Patent ID: 12237211

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

In accordance with some embodiments, a wafer bonding system is utilized. The wafer bonding system allows for the bonding of a first wafer to a second wafer in an airtight local process chamber by controlling the ambient pressure. The local process chamber is sealed by a gasket between wafer chucks holding the first and second wafers. The ambient pressure inside the local process chamber may be quickly adjusted and maintained, which can enable increased throughput and reduced cost for the bonding process. Reducing the pressure in the local process chamber enables the wafer bonding system to bond wafers together at a faster rate and increase the wafer per hour (WPH) processing rate. Increasing the pressure in the local process chamber reduces local stresses and bonding-induced distortion of the bonded wafers caused by uneven bonding wave velocity. The airtight seal of the local process chamber enables a gaseous purge capability to reduce moisture and decrease edge bubble defects.

FIG.1shows a top view of a wafer bonding system300that may be used to bond a wafer100with a wafer200. The process flow in accordance with the embodiments is briefly described below, and the details of the process flow and the wafer bonding system300are discussed, referencingFIGS.2through10. In some embodiments, the wafer bonding system300can be used to bond the wafers100and200through semiconductor-on-insulator (SOI) bonding, fusion bonding (e.g., hydrophilic bonding or hydrophobic bonding), eutectic bonding, hybrid bonding, or the like. However, any suitable method of bonding may be utilized.

The wafers100and200may be semiconductor wafers, such as silicon wafers, or semiconductor substrates, such as bulk semiconductors, a semiconductor-on-insulator (SOI) substrate, or the like, which may be doped (e.g., with a p-type or an n-type dopant) or undoped. Generally, an SOI substrate is a layer of a semiconductor material formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer, a silicon oxide layer, or the like. The insulator layer is provided on a substrate, typically a silicon or glass substrate. Other substrates, such as a multi-layered or gradient substrate may also be used. In some embodiments, the semiconductor material of the wafers100and200may include silicon; germanium; a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including silicon-germanium, gallium arsenide phosphide, aluminum indium arsenide, aluminum gallium arsenide, gallium indium arsenide, gallium indium phosphide, and/or gallium indium arsenide phosphide; or combinations thereof. In some embodiments, the wafers100and200comprise silicon, silicon germanium, combinations of these, or the like, and outer surfaces of the wafers100and200to be bonded may have a Si—O—Si crystalline structure.

In some embodiments, the wafers100and200are package components comprising a device wafer, a package substrate, an interposer wafer, or the like. In the embodiments in which the wafer100comprises a device wafer, the wafer100may include a semiconductor substrate, which may be, for example, a silicon substrate, although other semiconductor substrates are also usable. Active devices may be formed on a surface of the substrate, and may include, for example, transistors. Metal lines and vias may be formed in dielectric layers over the substrate, which may be low-k dielectric layers in some embodiments. The low-k dielectric layers may have dielectric constants (k values) lower than, for example, about 3.5, lower than about 3.0, or lower than about 2.5. The dielectric layers may also comprise non-low-k dielectric materials with dielectric constants (k values) greater than 3.9. The metal lines and vias may comprise copper, aluminum, nickel, tungsten, or alloys thereof. The metal lines and vias interconnect the active devices, and may connect the active devices to overlying metal pads formed on the dielectric layers. In some embodiments, the wafer100is an interposer wafer, which is free from active devices therein. The wafer100may or may not include passive devices (not shown) such as resistors, capacitors, inductors, transformers, and the like in accordance with some embodiments. In some embodiments, the wafer100is a package substrate. In some embodiments, the wafer100includes laminate package substrates, wherein conductive traces are embedded in laminate dielectric layers. In some embodiments, the wafers100and200are build-up package substrates, which comprise cores and conductive traces built on the opposite sides of the cores.

