WO2011128319A1

WO2011128319A1 - Method for handling a wafer using a support structure

Info

Publication number: WO2011128319A1
Application number: PCT/EP2011/055674
Authority: WO
Inventors: Ciaran Mcardle; Maria Parals Sendin; Terry Sterrett
Original assignee: Henkel Ag & Co. Kgaa; Henkel Corporation; Henkel Ireland Ltd.
Priority date: 2010-04-13
Filing date: 2011-04-12
Publication date: 2011-10-20
Also published as: TW201208071A

Abstract

The present invention relates to a method for handling a wafer using a support structure comprising a shape memory polymer film (3) The invention further relates to an assembly, comprising a wafer and a support structure, wherein the support structure comprises at least one shape memory polymer film.

Description

METHOD FOR HANDLING A WAFER USING A SUPPORT STRUCTURE

FIELD OF THE INVENTION

The present invention relates to a method for handling a wafer. The invention further relates to an assembly, comprising a wafer and a support structure, wherein the support structure comprises at least one shape memory polymer film.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

Thinning semiconductor wafers may be desirable, for example, because thinner wafers may have reduced thermal resistance and increased reliability.

Thinning the wafer may reduce the die stress from thermal effects, thereby increasing reliability. The thickness of the wafer may also affect the package size adversely. In some cases, the thickness of the wafer may adversely affect performance. Thus, for a variety of reasons it is desirable to thin semiconductor wafers.

Normally, the wafer thinning is performed by bringing the wafer surface in contact with a hard and flat rotating horizontal platter that contains a liquid slurry. The slurry may contain abrasive media with chemical etchants such as ammonia, fluoride, or combinations thereof. The wafer is maintained in contact with the media until an amount of substrate has been removed to achieve a targeted thickness.

It is known that adhesive materials can be used as support structures to hold wafers, such as bumped wafers, during the thinning processes.

In this context, US 7,226,812 B2 discloses a method for handling wafers in which a sacrificial polymer is spray-coated on the wafer bump side to form a thin layer. An adhesive layer is then used to attach the wafer bump side onto a wafer support structure over the sacrificial polymer to support the wafer in backside processing. After wafer thinning the wafer is exposed to heat to thermally decompose the sacrificial polymer into gases. The decomposition of the sacrificial polymer reduces the adhesion of the adhesive layer to the bump side of the wafer such that, when the support substrate is detached from the wafer, the adhesive layer is detached together with the support substrate from the wafer bump side. The method requires the application of shearing, wedging, and/or peeling forces in order to separate the support structure from thinned wafer surface. Additionally, the method does not allow to reuse the support structure. US2006/0046433 A1 teaches a method for handling a wafer comprising the steps of forming a plurality of protrusions on a wafer and engaging said protrusions in a fixture; and exposing the wafer to a grinding operation to thin said wafer. The fixture has a series of protrusions that form an interference fit with surface features extending outwardly from the non-thinned surface of the wafer to be thinned. In some embodiments, openings in a shape memory material may be utilized that, upon heating, more firmly engage the bumps on the wafer to be thinned.

As wafers are made thinner and thinner still, the severity of wafer damage may increase dramatically. At thicknesses below 100 μηη, wafers are prone to cracking, surface burnishing, and surface irregularities. Bumped wafers may have additional problems related to surface pitting in areas between bumps. Especially with very thin wafers, the wafers are prone to damage during removal from the thinning fixture.

Notwithstanding the state of technology it would be desirable to provide better methods for handling wafers, especially better methods for thinning wafers, in which the mechanical force needed to remove the wafer from the support structure is low and no subsequent cleaning step is needed to remove residues of the support structure from the wafer surface.

SUMMARY OF THE INVENTION

The present invention provides a method for handling a wafer, such as a method for thinning a wafer, in which a wafer, having a plurality of protrusion on its surface, is effectively held by a support structure to reduce the wafer damage during mechanical processes, such as grinding.

