NL2030087B1 - Method and device for temperature-controlled bonding of substrates with electromagnetic irradiation - Google Patents

Abstract

The invention relates to a method for bonding of two substrates (12, 14) comprising: providing a first substrate (12), a second substrate (14) aligned to the first substrate (12) and an 5 adhesive layer (20) disposed between the first (12) and the second substrate (14), wherein at least one of the substrates (12) is transparent for electromagnetic irradiation comprising electromagnetic waves of a wavelength at which the adhesive layer (20) is curable; irradiating electromagnetic waves through the transparent substrate (12), thereby at least partially curing the adhesive layer (20); controlling by means of a temperature control unit (24) the 10 temperature of at least one of the substrates (12, 14) such that the first and second substrate (12, 14) are at least partially bond together at said temperature. Furthermore, the invention relates to a device (10) for bonding of two substrates (12, 14). Fig 1 15

Description

P35429NLO0/ABL

Title: Method and device for temperature-controlled bonding of substrates with electromagnetic irradiation

The present invention relates to a method and a device for temperature-controlled bonding, in particular temporary bonding, of two substrates, in particular two wafers, with electromagnetic irradiation, in particular ultraviolet irradiation.

The packaging density of semiconductor devices significantly increased during recent years. Integrated circuit (IC) devices have become more powerful, yet smaller and thinner at the same time. However, the devices, which are often less than 100 um in thickness, are very fragile. Special processing techniques and substrate or wafer bonding techniques, such as permanent or temporary substrate/wafer bonding, have been developed to address the technical problems connected to manufacturing such devices.

The temporary wafer bonding process involves attaching a thin substrate wafer to a thicker and more rigid carrier wafer. The mechanical support provided by the carrier allows applying processing steps that would otherwise damage the substrate wafer. The bond between the two wafers is temporary. It is removed upon completion of the processing steps.

Typically, temporary bonding techniques make use of adhesive compounds applied between the wafers and thermally cured. A major shortcoming of this process is that the two wafers can have different thermal expansion coefficients. Heating the wafer stack to cure the adhesive leads to differential expansion of the wafers. This state of differential expansion is then preserved or locked in with the crosslinking or curing of the adhesive. Upon return to room temperature, this leads to wafer stack deformation like wafer bowing or warping due to the mechanical stresses.

Similar problems can arise with other bonding techniques, e.g. permanent bonding.

One solution to this problem is the application of optically sensitive adhesives, for example polymers that are curable by electromagnetic irradiation, e.g. ultraviolet (UV) irradiation. If electromagnetic (UV) curing is applied at room temperature, wafer bonding or warping can be reduced or even completely avoided.

However, electromagnetic irradiation sources typically also produce heat that may enter the processing chamber, thus heating the wafers and the adhesive during the curing process in an uncontrolled manner.

Hence, there is a need for a means to improve process control and reproducibility of electromagnetic irradiation based substrate/wafer bonding processes, e.g. temporary or permanently substrate/wafer bonding processes.

The object of the invention is solved by a method for bonding, in particular temporary bonding, of two substrates, in particular wafers, comprising: providing a first substrate, a second substrate aligned to the first substrate and an adhesive layer disposed between the first and the second substrate, wherein at least one of the substrates is transparent for electromagnetic irradiation, in particular ultraviolet (UV) irradiation, comprising electromagnetic waves of a wavelength at which the adhesive layer is curable; irradiating the electromagnetic irradiation, in particular the UV irradiation, through the transparent substrate, thereby at least partially curing the adhesive layer; controlling by means of a temperature control unit the temperature of at least one of the substrates such that the first and second substrate are at least partially bond together at said temperature.

Especially, the method is suitable for bonding substrates being wafers.

For example, the electromagnetic waves used in the method have a wavelength in the range of the UV spectrum. In other words, UV irradiation can be used in the method.

The method is especially suitable for temporary substrate bonding.

Alternatively, the method can be used for other substrate bonding techniques, e.g. permanent substrate bonding.

The temperature control allows to precisely regulate the substrate and/or adhesive layer temperature independent from additional heat sources, such as the electromagnetic irradiation source and/or parasitic absorption of electromagnetic waves within the substrates.

If the amount of heat introduced to the system is too high, the substrate stack can be cooled, e.g. during irradiation. For example, a bond can be formed at room temperature, locking the substrates together in their current state of expansion. Thus, subsequent thermal processing steps, such as additional thermal curing and/or post baking will result in reduced substrate stack deformation upon returning to room temperature after the bonding process is finished.