In some embodiments, the wafer bonding system300comprises loading stations302and304, transfer robots306to move wafers between areas of the wafer bonding system300, a controller380, and a bonding area320containing a pre-alignment module312, a surface treatment station314, a cleaning station322, and a bonding station400. However, more or fewer stations may be utilized within the wafer bonding system300. In some embodiments, the controller380comprises a programmable computer. The controller380is illustrated as a single element for illustrative purposes. In some embodiments, the controller380comprises multiple elements. The controller380may be connected to the transfer robots306and may be configured to move the wafers100and200through the bonding process.

To start the bonding process, the wafers that are to be bonded (for example, wafers100and200) are loaded into the wafer bonding system300through one or more of the loading stations302and304. For example, in some embodiments loading stations302are front opening unified pods (FOUPs) used to load wafers100(e.g., bottom wafers) and loading stations304are FOUPs used to load wafers200(e.g., top wafers). However, any suitable methods and loading stations may be utilized.

A transfer robot306adjacent to both the loading stations302and the bonding area320receives the wafers100and200from the loading stations302and304and places them into a load-lock308for the bonding area320. The bonding area320may be a vacuum environment (a vacuum chamber). Furthermore, the bonding area320may be surrounded by a chamber housing315(see below,FIG.2) made of material that is inert to the various process materials. As such, while the bonding area320may be any suitable material that can withstand the chemistries and pressures involved in the treatment process, in an embodiment the bonding area320may be steel, stainless steel, nickel, aluminum, alloys of these, combinations of these, and the like.

The bonding area320may also be connected to one or more vacuum pumps406(see below,FIG.2) for exhaust from the bonding area320. In an embodiment the vacuum pump406is under the control of the controller380, and may be utilized to control the pressure within the bonding area320to a desired pressure. Additionally, once the bonding process is completed, the vacuum pump406may be utilized to evacuate the bonding area320in preparation for removal of the wafers100and200.

In the bonding area320, the wafers100and200are transferred by a transfer robot306to a pre-alignment module312. In an embodiment the pre-alignment module312may comprise one or more rotating arms which can rotate the wafers100and200to any desired rotational position using, e.g., a notch located within the wafers100and200(see below,FIG.5C). However, any suitable angular position may be utilized.

Next, referring toFIG.2A, a transfer robot306within the bonding area320transfers the wafers100and200from the pre-alignment module312to the surface treatment station314. In some embodiments, the surface treatment station314is utilized to perform a surface treatment370, or surface activation, on the surfaces of the wafers100and200. In some embodiments, the surface treatment370includes a plasma activation step, a liquid activation step, combinations of these, or the like. However, any suitable surface treatment may be utilized.

Within the surface treatment station314is located a mounting platform345in order to position and control the wafers100and200during surface treatment370. The mounting platform345may hold one or more of the wafers100and200using a combination of clamps, vacuum pressure, and/or electrostatic forces, and may also include heating and cooling mechanisms in order to control the temperature of the wafers100and200during the processes.

Additionally, in embodiments in which the surface treatment370is a plasma activation treatment, the mounting platform345may further comprise a lower electrode319coupled to a first RF generator321. The lower electrode319may be electrically biased by the first RF generator321(which may be connected to and under control of the controller380) at a RF voltage during the surface treatment370. By being electrically biased, the lower electrode319is used to provide a bias to the incoming treatment gases and assist to ignite them into a treatment plasma. Additionally, the lower electrode319is also utilized to maintain the plasma during the surface treatment370.

Furthermore, while a single mounting platform345is illustrated inFIG.2A, this is merely intended for clarity and is not intended to be limiting. Rather, any number of mounting platforms345may additionally be included within the surface treatment station314. As such, multiple semiconductor substrates may be treated simultaneously.

Additionally, the surface treatment station314comprises a showerhead329. The showerhead329receives the treatment plasma and helps to disperse the treatment plasma into the surface treatment station314. In some embodiments, the showerhead329is designed to evenly disperse the treatment gases in order to minimize undesired process conditions that may arise from uneven dispersal and has a circular design with openings dispersed evenly around the showerhead329to allow for the even dispersal of the treatment plasma into the surface treatment station314. However, any suitable number and distribution of openings can be used.