The support structure comprises at least one shape memory polymer film having an inner surface and an outer surface, wherein the outer surface of the shape memory polymer forms the exterior surface of the support structure. Subsequent to processing, said support structure can be removed substantially completely from the wafer surface in a stress-free process by applying a low peel force. The support structure is therefore reusable, which reduces waste and overall cost.

In its broadest sense, the present invention provides a method for handling a wafer, comprising the steps of:

a) providing a wafer and a support structure,

wherein the wafer has a plurality of protrusions on its surface,

and wherein the support structure comprises at least one shape memory polymer film having an inner surface and an outer surface, wherein the outer surface of the shape memory polymer film forms the exterior surface of the support structure and said exterior surface exhibits a flatness index (Fl) of less than 0.02 and said shape memory polymer film has an average film thickness of 10 to 1200 μηι; b) bringing the protrusions into contact with the exterior surface of the support structure at a pressure of 0.1 to 100 MPa, preferably at a pressure of 1 to 100 MPa and at a temperature above the glass transition temperature (T_g) of the at least one shape memory polymer to form an assembly; c) cooling the formed assembly to a temperature of more than 20°C below the glass transition temperature (T_g) of the at least one shape memory polymer at a pressure of 0.1 to 100 MPa, preferably at a pressure of 1 to 100 MPa.

Another aspect of the present invention is an assembly, comprising a wafer and a support structure, wherein the assembly is obtained by the method of the present invention.

A further aspect of the present invention is an assembly, comprising a wafer and a support structure, wherein the wafer has a plurality of protrusions on its surface and the support structure comprises at least one shape memory polymer film having an inner surface and an outer surface, wherein said outer surface is attached to the protrusions of the wafer, with the proviso, that the outer surface of the shape memory polymer film forms the exterior surface of the support structure and said exterior surface exhibits a flatness index of less than 0.02 prior to the attachment of the wafer and said shape memory polymer film has an average film thickness of 10 to 1200 μηι.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, one aspect of the present invention is a method for handling a wafer, comprising the steps of:

a) providing a wafer and a support structure,

wherein the wafer has a plurality of protrusions on its surface,

The flatness index, as used in the present invention, is defined as the ratio of roughness of the exterior surface of the support structure to the average film thickness of the shape memory polymer film.

The flatness index can be determined as shown in Fig. 1 to 3. The term roughness, as used in the present invention, means the roughness average, which is defined as the arithmetic average of peak P_n and valley V_n distances D_n.

The term peak P_n, as used in the present invention, refers to any protruding area of the exterior surface, whereas the term valley V_n, as used in the present invention, refers to any recess between protruding areas of the exterior surface of the support structure.

The term distance D_n, as used in the present invention, means the difference in height of a peak P_n and neighbored valley V_n measured in a substantially orthogonal direction to the longitudinal extension 2 of the shape memory film 3.

In other words the distance D_n is defined as the distance, measured in a substantially orthogonal direction to the longitudinal extension 2, of two straight lines extending substantially parallel to the longitudinal extension 2, wherein one straight line subtends a peak P_n and the other straight line subtends a neighbored valley V_n.

The term longitudinal extension, as used in the present invention, refers to a plane extending substantially parallel to the surfaces of the support structure, wherein the asperity of the surfaces of the support structure is not taken into account.

The roughness can be measured, for example, along a line of usually 0.08 cm on the exterior surface of the support structure. Preferably the roughness, as used in the present invention, is the arithmetic average of 15 different measurements at 15 different positions on the exterior surface of the support structure. The roughness can be determined at a measurement speed of 0.5 mm/s by using a Solarius non-contacting Laser Profilometer equipped with an AF2000 autofocus sensor. The obtained data can be analyzed by using solar map universal 3.1 .10 image analysis software (Gaussian filter 0.8 mm), wherein a microroughness filtering is used, with a cutoff of 2.5 m.