On the other hand, if the heat introduced by the curing setup is small, the substrates and the adhesive layer may even be actively heated to achieve shorter processing times and higher throughput.

The bonding step can at least comprise an, in particular horizontal and/or lateral alignment of the substrates and/or an, in particular vertical pressing of the substrates against each other, e.g. via compression between chucks, such that an aligned substrate stack is formed.

The proposed method furthermore allows changing the electromagnetic irradiation source, in particular spectral range and/or intensity, during curing while keeping the desired processing temperature constant.

In one variant, the transparent substrate is located on a transparent chuck through which the electromagnetic irradiation, in particular the UV irradiation, is irradiated. This allows to couple the electromagnetic irradiation into the transparent substrate and adhesive layer through the chuck, particularly also from the bottom side of the curing chamber. Furthermore, the transparent chuck itself can act as light guiding element, spreading and/or homogenizing the electromagnetic irradiation.

In a simple preferred arrangement, the first or second substrate is located on a chuck, wherein the temperature control unit controls the temperature of the chuck. The substrates are typically very thin and thermally highly conductive. Thus, the temperature control of the chuck enables fast temperature adjustment of the contacting substrate and therefore appropriate reaction to sudden process parameter changes.

Additionally, the temperature of the adhesive layer can be influenced by heating and/or cooling of the chuck, thereby increasing the process control during curing.

In another embodiment, the first or second substrate is located on at least one support such that both substrates are exposed to a gas, wherein the temperature control unit controls the temperature and/or flow rate of the gas. This arrangement is in particular useful for substrates with highly sensitive surfaces, because direct contact can be avoided. Furthermore, it enables introducing and/or removing heat homogeneously throughout the whole exposed surface of both substrates, thereby avoiding undesired temperature gradients within the substrate stack.

In a further preferred embodiment of the method, the temperature of at least one of the substrates (12, 14) and/or the adhesive layer is increased by controlling a temperature control means, in particular a Peltier element and/or an electric heating element, via the temperature control unit. The heating allows to achieve higher throughput speeds.

In particular, the steps of irradiating the electromagnetic waves and controlling the temperature of at least one of the substrates are performed simultaneously and/or in the one and same curing chamber. Thus, the processing time and/or the number of chambers as well as the number of additional transportation of the substrates between the chambers can be reduced.

The object of the invention is also solved by a device for bonding, in particular temporary bonding, of two substrates, in particular two wafers, comprising: a curing chamber with a substrate space for receiving a first substrate, a second substrate aligned to the first substrate and an adhesive layer disposed between the first and the second substrate, wherein at least one of the substrates is transparent for electromagnetic irradiation, in particular ultraviolet (UV) irradiation, comprising waves of a wavelength at which the adhesive layer is curable; an electromagnetic irradiation source, in particular UV irradiation source, which is located such that the emitted electromagnetic irradiation can pass through the transparent substrate onto the adhesive layer; and a temperature control unit adapted to control the temperature of at least one of the substrates such that the first and second substrate are at least partially bond together at said temperature. The advantages and features that were discussed for the method also apply for the device.

Especially, the device is suitable for bonding substrates being wafers.

For example, the electromagnetic waves used in the device have a wavelength in the range of the UV spectrum. In other words, UV irradiation can be used in the device.

The proposed device is especially suitable for temporary substrate bonding.

Alternatively, the device can be used for other substrate bonding techniques, e.g. permanent substrate bonding.

In one variant, the device comprises a chuck for holding the first or second substrate.

Preferably, the chuck is transparent for the electromagnetic irradiation, in particular the UV irradiation. This enables irradiating the substrates and/or adhesive layer through the chuck, in particular also from the bottom side.

In a preferred embodiment, the device comprises a temperature control means, in particular a Peltier element and/or an electric heating or cooling element and/or liquid filled channels, wherein the temperature of the chuck is controllable by the temperature control unit via the temperature control means. The temperature control means can be for example located at the chuck surface or embedded inside of the chuck. Fast heat transfer and therefore also temperature adjustment of the substrates and/or adhesive layer is achieved by a direct contact between chuck and contacting substrate, the high thermal conductivity of the chuck and the substrates as well as the short distances between the temperature control means, the substrates and the adhesive layer.

In another embodiment, the device comprises at least one support for carrying the first or second substrate such that both substrates are exposed to a gas the temperature of which is controlled by the temperature control means. The advantages of contact free and homogeneous substrate temperature control by a gas flow that were described for the method are enabled by this configuration and also apply to the device.