The surface treatment station314also comprises an upper electrode327, for use as a plasma generator. In an embodiment the plasma generator may be a transformer coupled plasma generator and may be, e.g., a coil. The coil may be attached to a second RF generator323that is utilized to provide power to the upper electrode327(which may be connected to and under control of the controller380) in order to ignite the plasma during introduction of the treatment gases.

However, while the upper electrode327is described above as a transformer coupled plasma generator, embodiments are not intended to be limited to a transformer coupled plasma generator. Rather, any suitable method of generating the plasma, such as inductively coupled plasma systems, magnetically enhanced reactive ion etching, electron cyclotron resonance, a remote plasma generator, or the like, may be utilized. All such methods are fully intended to be included within the scope of the embodiments.

In the surface treatment370, the exposed surfaces of the wafers100and200are activated. For example, in an embodiment, the bonding area may initially be purged with an inert gas ambient such as e.g. Ar, N2, the like, or a combination thereof. Once purged a process gas used for generating the plasma may be nitrogen (N2), oxygen (O2), or an N2/O2mixture and may be introduced into the surface treatment station314through the showerhead329. However, any suitable process gas may be used to generate the plasma.

FIG.2Billustrates the effect of the surface treatment370on the surfaces of the wafers100and200, in accordance with some embodiments in which the wafers100and200are silicon wafers to be subsequently bonded by oxide-oxide bonding. The surface treatment370acts to remove oxygen atoms from silicon atoms on top surfaces of a silicon oxide layer on the wafers100and200. This activates the surfaces of the wafers100and200in preparation for subsequent oxide-oxide bonding.

Referring toFIG.1andFIG.3A(withFIG.3Aillustrating a view of the cleaning station322inFIG.1), once the surface treatment370has been performed, a transfer robot306transfers the wafers100and200to the cleaning station322. The cleaning station322may be used to perform a cleaning step on the wafers100and200to remove metal oxides, chemicals, particles, and other undesirable substances from the surfaces of the wafers100and200prior to bonding.

In an embodiment the cleaning station322comprises a mounting station347and a faucet360. The mounting station347may be similar to the mounting platform345described above with respect toFIG.2. For example, the mounting station347may hold one or more of the wafers100and200using a combination of clamps, vacuum pressure, and/or electrostatic forces, and may also include heating and cooling mechanisms. However, any suitable devices for holding the wafers100and200may be utilized.

The faucet360is positioned over the mounting station347in order to dispense one or more cleaning agents over wafers100and200when the wafers100and200are mounted in the mounting station347. During the cleaning step, the wafers100and200are mounted in the mounting station347and a cleaning agent362is then dispensed from the faucet360over the wafers100and200. In some embodiments, the cleaning agent362is deionized (DI) water. In other embodiments the cleaning agent362comprises, in addition to DI water, a chemical such as NH3, H2O2, citric acid, or the like. However, any suitable cleaning agent362may be utilized.

FIG.3Billustrates the effect of the cleaning agent362on the surfaces of the wafers100and200, in accordance with some embodiments in which the wafers100and200are silicon wafers to be subsequently bonded by oxide-oxide bonding and the cleaning agent362comprises water. Silanol groups form on the activated surface of the wafers100and200and water molecules attach to the silanol groups, which is advantageous for subsequent oxide-oxide bonding between the wafers100and200.