The average film thickness T of the shape memory polymer film can be determined as shown in Fig. 3. The average film thickness T is the arithmetic average of a multitude of film thickness values T_n, wherein each film thickness value T_n is measured in a substantially orthogonal direction to the longitudinal extension 2 of the shape memory film 3 between two opposing points, wherein one point is located on the inner surface of the shape memory polymer film, and the corresponding point is located on the opposing outer surface of the shape memory polymer film.

The term multitude, as used above, can refer, for example, to at least 10 different measuring points at 10 different positions along a profile section of the support structure 1.

The average film thickness T can be determined, for example, by using a section of the support structure (1 cmx1 cm in size) divided into 5 equal strips (each 2 mm) and in each case the average film thickness T of the shape memory polymer film is determined along the profile. The average film thickness T along the profile can be determined by scanning electron microscopy (SEM) preferably using a scanning electron microscope JEOL JSM-6060 SEM.

The exterior surface of the support structure can be regarded as a flat or as a non- microstructured surface. The term "microstructured" refers to surfaces, comprising

microstructured features, wherein the term "microstructured features" refers to features of a surface that have at least one, preferably all three dimensions (e.g., height, length, width, or diameter) of less than one millimeter.

In one embodiment step b) of the inventive method comprises contacting the exterior surface of the support structure and the wafer at a temperature above the glass transition temperature (T_g) of the shape memory polymer for 30 seconds to 100 minutes, such as 1 to 5 minutes, at a pressure of 4 to 50 MPa.

Preferably, the exterior surface of the support structure and the protrusions are contacted at a temperature of 90°C to 180°C, more preferably at a temperature of 100°C to 160°C, and most preferably at a temperature of 1 10°C to 140°C and/or at a pressure of 4 to 30 MPa, more preferably at a pressure of 5 to 10 MPa and most preferably at a pressure of 5.5 to 8 MPa. Contacting the exterior surface of the support structure and the protrusions at a temperature of 100°C to 160°C and/or at a pressure of 5.5 to 8 MPa is/are advantageous, because these process conditions cause a better adhesion between the wafer and the support structure.

In a further embodiment step c) of the inventive method comprises cooling the formed assembly to a temperature of more than 20°C below the glass transition temperature (T_g) of the at least one shape memory polymer at a pressure of 4 to 50 MPa.

Preferably, the formed assembly is cooled to a temperature of more than 20°C, preferably of more than 30°C, and more preferably of more than 40°C below the glass transition temperature (T_g) of the at least one shape memory polymer at a pressure of 4 to 30 MPa, more preferably at a pressure of 5 to 10 MPa and most preferably at a pressure of 5.5 to 8 MPa.

In certain embodiment the inventive method comprises the additional step d) of exposing the wafer to a grinding operation to thin said wafer, preferably to a thickness of less than 100 μηη.

In order to remove/release the support structure from the wafer surface, the inventive method can comprises the additional step of exposing the support structure to a temperature above the glass transition temperature (T_g) of the shape memory polymer for a period of time sufficient to remove said support structure form the wafer.

In this context, it is advantageous to use support structures having an exterior surface which exhibits a flatness index (Fl) of less than 0.02, because the flat nature of said surface allows the removal of the support structure in a very efficient way. Support structures having an exterior surface which exhibits a flatness index (Fl) of less than 0.02 can be removed substantially completely from the wafer surface in a stress-free process by applying a low peel force.

Preferably the support structure is removed by exposing it to a temperature of at least 10°C, more preferably to a temperature of at least 20°C, and particularly preferably to a temperature of at least 30°C above the glass transition temperature (T_g) of the shape memory polymer.

By applying an appropriate temperature, the support structure of the present invention can be removed substantially completely from the wafer surface in short time periods, preferably in less than 15 minutes, more preferably in less than 10 minutes, and particularly preferably in less than 5 minutes.

The term "substantially completely", as used in the present invention, preferably means, that less than 5 wt.-%, preferably less than 1 wt.-%, more preferably less than 0.5 wt.-%, and particularly preferably less than 0.1 wt.-%, based on the total weight of the support structure, remain on the wafer surface after the support structure is removed from the wafer surface.

