oxidation protection layer (852 and 858) may include Au, Pt or Pd. The first adhesive layer (602, 702 and 802) and the second adhesive layer (604, 704 and 804) may include any one or more of Ti, Cr, W, Ni, Cr, Pd, Pt or V.

[0121] Notwithstanding the materials, along with their respective parameters, presented thus far, a hermetic seal according to various embodiments may be manufactured using the following materials having the following respective parameters.

[0122] The hermetic seal may be fabricated on an 8" wafer as a square ring (see

Figure 1) with a length 101 of 11mm.

[0123] 300 A thick Si0₂ (not shown) and 1500 A thick SiN (not shown) may be formed on a silicon wafer (not shown) in turn by thermal oxidation and a low pressure chemical vapor deposition process. The Si0₂ and SiN act as a hard mask for cavity etching. The hard mask may be patterned by photolithography using dry film as a photoresist material. With reference to Figure 9, a cavity 902 with an area of 6x6 mm² and 250 μηι in depth may be formed using a wet-etching process with KOH inside the square ring 904 on both a cap wafer and a bottom wafer. It will be appreciated that only a quarter model is shown in Figure 9, where the cap wafer and the bottom wafer are not shown for the purpose of simplicity.

[0124] Figure 10 illustrates Ti-Cu-Ni-Au metallization being sputtered to form high melting point components for diffusion bonding.

[0125] The Ti layers (1002 and 1004) may each have a width (1002W and 1004W) of around 300μιη and a thickness of around 0.05μπι. The Ti layer acts as the adhesive layer.

[0126] Stacks 1040 and 1060, with lOOum width, of Cu/Ni/Au metal layers of around 2μη , 0.05μπι and 0.03μπι thickness respectively may be deposited. The Cu layer provides the high melting point material for the hermetic seal. Ni is the buffer layer for bonding process. The Au layer is necessary for wetting and to prevent metals from oxidizing before solder deposition.

[0127] Au/Sn/Au layers (not shown) of around 0.02μπι, 3.5μηι and 0.03μιη may be further deposited, in an e-beam evaporation chamber, on each stack (1040 and 1060) and exposed portions of the Ti layers (1002 and 1004) to provide the material to form a solder joint. After deposition and dry film stripping, 0₂ plasma descum may be conducted before the bonding process to remove oxide layer and organic contaminants to obtain a clean surface.

[0128] The two wafers 1016 and 1018 may then be bonded using wafer bonder (EVG) in a controlled N₂ atmosphere. The bonding force may be around 6MPa, or may be around l-20MPa. The bonding temperature may be around 280°C or may be about 20-80°C above the melting temperature of solder. The bonding time may be around 20 min at peak temperature. The intermetallic compound formed by bonding the two wafers 1016 and 1018 may have a melting point higher than 300°C.

Experimental results

[0129] Figures 11 A to 11C show SEM and TEM pictures of a hermetic seal according to an embodiment.

[0130] Figure 11 A shows a centre portion of the hermetic seal, i.e. the composite joint of the hermetic seal.

[0131] Figure 1 IB shows a side portion of the hermetic seal, i.e. the solder joint of the hermetic seal. From Figure 11B, it was observed that there are no voids between solder 1102 and a Ti interface 1104. Thus, from Figure 1 IB, the Ti interface 1104 acts as an adhesive layer for wetting by the solder 1102.

[0132] Figure 11C shows a bonding interface between solder 1108 and Ti 1106 at the solder joint. The IMC formed at the Ti/Sn interface is nearly invisible, from the TEM analysis. However, it is known from Sn and Ti binary phase diagrams (an example being shown in Figure 11D) that Sn and Ti, under soldering conditions, would form an IMC. Moreover, Li et al. [J. F. Li, S. H. Mannan, M. P. Clode, Scripta Mater. 54 (2006) 1773-1178] reported that the growth rate of IMC, formed by Ti/Sn soldering, is quite slow. Thus, it can be concluded that a thin TiSn EVIC is slowly formed from the portion of the solder 1108 and the portion of the Ti 1106 at the bonding interface.