Next, referring toFIG.1andFIGS.4A-4C(withFIG.4Aillustrating a close-up view of the bonding station400inFIG.1,FIG.4Billustrating a bottom view of top wafer chuck410, andFIG.4Cillustrating a top view of bottom wafer chuck418), a transfer robot306within the bonding area320transfers the wafers100and200from the cleaning station322to the bonding station400. The bonding station400comprises a chamber405, one or more gas outlet(s)404, and one or more gas inlet(s)402. An ambient pressure inside the chamber405can be controlled by flowing gas/air into the chamber405through the gas inlet(s)402and removing gas/air from the chamber405via the gas outlet(s)404through the use of one or more vacuum pumps connected to the gas outlet(s)404. The bonding station400comprises a top wafer chuck410and a bottom wafer chuck418that can be positioned to face each other. The top wafer chuck410and the bottom wafer chuck418are moveable relative to each other in order to move wafers mounted on the top wafer chuck410and the bottom wafer chuck418together for bonding. In some embodiments, the top wafer chuck410and the bottom wafer chuck418are used to bond two semiconductor wafers (e.g., the wafer100to the wafer200) or two package components together. The top wafer chuck410is attached to a top arm408, and the bottom wafer chuck418is attached to a bottom arm420.FIG.4Dis illustrated in accordance with some embodiments in which the top wafer chuck410is mounted on a top plate440on the top arm408and the bottom wafer chuck418is mounted on a bottom plate441on the bottom arm420.

The top wafer chuck410and the bottom wafer chuck418are used in order to hold and control the orientation and movement of the wafers100and200during the bonding process. In some embodiments, the top wafer chuck410and the bottom wafer chuck418comprise any suitable material that may be used to hold one of the wafers100and200. For example, silicon based materials, such as glass, silicon oxide, silicon nitride, or other materials, such as aluminum oxide, combinations of any of these materials, or the like may be used. Additionally, the top wafer chuck410and the bottom wafer chuck418have diameters that are suitable to hold one of the wafers100and200. As such, while the size of the top wafer chuck410and the bottom wafer chuck418will be in some ways dependent upon the size of the wafers100and200, the top wafer chuck410and the bottom wafer chuck418can have diameters in a range of 250 mm to 300 mm. However, any suitable dimensions may be utilized.

A gasket450is disposed between the top wafer chuck410and the bottom wafer chuck418in order to form an airtight seal around a local process chamber460, also referred to as a sealed region, between the top wafer chuck410and the bottom wafer chuck418when top wafer chuck410and the bottom wafer chuck418are moved closer together for a subsequent bonding process (see below,FIG.5A). In some embodiments, the gasket450has a round or circular profile. The gasket450may comprise a compressible material, such as polytetrafluoroethylene, polybutadiene, silicone rubber, butyl rubber, nitrile rubber, natural rubber, fluoropolymer elastomers, the like, or a combination thereof. In some embodiments in accordance withFIGS.4A-4B, the gasket450is attached to the top wafer chuck410. In some embodiments, the gasket450is attached to the bottom wafer chuck418. As illustrated inFIGS.4A-4B, the gasket450is attached to the bottom surface of the top wafer chuck410using a suitable airtight adhesive such as an epoxy. However, any suitable adhesive may be used. In some embodiments illustrated in accordance withFIG.4D, a gasket450′ is attached to the top plate440adjacent to the top wafer chuck410. In some embodiments, the gasket450′ is attached to the bottom plate441adjacent to the bottom wafer chuck418.

A pressure regulator comprising one or more gas inlet(s)430and/or one or more gas outlet(s)432can be used to control an ambient pressure inside the local process chamber460between the top wafer chuck410and the bottom wafer chuck418when top wafer chuck410and the bottom wafer chuck418are moved closer together for a subsequent bonding process (see below,FIG.5A). In some embodiments, as illustrated in accordance withFIG.4A, the gas inlet(s)430and the gas outlet(s)432pass through the bottom wafer chuck418. In some embodiments, one or more of the gas inlet(s)430or the gas outlet(s)432pass through the top wafer chuck410.