The term "peel force" as used in the present invention preferably refers to the 90° peel force needed for peeling the adhered surfaces (exterior surface of the support structure and wafer surface) apart. Said 90° peel force can be determined at 23°C according to ASTM D6862-04 test method using a TXT plus tensile tester (available from Stable Micro Systems, Surrey UK) using 5 Kg load cell and a crosshead speed of 25 mm/min.

As used in the present invention a peel force, such as the 90° peel force, is regarded as being low, if the peel force, such as the 90° peel force, needed to separate the support structure and the wafer at 23°C is less than 0.01 N/mm, preferably less than 0.005 N/mm, more preferably less than 0.002 N/mm, and particularly preferably less than 0.001 N/mm.

It is desirable that the flatness index (Fl) of the exterior surface of the support structure of the present invention is less than 0.01 , more preferably less than 0.005, and particularly less than 0.001 , because the very flat nature of said surface allows the substantially complete removal of the support structure from the wafer surface in a stress-free process by applying a very low peel force, such as a 90° peel force of less than 0.005 N/mm.

As noted above, the exterior surface the support structure of the present invention is formed by a shape memory polymer film.

The term "shape memory polymer" as used in the present invention refers to polymeric materials that are stimuli-responsive. Upon application of an external stimulus they have the ability to change their shape. A change in shape initiated by a change in temperature can be referred to as a thermally induced shape memory effect.

The at least one shape memory polymer of the present invention preferably has a glass transition temperature (T_g) of between 40°C and 200°C, preferably between 50°C and 150°C and more preferably between 100°C and 120°C.

The glass transition temperature (T_g) is preferably determined by DMA (Dynamic mechanical analysis), preferably using a Rheometrics Solids Analyzer (RSA-3) from TA Instruments-Waters LLC, New Castle. The glass transition temperature (T_g) can be determined as described in the examples of the present invention. Alternatively, the glass transition temperature (T_g) of the shape memory polymer can be determined by DSC (Differential Scanning Calorimetry), preferably using a TA Instruments Q20 under the following conditions: a small sample of around 10 mg of the shape memory polymer was loaded into the instrument at 20°C and cooled down to -20°C. After equilibration at this temperature for 20 minutes the sample was heated to 200° C at a heating rate of 5 °C/minute.

In certain embodiments of the present invention the shape memory polymer is a reaction product formed by curing a curable composition, comprising

i) at least one crosslinkable component which forms an elastomer when cured; and

ii) distributed within said crosslinkable component in a shape-holding amount a polymeric powder which remains discrete in the cured elastomer and has a melt temperature below the degradation temperature of the cured elastomer.

The at least one crosslinkable component of the present invention is preferably selected from curable silicone compositions. Various types of curable silicone compositions may be employed. For example, heat curing silicone compositions, moisture curing silicone compositions and photocuring silicone compositions may be employed. Polymodal curing silicone compositions, for example photo and moisture dual curing compositions or heat and moisture dual curing silicone compositions are also useful.

Desirably, the curable silicone compositions are heat cure compositions. In particular, such heat cure silicone compositions include reactive polyorganosiloxanes containing reactive functional groups such as vinyl or allyl groups, or (meth)acrylate groups. In addition to the reactive polyorganosiloxanes, such heat curing compositions also include a silicon hydride cross-linker and an organo-metallic hydrosilation catalyst.