[0133] Figures 12A to 12B show SEM pictures of a known hermetic seal.

[0134] Figure 12A shows a center region of the known hermetic seal. The joint 1202 was composed of Cu-Sn IMC and residue Cu.

[0135] Figure 12B shows a side portion of the known hermetic seal. At the side portion, there was observed an approximate 25μπι of overflow solder 1204. The overflow solder 1204 (squeezed out when forming the known hermetic seal) does not wet a substrate 1208, such as SiN.

[0136] Tests were conducted on a hermetic seal in accordance to an embodiment and other known hermetic seals. In more detail, the tests were: Pressure cooker test (PCT) conducted at 121°C and under 2 arm for 300 hrs. High humidity (HH) storage conducted at 85°C and under 85 RH for 1000 hrs. High temperature storage (HTS) test conducted at 125°C for up to lOOOhrs. Temperature cycling (TC) test conducted from around 45 to around 125°C for up to lOOOhrs. The results of the tests are discussed below with reference to Figures 13 A and 13B.

[0137] In Figures 13A and 13B, row 1304 refers to the hermetic seal shown in Figures 12A and 12B; row 1302 refers to the hermetic seal in accordance to an embodiment; row 1306 refers to a hermetic seal presented in the publication "W. K. Choi, et al., Cu-Sn solder sealing for hermetic optical MEMS devices package, IMAPS, 2006"; and row 1308 refers to a hermetic seal presented in the publication "Wang, et al., Application of Au-Sn eutectic bonding in hermetic radio frequency microelectromechanical system wafer level packaging, J. Electron. Mater. 35(3) (2006) 425-431".

Hermeticity measurement [0138] Figure 13 A shows the results of hermeticity measurement for tested bonded dies.

[0139] The hermeticity of the ring seals was evaluated by helium leak rate tests based on the MIL-STD-883 standard. According to MIL-STD-883 criteria, helium rate smaller than 5* 10^~8 atnv cc/sec was accepted.

[0140] 21 samples were used for each test result shown in Figure 13 A. Each test was conducted as follows. According to an internal cavity volume of the bonded die (approximately 0.02 cm³), the pressure of helium gas in a bombing chamber was set as 0.5Mpa (75 Psi), and exposure time was 2 hours. After bombing, the dies were put into a helium leak detector to measure their helium leak rates.

[0141] From the leak rate test results shown in row 1302, hermeticity improved by using the hermetic seal in accordance to an embodiment. Comparing with 1306, in which chip to chip bonding using Cu/Sn, 1302 using 8 inch wafer to wafer bonding obtained better hermeticity after bonding. Comparing with 1308, in which 4 inch wafer bonding using Au/Sn system, 1302 obtains about the same hermeticity results. The results show that a hermetic seal, formed by low cost Cu/Sn system in accordance to an embodiment, can replace more expensive Au/Sn MEMS hermetic sealing applications.

Mechanical test

[0142] Figure 13B shows mechanical test results of bonded dies.

[0143] To evaluate the mechanical properties of seal ring joints, 5 samples were used for shear tests. The shear test was conducted with a shear tester (BT4000 Dage) using a speed of 50 μηι/s. In order to know the shear strength under high temperature, some samples were also tested at 250°C. The shear test results showed that both 1304 and 1302 have a robust bonding strength even tested under 250°C.

Stress simulation

[0144] Mises stress profile in joints were simulated and the results shown in Figure

14. The reflow temperature was loaded from 125 to -40°C. Three kinds of seal joints with 300μπι width were used for simulation, i.e., complete IMC joint, composite seal joint with 200μπι IMC joint and 50μπι solder joint, and composite seal joint with

ΙΟΟμιη IMC joint and ΙΟΟμπι solder joint at edges.

[0145] The simulation results showed that:

• Mises stress was highest in a joint which only used a pure IMC joints with no solder and occurred at the corners of the joints • In the joints having both IMC and solder, the location of highest Mises stress was at the interface between the IMC and Sn.