In a simple arrangement, the electromagnetic irradiation source is located outside of the curing chamber. Thus, transfer of waste heat from the electromagnetic irradiation source to the curing chamber is avoided, allowing to, e.g. temporarily or permanently, bond highly temperature sensitive and/or very thin substrates.

In another variant, the electromagnetic irradiation source is located inside of the curing chamber. This enables to make use of the electromagnetic irradiation source waste heat for curing and/or post baking the adhesive layer. The temperature control unit thereby ensures that the waste heat does not result in undesired temperature changes of the adhesive layer.

In a preferred embodiment, the electromagnetic irradiation source comprises an array of light emitting diodes (LEDs). Compared to conventional lamps, such as mercury vapor lamps, LEDs are more energy efficient and produce less waste heat, thereby facilitating the temperature control.

To enhance electromagnetic irradiation homogeneity, a beam shaping means can be located between the electromagnetic irradiation source and the transparent substrate.

Thereby, homogeneous curing is achieved and mechanical stresses within the substrate stack are minimized.

In particular, the device is adapted to irradiate electromagnetic waves and to control the temperature of at least one of the substrates simultaneously and/or in the one and same curing chamber. Thus, the processing time and/or the number of chambers as well as the number of additional transportation of the substrates between the chambers can be reduced.

Further advantages and features will become apparent from the following description of the invention and from the appended figures, which show a non-limiting exemplary embodiment of the invention and in which: - Fig. 1 schematically shows a side view of a first embodiment of a device for temporary bonding of two wafers with an LED array and a temperature controllable chuck; - Fig. 2 schematically shows a side view of a second embodiment of a device for temporary bonding of two wafers with two LED arrays, a beam shaping means and a temperature controllable chuck; - Fig. 3 schematically shows a side view of a third embodiment of a device for temporary bonding of two wafers with a UV lamp, support pins for carrying wafers and a gas temperature control; and - Fig. 4 schematically shows a side view of a fourth embodiment of a device for temporary bonding of two wafers with an LED array and a transparent chuck.

All embodiments in the following are related to temporary wafer bonding with UV irradiation. However, other bonding techniques, e.g. permanent wafer bonding, substrates, e.g. glass or plastic substrates, or irradiation forms, e.g. infrared irradiation, are also conceivable.

Accordingly, substrates and wafers have the same reference signs 12 and 14.

Fig. 1 schematically shows a first embodiment of a device 10 for bonding, in particular temporary bonding, of two wafers 12, 14. The device 10 has a curing chamber 16 having walls which confine a substrate space 18.

In the described embodiment, a wafer stack is located in the substrate space 18. The wafer stack consists of a transparent carrier wafer 12, a thin substrate wafer 14 aligned in parallel to the carrier wafer 12, and a UV curable adhesive layer 20 located between both wafers 12, 14.

The device 10 furthermore comprises a chuck 26 for holding the wafer stack. The wafer 14 or wafer stack can for example be fixed to the chuck 26 by vacuum, electrostatic forces and/or mechanical clamps.

Furthermore, the device 10 comprises a UV irradiation source 22 which is located such that emitted UV irradiation can pass through the transparent wafer 12 onto the adhesive layer 20.

In the described embodiment, the UV irradiation source 22 is located inside the curing chamber 16 and consists of an array of LEDs 28. The LEDs are adapted to emit UV irradiation in a spectral range in which the uncured adhesive material is absorbing.

The adhesive material may for example comprise a polymer and a photoactive component for starting a curing reaction, in particular a crosslinking and/or polymerization reaction.

Of course, the polymer can also have distinct absorption bands, which may or may not coincide with the photoactive component.

Suitable LEDs 28 might be for example Indiumgalliumnitride LEDs emitting at a wavelength of 365 nm.

Despite the typically narrow emission spectra of such LEDs 28, there is still generation of heat from the irradiation source 22 as well as parasitic absorption of the UV irradiation in the walls of the curing chamber 16 and the wafers 12, 14.

To avoid this additional heat from having an uncontrolled effect on the curing of the adhesive layer 20, the device 10 is equipped with a temperature control unit 24.

In the embodiment depicted in Fig. 1, the temperature control unit 24 is adapted to control during irradiation the temperature and/or flow rate of a liquid flowing through channels 30 inside of the chuck 26. It thereby dictates the temperature of the chuck 26 itself as well as the wafer stack in direct contact with chuck 26 such that the temperature of the wafers 12, 14 can be set to and/or kept at a desired value during UV curing of the adhesive layer 20.