Examples of useful reactive polyorganosiloxanes which may be employed in heat curing silicone compositions include those which conform to formula (I) below:

formula (I) where R¹, R², R³ and R⁵ can be the same or different and are substituted or unsubstituted hydrocarbon or hydrocarbonoxy radicals from C1-2₀, provided that at least two of these R groups, and desirably more than two, are reactive functional groups such as olefinic groups, including vinyl, (meth)acrylate, maleate and cinaminate groups. For example, when one or more of the aforementioned R groups (R¹, R², R³ and R ⁵) is not one of the required reactive functional groups, they can be chosen from alkyl radicals such as methyl, propyl, butyl and pentyl; alkenyl radicals such as vinyl and allyl; cycloalkyl radicals such as cyclohexyl and cycloheptyl; aryl radicals such as phenyl; arylalkyl radicals such as betaphenylethyl; alkylaryl radicals; and hydrocarbonoxy radicals such as alkoxy, aryloxy, alkaryloxy, aryalkoxy, and desirably methoxy, ethoxy or hydroxy, and the like. Any of the foregoing radicals having some or all of the hydrogen atoms replaced, for example, by a halogen such as fluorine or chlorine, are also useful. One or more of the aforementioned R groups can also be hydrogen, provided the required reactive functional group is present as indicated and the presence of the hydrogen does not

deleteriously interfere with the ability of the polyorganosiloxane to perform in the present invention. R³ in the above formula desirably is:

O R⁴

R⁶— O— C ii C I =CH where R⁶ is a substituted or unsubstituted hydrocarbon radical Ci_-2o and desirably is an alkyl group such as propyl; and R⁴ is H or CH₃.

The number of repeating units in the reactive polyorganosiloxanes can be varied to achieve specific molecular weights, viscosities and other chemical or physical properties. Generally n is an integer such that the viscosity is from about 25 cps to about 2,500,000 cps at 25°C, such as when n is from 1 to 1200 and desirably from 10 to 1000.

The reactive polyorganosiloxanes of the present invention may include as part of their backbone one or more divalent substituted or unsubstituted C i_₂o aliphatic, cycloaliphatic or aromatic hydrocarbon radicals, which may be interrupted with a heteroatom-containing linkage. The heteroatom may include N, O or S. Among the useful divalent hydrocarbon radicals include alkylenes, polyolefins, polyethers, polyesters, polyurethanes and combinations thereof.

Desirably the crosslinkable component includes a compound (reactive polyorganosiloxane) having the formula (II): MA R- 5 MA

(CH₃0)_2-c SiO (SiO)_n— Si (OCH₃)_2- ^■,c

(CH₃)_C R 5 (CH₃)_C

formula (II) wherein MA is a methacryloxypropyl group, n is from 1 to 1200 and c is 0 or 1 ; and R⁵ is a substituted or unsubstituted Ci_-2o hydrocarbon or Ci_-2o hydrocarbonoxy radical.

The reactive polyorganosiloxanes should be present in amounts of about 50 to about 95%, and desirably in amounts of about 60 to about 80% by weight, based on the total amount of the curable composition.

Silicon hydride crosslinker compounds may also be incorporated, and are particularly useful in heat curing compositions. These materials may be selected from a wide variety of compounds, although the crosslinker desirably conforms to formula (III) below:

R¹⁰ R¹⁰ R¹⁰

R ' Si— O (Si— 0)_x (SiO)y—

I I

_R10 _R10 _R10

formula (III) where at least two of R⁷, R⁹ and R¹⁰ are H; otherwise R⁷, R⁹ and R¹⁰ can be the same or different and can be substituted or unsubstituted hydrocarbon radical from Ci_-2o Such hydrocarbon radicals including those as previously defined for formula (I) above, thus, the SiH group can be terminal, pendent or both; R¹⁰ can also be a substituted or unsubstituted hydrocarbon radical from C 1-20 such hydrocarbon radicals including those as previously defined for formula I above, and desirably is an alkyl group such as methyl; x is an integer from 10 to 1000; and y is an integer from 1 to 20. Desirably R groups which are not H are methyl. The silicon hydride crosslinker should be present in amounts sufficient to achieve the desired amount of crosslinking and desirably in amounts of about 1 to about 10 wt.-% of the curable composition.

Useful organo-metallic hydrosilation catalyst may be selected from any precious metal or precious metal-containing catalyst effective for initiating a thermal hydrosilation cure reaction. Especially useful are the platinum and rhodium catalysts which are effective for catalyzing the addition reaction between silicone-bonded hydrogen atoms and silicone-bonded olefinic groups.