• The shorter the length of the IMC joint, the shorter was the length of the bond experiencing maximum stress

• Stress at the edges of the joint was decreased by about 120MPa when using an IMC-Sn joint compared to a joint using only an IMC joint without solder.

• Max. stress in a joint with IMC and Sn was not significantly dependant on joint length.

[0146] From the above, a hermetic seal in accordance to an embodiment harnesses advantages associated with using an IMC joint and a solder joint, and yet overcomes shortages associated with the joints. Another advantage is that in various embodiments, a large number of solder and material systems such as Cu/Sn, Cu/In, Cu/InSn are available to manufacture the hermetic seal. Therefore, a large bonding temperature range is usable, depending on the material used. A suitable application of the composite seal joint, in various embodiments, may be used to hermetically seal MEMS devices.

[0147] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A hermetic seal comprising

a first adhesive layer;

a second adhesive layer;

a first joint structure;

a second joint structure; and

a third joint structure disposed between the first joint structure and the second joint structure, the material of the third joint structure having a higher melting temperature than the material of the first joint structure and the second joint structure, wherein the first joint structure, the second joint structure and the third joint structure are respectively disposed on a common plane which is parallel to both the first adhesive layer and the second adhesive layer, and

wherein the first joint structure, the second joint structure and the third joint structure forms a joint arrangement that is sandwiched between the first adhesive layer and the second adhesive layer.

2. The hermetic seal of claim 1, wherein the joint arrangement is in contact with both the first adhesive layer and the second adhesive layer.

3. The hermetic seal of claim 2, wherein one surface of the joint arrangement forms an interface with the first adhesive layer, while an opposite surface of the joint arrangement forms an interface with the second adhesive layer.

4. The hermetic seal of claims 2 or 3, wherein one surface of each joint structure forms an interface with the first adhesive layer, while an opposite surface of each joint structure forms an interface with the second adhesive layer.

5. The hermetic seal of any one of the preceding claims, wherein the first joint structure and the second joint structure are in contact with opposite sides of the third joint structure.

6. The hermetic seal of any one of the preceding claims, wherein a perimeter of the joint arrangement is located within the perimeter of the first adhesive layer and the perimeter of the second adhesive layer.

7. The hermetic seal of any one of the preceding claims, wherein the first adhesive layer and the second adhesive layer comprise any one or more of Ti, Cr, W, Ni, Cr, Pd, Pt and V.

8. The hermetic seal of any one of the preceding claims, wherein the third joint structure comprises at least one intermetallic compound.

9. The hermetic seal of claim 8, wherein the at least one intermetallic compound comprises any one or more of Cu, Au, Ag, Ni, Pt, Pd, Ti, Cr and any one or more of

Sn, In, or alloys SnBi, InSn, SnAg or SnCu.

10. The hermetic seal of any one of the preceding claims, wherein the first and the second joint structures comprise solder.

11. The hermetic seal of claim 10, further comprising an intermetallic compound layer between the first adhesive layer and the first and the second joint structures; and an intermetallic compound between the second adhesive layer and the first and the second joint structures.

12. The hermetic seal of claims 10 or 11, wherein the solder comprises any one or more of Sn, In, or alloys of InSn, SnAg, SnCu or SnBi.

13. The hermetic seal of any one of the preceding claims, wherein each of the j oint structures is about the same thickness.

14. The hermetic seal of any one of the preceding claims, further comprising

a first substrate upon which the first adhesive layer is formed; and

a second substrate upon which the second adhesive layer is formed.

15. A semiconductor package comprising the hermetic seal of any one of the preceding claims.

16. The semiconductor package of claim 15, further comprising a MEMs device surrounded by the hermetic seal of any one of the preceding claims.

17. A method of forming a hermetic seal, the method comprising:

providing a first adhesive layer;

providing a second adhesive layer;

forming a joint arrangement between the first adhesive layer and the second adhesive layer, the joint arrangement comprising:

a first joint structure;

a second joint structure; and

a third joint structure disposed between the first joint structure and the second joint structure, the material of the third joint structure having a higher melting temperature than the material of the first joint structure and the second joint structure,

wherein the first joint structure, the second joint structure and the third joint structure are respectively disposed on a common plane which is parallel to both the first adhesive layer and the second adhesive layer.