Furthermore, the bonding station400comprises one or more push pins412. In some embodiments, the one or more push pins412are positioned to extend through top wafer chuck410. The one or more push pins412are each surrounded by an airtight seal (not illustrated) so that the local process chamber460may be sealed by the gasket450when the top wafer chuck410and the bottom wafer chuck418are moved together. The one or more push pins412are subsequently used to warp or bend one or more of the wafers100and200(see below,FIG.6A). By warping the wafers100and200, physical contact is initially made at a center of the wafers100and200before allowing the wafers100and200to bond at the edges. The bottom surface of the top wafer chuck410has a plurality of vacuum zones442that are connected to one or more vacuum pumps406through a series of pipes422. Each vacuum zone442is connected to a respective pipe422(not individually illustrated). The top surface of the bottom wafer chuck418has a plurality of vacuum zones444that are connected to one or more vacuum pumps406through respective pipes422.

During operation, the vacuum pump406will evacuate any gases from the vacuum zones442and444across the bottom surface of the top wafer chuck410and across the top surface of the bottom wafer chuck418, respectively, thereby lowering the pressure (also referred to as the chuck pressure) within these vacuum zones442and444. When the wafer200is placed against the bottom surface of the top wafer chuck410and the chuck pressure within the vacuum zones442at the bottom surface of the top wafer chuck410has been reduced by the vacuum pump406, the pressure difference (e.g., the difference between the pressure in the chamber405and the chuck pressure) between the side of the wafer200facing the vacuum zones442at the bottom surface of the top wafer chuck410and the side of the wafer200facing away from the vacuum zones442at the bottom surface of the top wafer chuck410will hold the wafer200against the bottom surface of the top wafer chuck410.

Likewise, when the wafer100is placed against the top surface of the bottom wafer chuck418and the chuck pressure within the vacuum zones444at the top surface of the bottom wafer chuck418has been reduced by the vacuum pump406, the pressure difference (e.g., the difference between the pressure in the chamber405and the chuck pressure) between the side of the wafer100facing the vacuum zones444at the top surface of the bottom wafer chuck418and the side of the wafer100facing away from the vacuum zones444at the top surface of the bottom wafer chuck418will hold the wafer100against the top surface of the bottom wafer chuck418. The pressures of the vacuum zones444may be controlled individually by the controller380to adjust for any warpages of the wafer100.

At the bonding station400, the wafers100and200are mounted on the top wafer chuck410and the bottom wafer chuck418. Once in place the top wafer chuck410and the bottom wafer chuck418may align the wafers100and200for bonding. In a particular embodiment the bonding station400may align the wafers100and200to an alignment accuracy in a range of 10 nm to 100 μm. However, any suitable alignment may be performed.

InFIGS.5A-5C, the top wafer chuck410and the bottom wafer chuck418move the wafers100and200together for bonding and the gasket450forms an airtight seal around the local process chamber460between the top wafer chuck410and the bottom wafer chuck418. The wafers100and200are moved together to a separation by a distance D1in a range of 10 μm to 1 mm. As illustrated inFIGS.5B-5C, the gasket450makes airtight contact with the bottom surface of the top wafer chuck410and the top surface of the bottom wafer chuck418. The gasket450is compressible to a thickness equivalent to the distance D1in a range of 10 μm to 1 mm.

Ambient pressure in the local process chamber460is controlled by flowing gas/air into the local process chamber460through the gas inlet(s)430and removing gas/air from the chamber405via the gas outlets404through the use of one or more vacuum pumps connected to the gas inlet(s)430and the gas outlet(s)432. In some embodiments, the local process chamber460has a volume in a range of 750 mm3to 123000 mm3, and the pressure inside the local process chamber460can be controlled, such as by the controller380regulating the pressure using the gas inlet(s)430and the gas outlet(s)432, at a faster speed than the pressure of the chamber405containing the top wafer chuck410and the bottom wafer chuck418. This may enable the pressure inside the local process chamber460to be quickly adjusted and maintained, which may enable increased throughput and reduced cost in comparison with controlling the pressure inside the larger chamber405. The airtight seal established by the gasket450around the local process chamber460allows for a gaseous purge capability in the local process chamber460. In some embodiments, an N2purge is performed using the gas inlet(s)430after forming the airtight seal in order to remove moisture from the local process chamber460. This may reduce edge bubble defects between the subsequently bonded wafers100and200caused by moisture condensation by the Joule-Thomson effect.