Other classes of catalysts useful in the present invention include organo rhodium and platinum alcoholates. Complexes of ruthenium, palladium, oznium and arridium are also contemplated. The organo metallic hydrosilation catalyst may be used in any effective amount to effectuate thermal curing. Desirably the catalyst is present in amounts of 0.025 to about 1 .0 wt.-%, based on the total amount of the curable composition. Combinations of catalysts are contemplated.

Desirably, the at least one crosslinkable component of the present invention is present in amounts of 20 to 80 wt.-% and more desirably in amounts of 40 to 60 wt.-%, based on the total amount of the curable composition.

The curing (vulcanization) reaction can be defined as any treatment that increases the viscosity of the elastomers, increases the tensile strength and modulus, and strain-to-failure. This process can be described as a crosslinking reaction between polymer molecules, which also includes chain extension as well as crosslinking. Included among the useful silicone polymers are liquid vinyl containing esters or ethers.

The polymeric particles of the present invention remain discrete in the cured elastomer and have a melt temperature below the degradation temperature of the cured elastomer.

As used in the present invention the term "melt temperature" of the polymeric particles preferably refers to the temperature at which the polymeric particles undergo a change of state from a solid to liquid. The melt temperature can be determined by DSC where the melt temperature is defined as the inflection point of a DSC curve.

As used in the present invention the term "degradation temperature" of the cured elastomer refers to the temperature at which the elastomer undergoes a weight loss of more than 10 wt.-%, preferably more than 20 wt.-%. The degradation temperature can be determined by TGA

(Thermogravimetric Analysis).

Among the useful polymeric particles, such as polymeric powders, are polyolefins or

copolyolefins such as polyethylene, polypropylene, polyethylene-co-propylene, polybutadiene (72% cis, 28% trans), polycapralactone, isotactic poly(l -butene), syndiotactic polypropylene, poly(l -decene), poly(ethylene-co-l -butene), poly(ethylene-co-vinylacetate) , polybutylene adipic acid), poly(omethyl styrene-co-methylstyrene), polyethylene oxide, trans 1 ,4-polybutadiene or trans 1 ,4-polyisoprene. The particle size of the polymeric particles of the present invention may vary widely from 50 nm, up to about 100 microns. Desirably, the polymeric particles have a size range of about 5 to about 10 μηι.

The particle size can be determined by laser diffraction using a Mastersizer 2000 (produced by Malvern instruments Ltd, calculation according to Mie).

The term "particle size" as used in the present invention refers to the d50 volume average particle diameter. D50 represents a particle diameter defining that 50% of the particles are greater than this, and another 50% of the particles are smaller than this.

It is desirable that the polymeric particles are distributed within the crosslinkable component in a shape-holding amount, preferably in an amount of 1 to 80 wt.-%, more preferably in an amount of 20 to 60 wt.-%, and more preferably in an amount of 30 to 50 wt.-%, based on the total amount of the curable composition.

The shape memory polymers of the present invention can be prepared according to any method. A particular preferred method to prepare the shape memory polymers of the present invention is described in US patent application No. 2004/0266940 A1 , which is expressly incorporated herein by reference.

In a further embodiment of the present invention, the shape memory polymer is a shape memory epoxy polymer. Preferred shape memory epoxy polymers are selected from reaction products formed by curing a curable composition, comprising an aromatic diepoxide (rigid epoxy), an aliphatic diepoxide (flexible epoxy), and a diamine curing agent.

Suitable aromatic diepoxides include the diglycidyl ether of bisphenol A epoxy monomer, which is commercially available under the tradename EPON 826 from Hexion Speciality Chemicals; suitable aliphatic diepoxides include neopentyl glycol diglycidyl ether (NGDE), which is commercially available from TCI America; suitable diamine curing agents include poly(propylene glycol)bis(2-aminopropyl)ether, which is commercially available under the tradename Jeffamine D-230 from Huntsman.