It is thereby conceivable that the control unit 24 is adapted to either heat or cool the chuck 26 dependent on the processing situation, in particular the current temperature of the wafer stack and/or chuck 26.

It is furthermore conceivable, that the control unit 24 is adapted control the temperature of the chuck 26 and/or wafer stack and/or adhesive layer 20 according to a desired preset value, in particular independent from the irradiation source 22.

A second embodiment of a device 10 for temporary bonding of two wafers 12, 14 is shown in Fig. 2. The second embodiment corresponds in several essential features to the first embodiment, so that only the differences will be discussed below. Identical and functionally identical parts are provided with the same reference symbols.

In the embodiment shown in Fig. 2, the chuck 26 is also equipped with a temperature control means 30. However, in contrast to the first embodiment, the temperature control means 30 is an electric heating element with a heating coil. In this particular embodiment, active cooling is not possible.

Of course, the nature of the temperature control means 30 is not limited to the described embodiments. In particular, it is also conceivable, that the temperature control means 30 is a Peltier element which allows direct electric cooling and/or heating of the chuck 26.

The embodiment shown in Figure 2 further comprises beam shaping means 32 located between the UV irradiation source 22 and the transparent wafer 12. The beam shaping means 32 may comprise optical lenses to parallelize the UV irradiation as well as scattering elements for homogenizing the intensity distribution.

A third embodiment of a device 10 for temporary bonding of two wafers 12, 14 is shown in Fig. 3. The third embodiment corresponds in several essential features to the first and second embodiment, so that only the differences will be discussed below. Identical and functionally identical parts are provided with the same reference symbols.

The device 10 shown in Fig. 3 does not comprise a conventional chuck 26 for holding wafers 12, 14. Instead, the curing chamber 16 is equipped with support pins for carrying one of the wafers 12, 14.

In this embodiment, the support is formed by a plurality of support pins 34 on which the wafers 12, 14 are placed. Accordingly, the surface of both wafers 12, 14 which is not in contact with the adhesive layer 20 is directly exposed to a gas flow. The temperature and/or flow rate of the gas can thereby be controlled by the temperature control unit 24. Dashed arrows indicate the gas flow direction in Fig. 3.

Furthermore, in the described embodiment, the UV irradiation source 22, which can for example be a mercury vapor lamp or a xenon lamp, is located outside of the curing chamber 16. Thereby preventing heat from the UV irradiation source 22 from coupling into the curing chamber 16.

A fourth embodiment of a device 10 for temporary bonding of two wafers 12, 14 is shown in Fig. 4. Again, this embodiment corresponds in several essential features to the previously described embodiments. Only the differences will be discussed below.

In Fig. 4, the chuck 26 is transparent for UV irradiation. The irradiation emitted from an array of LEDs 28 located at the bottom of the curing chamber 16 can pass through the chuck 26 and the transparent wafer 12 onto the adhesive layer 20. In the described embodiment, the transparent chuck 26 comprises a thin scattering layer 32 to homogenize the UV irradiation.

All different device embodiments shown in Fig 1 to Fig 4 are intended to be used for temporary bonding of two wafers 12, 14. To achieve this purpose, a method according to the invention can be applied, which will be described in the following.

In a first step of the method, the wafer stack consisting of the substrate wafer 14, the transparent carrier wafer 12 and the UV curable adhesive layer 20 is provided to the curing chamber 16.

It is conceivable, that the wafer stack is assembled and aligned inside the curing chamber 16 or before providing it to the curing chamber 16 in a separate process module.

In a second step, the UV irradiation source 22 irradiates UV irradiation through the transparent wafer 12, thereby at least partially curing the adhesive layer 20. Even if the adhesive layer 16 is not fully cured, the wafers 12, 14 are already fixed to each other, thereby reducing and/or preventing temperature induced deformation, in particular during subsequent processing steps.

In a third step, the temperature control unit 24 controls the temperature of at least one of the wafers 12, 14 during irradiation such that the wafers 12, 14 are bond together at said temperature.

It is conceivable, that the temperature control allows to heat up and/or to cool down the wafers 12, 14 and/or adhesive layer 20 independent from additional heat sources and/or other process parameters. Thus, uncontrolled curing of the adhesive layer 20, for example due to waste heat from the UV irradiation source 22, can be avoided.

In one embodiment of the method, the temperature of the wafers 12, 14 and/or adhesive layer 20 is increased by controlling a temperature control means 30, in particular a

Peltier element and/or an electric heating element, via the temperature control unit 24. This allows to decrease the curing time and thus to achieve higher throughput rates.