While the pressure inside the local process chamber460is adjusted, the controller380may maintain the pressure differential between the vacuum levels of the vacuum zones442and444and the pressure of the local process chamber460as constant. This may keep the force holding the wafers100and200on the bottom wafer chuck418and the top wafer chuck410, respectively, from changing, which may reduce distortion of the wafers100and200during bonding.

FIG.6Aillustrates the initiation of a bonding process of the wafers100and200. One or more of the push pins412are utilized to warp or deform one or more of the wafers100and/or200to initiate the bonding process. In some embodiments, the bonding process is performed by bringing the wafers100and200into contact by utilizing a combination of the top wafer chuck410, the bottom wafer chuck418, and the push pin412to apply pressure against the wafers100and200at a first point P1. For example, the push pin412may be extended through the top wafer chuck410to deform the wafer200and bring the wafer200into contact with the wafer100at the first point P1. The bonding then proceeds in a wave (also referred to as a bonding wave) from the first point P1and moving outwards towards the edges of the wafers100and200.

During the bonding process, the gasket450maintains an airtight seal of the local process chamber460between the top wafer chuck410and the bottom wafer chuck418. The ambient pressure inside the local process chamber460can be maintained constant (a static mode) during the bonding process, or the ambient pressure inside the local process chamber460can be changed over time (a dynamic mode) as the bonding process proceeds.

In the static mode, the pressure in the local process chamber460is adjusted prior to the initiation of the bonding process and maintained at a constant pressure until the completion of the bonding process. In some embodiments, the pressure is set to a low pressure, such as a pressure less than 1 atmosphere, which may significantly increase the bonding wave velocity and allow for high throughput. In some embodiments, the pressure is set to a high pressure, such as a pressure greater than 1 atmosphere, which may decrease the bonding wave velocity and reduce distortion of the bonded wafers100and200caused by uneven bonding wave velocity.

In the dynamic mode, the pressure in the local process chamber460is changed over time during the bonding process. For example, the pressure may be set to less than 1 atmosphere prior to the initiation of the bonding process in order to reduce an initiation delay of the bonding wave, and the pressure is increased to greater than 1 atmosphere after the initiation of the bonding process to decrease the bonding wave velocity and reduce distortion of the bonded wafers100and200caused by uneven bonding wave velocity. In some embodiments, other dynamic pressure profiles are used in order to compensate for customized distortion patterns. For example, one dynamic pressure profile includes starting with a pressure greater than 1 atmosphere prior to the initiation of the bonding process, decreasing the pressure to less than 1 atmosphere as the bonding process is initiated by bringing the wafers100and200into contact with the push pin412, and increasing the pressure to greater than 1 atmosphere as the bonding wave propagates across the wafers100and200.

FIG.6Billustrates a formation of bonds between the wafers100and200across the bonding interface between the wafers100and200, in accordance with some embodiments in which the bonding process includes oxide-oxide bonding. As the bonding wave proceeds outwards from the first point P1, hydrogen bonds between hydrogen and oxygen atoms of water molecules attached to silanol groups on the surfaces of the wafers100and200may be formed, such as through Van der Waals forces.