18. The method of claim 17, wherein forming the joint arrangement comprises:

providing a layer arrangement on the first adhesive layer, the layer arrangement comprising:

a first layer formed on the first adhesive layer; and

a second layer formed over the first layer, the material of the first layer having a higher melting temperature than the material of the second layer; and

bonding the layer arrangement on the first adhesive layer to the second adhesive layer to create the third joint structure, the third joint structure being a compound comprising a portion of the first layer and a portion of the second layer, while a remaining portion of the second layer forms the first and second joint structures.

19. The method of claim 18, wherein the third joint structure further comprises a portion of the second adhesive layer.

20. The method of claim 17, wherein forming the joint arrangement comprises: providing a layer arrangement on the first adhesive layer, the layer arrangement comprising:

a first layer formed on the first adhesive layer; and

a second layer formed over the first layer, the material of the first layer having a higher melting temperature than the material of the second layer;

providing a layer arrangement on the second adhesive layer, the layer arrangement comprising:

a first layer formed on the second adhesive layer; and a second layer formed over the first layer, the material of the first layer having a higher melting temperature than the material of the second layer; and

bonding the layer arrangement on the second adhesive layer to the layer arrangement on the first adhesive layer to create the third joint structure, the third joint structure being a compound comprising a portion of the first layer formed on the first adhesive layer, a portion of the first layer formed on the second adhesive layer and a portion of the second layer of each of the layer arrangements, while a remaining portion of the second layer of each of the layer arrangements forms the first and second joint structures.

21. The method of claims 18 to 20, wherein bonding of the layer arrangement further comprises displacing the portion of the second layer, that forms the first and second joint structures, to opposite sides of the created third joint structure, to laterally surround me created third joint structure.

22. The method of claims 18 to 21, wherein the second layer covers only the first layer to which the second layer is formed on.

23. The method of claims 18 to 21, wherein the second layer extends to cover the adhesive layer to which the second layer is formed on.

24. The method of claims 18 to 23, wherein the layer arrangement further comprises a buffer layer formed between the first layer and the second layer.

25. The method of claims 18 to 24, wherein the layer arrangement further comprises an oxidation protection layer formed over the first layer.

26. The method of claims 17 to 25, further comprising a wetting layer formed on both the first adhesive layer and the second adhesive layer.

27. The method of claim 26, wherein the wetting layer comprises any one or more of Au, Pt, Pd or Ag.

28. The method of claims 18 to 28, wherein compounds for the first layer and the second layer are chosen such that the created third joint structure comprises at least one intermetallic compound.

29. The method of claims 18 to 28, wherein the first layer comprises any one of Cu, Au, Ag, Ni, Pt, Pd, Ti or Cr.

30. The method of claims 18 to 29, wherein the second layer comprises solder.

31. The method of claim 30, wherein the solder comprises any one of Sn, In, or alloys of InSn, SnAg, SnCu or SnBi.

32. The method of claims 24 to 31, wherein the buffer layer comprises any one of Ni, Cr, Ti or Pt.

33. The method of claims 25 to 32, wherein the oxidation protection layer comprises Au, Pt or Pd.

34. The method of claims 17 to 33, wherein the first adhesive layer and the second adhesive layer comprise any one or more of Ti, Cr, W, Ni, Cr, Pd, Pt or V.

35. The method of claims 17 to 34, wherein forming the joint arrangement is performed under pressure of around l-20MPa.

36. The method of claims 17 to 35, wherein forming the joint arrangement is performed at a temperature of about 20-80°C higher than the melting temperature of solder.

37. The method of claims 17 to 36, wherein forming the joint arrangement is performed for a duration of around 1 to 30 minutes.

38. The method of claims 17 to 37, wherein the first adhesive layer is provided on a first substrate; and the second adhesive layer is provided on a second substrate.