In a further embodiment of the method, in particular suitable for temporary bonding of two wafers 12, 14 using a device 10 as shown in Fig. 1 and/or Fig 2, the control unit 24 controls the temperature of a chuck 26 located inside the curing chamber 16 and thus also a wafer stack in contact with the chuck 26.

In an alternative embodiment of the method, in particular suitable for temporary bonding of two wafers 12, 14 using a device 10 as shown in Fig. 3, the temperature control unit 24 controls the temperature and/or flow rate of a gas to which the wafers 12, 14 are exposed.

In yet another embodiment of the method, in particular suitable for temporary bonding of two wafers 12, 14 using a device 10 as shown in Figure 4, the UV irradiation is irradiated through a transparent chuck 26.

Claims (16)

CONCLUSIONS A method of bonding two substrates (12, 14) comprising: providing a first substrate (12), a second substrate (14) aligned with the first substrate (12), and an adhesive layer (20) disposed between the first (12) and second substrates (14), at least one of the substrates (12) being transparent to electromagnetic radiation comprising electromagnetic waves having a wavelength at which the adhesive layer (20) can cure; irradiating the electromagnetic radiation through the transparent substrate (12), thereby at least partially curing the adhesive layer (20); controlling the temperature of at least one of the substrates (12, 14) by means of a temperature control unit (24) such that the first and second substrates (12, 14) at least partially adhere to each other at said temperature. The method of claim 1, wherein the transparent substrate (12) is on a transparent head (26) and wherein the electromagnetic radiation is irradiated through the transparent head (26). The method of claim 1 or claim 2, wherein the first or second substrate (12, 14) is on a head (26), the temperature control unit (24) controlling the temperature of the head (26). The method of claim 1, wherein the first or second substrate (12, 14) is on at least one support (34) such that both substrates (12, 14) are exposed to a gas, the temperature control unit (24) controlling the temperature and/or flow rate of the gas. A method according to any one of the preceding claims, wherein the temperature of at least one of the substrates (12, 14) and/or the adhesive layer (20) is increased by controlling a temperature control means (30), in particular a Peltier element and/or an electric heating element, via the temperature control unit (24). A method according to any one of the preceding claims, wherein the steps of irradiating the electromagnetic waves and controlling the temperature of at least one of the substrates (12, 14) simultaneously and/or in one and the same curing chamber (16) are carried out. An apparatus for bonding two substrates (12, 14), comprising: a curing chamber (16) having a substrate space (18) for receiving a first substrate (12), a second substrate (14) aligned with the first substrate (12) and an adhesive layer (20) disposed between the first and second substrates (12, 14), at least one of the substrates (12) being transparent to electromagnetic radiation comprising waves having a wavelength at which the adhesive layer (20) can cure; an electromagnetic radiation source (22) arranged so that the emitted electromagnetic radiation can pass through the transparent substrate (12) onto the adhesive layer (20); and a temperature control unit (24) configured to control the temperature of at least one of the substrates (12, 14) such that the first and second substrates (12, 14) at least partially adhere to each other at said temperature. An apparatus according to claim 7, comprising a head (26) for holding the first or second substrate (12, 14). An apparatus according to claim 8, wherein the head (26) is transparent to the electromagnetic radiation. Device according to claim 8 or 9, wherein the head (26) comprises a temperature control means (30), in particular a Peltier element and/or an electrical heating or cooling element and/or liquid-filled channels, wherein the temperature of the head (26) is controllable by the temperature control unit (24) via the temperature control means (30). An apparatus according to claim 7, comprising at least one support (34) for supporting the first or second substrate (12, 14) such that both substrates (12, 14) are exposed to a gas whose temperature is controlled by the temperature control unit (24). An apparatus according to any one of claims 7 to 11, wherein the electromagnetic radiation source (22) is located outside the curing chamber (18). An apparatus according to any one of claims 7 to 11, wherein the electromagnetic radiation source (22) is located in the curing chamber (16). An apparatus according to any one of claims 7 to 13, wherein the electromagnetic radiation source (22) comprises an array of light emitting diodes. An apparatus according to any one of claims 7 to 14, comprising a beamforming means (32) located between the electromagnetic radiation source (22) and the transparent substrate (12). An apparatus according to any one of claims 7 to 15, wherein the apparatus is arranged to irradiate the electromagnetic waves and to measure the temperature of at least one of the substrates (12, 14) simultaneously and/or in one and control the same curing chamber (16).