FIG.7illustrates a bonding wave500propagating outwards from the first point P1between the wafers100and200. The bonding wave velocity is proportional to the ambient pressure and in some embodiments can be described by the expression
V∝P−1/2
where V is the bonding wave velocity of the bonding wave and P is the ambient pressure. By adjusting the ambient pressure of the local process chamber460, the bonding wave velocity may be controlled. For example, in the static mode, a pressure less than 1 atmosphere such as lower than 760 torr (e.g., a vacuum, or the like) may allow for high throughput by increasing the bonding wave velocity. The increased bond wave velocity V leads to a reduced bonding time needed to bond the wafers100and200, which allows the wafer bonding system300to bond wafers together at a faster rate and therefore increase the wafer per hour (WPH) processing rate. A pressure greater than 1 atmosphere may reduce local stresses and bonding-induced distortion of the bonded wafers100and200caused by uneven bonding wave velocity by decreasing the bonding wave velocity. This leads to improved bonding alignment between the wafers100and200. In the dynamic mode, the ambient pressure of the local process chamber460may be increased or decreased as the bonding wave500propagates to enable bonding wave velocity control at different radial distances.

FIG.8illustrates the bonding wave500between the wafers100and200reaching a distance D2from the point P1, in accordance with some embodiments. Once the bonding wave500reaches a distance D2in a range of 50 mm to 120 mm depending on film properties and patterning scheme, the vacuum zones442on the top wafer chuck410are deactivated. This releases the wafer200from the top wafer chuck410and allows the bonding wave500to propagate to the edges of the wafers100and200. In some embodiments, the ambient pressure of the local process chamber460is controlled to be higher than 1 atmosphere as the bonding wave500propagates to the edges of the wafers100and200, which may reduce local stresses and bonding-induced distortion, improve bonding alignment, and decrease edge bubbles formed between the wafers100and200due to moisture condensation from the Joule-Thomson effect.

FIG.9Aillustrates the wafers100and200after the bonding wave500has propagated to the edges of the wafers100and200. Subsequently, in some embodiments an anneal is performed to form permanent adhesion (e.g., fusion bond) of the wafers100and200together by forming chemical bonds between the oxide surfaces. For example,FIG.9Billustrates the atoms (such as oxygen atoms) on the interface of the wafers100and200forming chemical or covalence bonds (such as Si—O—Si bonds) with the atoms (such as silicon atoms) in the wafers100and200. Slight variations in surfaces of the bonding structures can be overcome through the annealing process. In some embodiments a bond strength of about 0.5 to 10 J/m2can be exerted to hold the wafers100and200together.

After the bonding process is completed, the one or more push pins412is retracted and the top wafer chuck410and bottom wafer chuck418are separated, breaking the seal of the local process chamber460. The bonded wafers100and200are then removed from the bottom wafer chuck418, such as by a transfer robot306. The bonded wafers100and200may then be transferred back to the loading stations202or204by the transfer robot306, where the bonded wafers100and200are unloaded from the wafer bonding system300.

FIG.10illustrates a method1000of bonding two wafers100and200as illustrated inFIGS.1through9B. In step1010, a wafer200is mounted on a top wafer chuck410and a wafer100is mounted on a bottom wafer chuck418, as described above with respect toFIG.4A. In step1020, an airtight seal is formed around the local process chamber460between the top wafer chuck410and the bottom wafer chuck418by the gasket450, as described above with respect toFIGS.5A-5C. In step1030, the ambient pressure in the local process chamber460is adjusted, as described above with respect toFIGS.5A-5C. In step1040, the wafer200is brought into physical contact with the wafer100by using a push pin412, as described above with respect toFIG.6A. In step1050, the ambient pressure in the local process chamber460is controlled while the bonding wave500propagates, as described above with respect toFIGS.6A-7. In step1060, the push pin412is retracted and the bonded wafers100and200are removed from the bottom wafer chuck418, as described above in respect toFIGS.9A-9B.

Embodiments may achieve advantages. A wafer bonding system bonds a first wafer to a second wafer in an airtight local process chamber sealed by a gasket between wafer chucks holding the first and second wafers. Increased throughput and reduced cost for the bonding process are enabled by quickly adjusting and maintaining the ambient pressure inside the smaller volume of the local process chamber. By reducing ambient pressure in the local process chamber, bonding wave velocity is increased, which may raise the wafer per hour (WPH) processing rate. By increasing the ambient pressure in the local process chamber, the bonding wave velocity is decreased, which reduces local stresses and bonding-induced distortion of the bonded wafers caused by uneven bonding wave velocity. Edge bubble defects can be decreased by gaseous purges to reduce moisture enabled by the airtight seal of the local process chamber.

In accordance with an embodiment, a method of forming a semiconductor device includes: mounting a bottom wafer on a bottom chuck; mounting a top wafer on a top chuck, wherein one of the bottom chuck and the top chuck has a gasket; moving the top chuck towards the bottom chuck, wherein the gasket forms a sealed region between the bottom chuck and the top chuck around the top wafer and the bottom wafer; adjusting an ambient pressure in the sealed region; and bonding the top wafer to the bottom wafer. In some embodiments of the method, the ambient pressure is adjusted to a pressure lower than 760 torr. In some embodiments of the method, the ambient pressure is adjusted to a pressure greater than 1 atmosphere. In some embodiments of the method, the top wafer is mounted to the top chuck using a vacuum zone. In some embodiments of the method, while adjusting the ambient pressure in the sealed region, a vacuum pressure of the vacuum zone is adjusted to maintain a constant pressure differential between the ambient pressure and the vacuum pressure. In some embodiments of the method, the ambient pressure is held constant while bonding the top wafer to the bottom wafer. In some embodiments of the method, the ambient pressure is changed while bonding the top wafer to the bottom wafer.

In accordance with another embodiment, a method of forming a semiconductor device includes: forming an airtight seal around a local process chamber, the local process chamber being bounded by a first wafer chuck, a second wafer chuck, and a gasket, a first wafer being held by the first wafer chuck, a second wafer being held by the second wafer chuck; setting an ambient pressure in the local process chamber to a first pressure; bringing the first wafer and the second wafer into physical contact; changing the ambient pressure in the local process chamber to a second pressure while a bonding wave propagates between the first wafer and the second wafer; and removing the bonded first wafer and second wafer from the local process chamber. In some embodiments, the method further includes purging the local process chamber with N2. In some embodiments of the method, the gasket includes polytetrafluoroethylene. In some embodiments of the method, the gasket is mounted on the first wafer chuck, the gasket surrounding the first wafer. In some embodiments of the method, forming the airtight seal includes moving the first wafer chuck towards the second wafer chuck, the moving the first wafer chuck towards the second wafer chuck bringing the gasket into physical contact with the second wafer chuck. In some embodiments of the method, bringing the first wafer and the second wafer into physical contact includes extending a push pin through the first wafer chuck. In some embodiments of the method, the push pin is surrounded by an airtight seal.

In accordance with yet another embodiment, a wafer bonding system includes: a first wafer chuck in a chamber, the first wafer chuck having a first surface to support a first wafer; a second wafer chuck having a second surface to support a second wafer, the second surface being opposite the first surface, the second wafer chuck and the first wafer chuck being movable relative to each other; a gasket between the first wafer chuck and the second wafer chuck, the gasket forming an airtight seal around a local process chamber between the first wafer chuck and the second wafer chuck; and a pressure regulator, the pressure regulator configured to control an ambient pressure in the local process chamber. In some embodiments of the wafer bonding system, the gasket includes a compressible material. In some embodiments of the wafer bonding system, the compressible material is polytetrafluoroethylene, silicone rubber, butyl rubber, or a fluoropolymer elastomer. In some embodiments of the wafer bonding system, the gasket is compressible to a thickness in a range of 10 μm to 1 mm. In some embodiments of the wafer bonding system, the gasket has a round profile. In some embodiments of the wafer bonding system, the pressure regulator includes a gas inlet and a gas outlet.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.