A particular preferred method to prepare the shape memory epoxy polymers of the present invention is described in US patent application No. 2008/0262188 A1 , which is expressly incorporated herein by reference. In one embodiment of the invention the protrusions on the wafer surface, preferably on the front side of the wafer, are solder bumps. As used in the present invention the term "solder bumps" include solder formations of substantial size that are employed both to bond planar structures together and to interconnect these structures electrically. Desirably, the solder bumps have a maximum length in any direction of space in the range of 20 μηη to 500 μηη. Preferably the maximum length of the solder bumps in any direction of space is in the range of 25 μηη to 100 μπΊ for first level semiconductor packaging applications and 150 μηη to 400 μηη for certain first level and all second level semiconductor packaging applications, where first level applications relate to direct attachment of semiconductor solder bumps to package substrate surface and second level applications relating to attachment of full package assembly to printed circuit board surface. The length of the solder bumps is determined by scanning electron microscopy (SEM) preferably using a scanning electron microscope JEOL JSM-6060 SEM.

The shape memory polymer of the support structure is a film, which has an average film thickness of 10 to 1200 μηη. Support structures, comprising shape memory polymer films are used to support the wafer during backside grind. The shape memory polymer film protects the protrusions, such as bumps, on the surface of the wafer during the backside grinding process, wherein the thickness of the shape memory polymer film is typically at least two times larger than the height (maximum length in any direction of space) of the protrusions, such as bumps, in order ensure an effective protection of the protrusions and other extruding structures on the front side of the wafer

It has been found that, particularly in the range of an average film thickness of 10 to 1200 μηη, more preferably in the range of 15 to 500 μηη, and particularly preferably in the range of 30 to 300 μηη, the protrusions of wafer surface are effectively protected during the grinding process and that the support structure can be substantially completely removed from the wafer surface in a stress-free process by applying a low 90° peel force of less than 0.005 N/mm.

It should be noted that the support structure of the present invention can or can not comprise at least one carrier film.

In cases, when the support structure comprises at least one carrier film, said carrier film is attached to the inner surface of the shape memory polymer of the support structure.

It is advantageous to use support structures comprising at least one carrier film in the method of the present invention, because the carrier film reduces the peel force needed to remove the support structure form the wafer surface. Additionally, the carrier film imparts structural integrity and/or stiffness to the support structure and/or ensures the reusability of the support structure.

The at least one carrier film of the present invention may be UV transparent and/or may comprise at least one polymer selected from polyethylenes, polypropylenes, polycarbonates, polyesters, polyethyleneterephthalat.es, polyvinylchlorides, copolymers of ethylene and vinyl acetate and/or combinations thereof. The carrier film can comprise or consist of one, two or more than two different layers, wherein each layer can comprise or consist of at least one of the aforementioned polymers.

It is desirable that the average thickness of the carrier film is in the range of 50 to 200 μηη, preferably in the range of 70 to 175 μηη, and more preferably in the range of 90 to 140 μηη.

The average film thickness of the carrier film is defined as the arithmetic average of a multitude of film thickness values, wherein each film thickness value is measured in a substantially orthogonal direction to the longitudinal extension of the carrier film between two opposing points, wherein one point is located on the inner surface of the carrier film, and the corresponding point is located on the opposing outer surface of the carrier film.

The term multitude, as used above, can refer, for example, to at least 10 different measuring points at 10 different positions along a profile section of the support structure.

The average film thickness of the carrier film can be determined, for example, by using a section of the support structure (1 cmx1 cm in size) divided into 5 equal strips (each 2 mm) and in each case the average film thickness of the carrier film is determined along the profile. The average film thickness of the carrier film along the profile can be determined by scanning electron microscopy (SEM) preferably using a scanning electron microscope JEOL JSM-6060 SEM.

A further aspect of the invention is an assembly, comprising a wafer and a support structure, wherein the assembly is obtained by the method of the present invention.

Another aspect of the present invention is an assembly, comprising a wafer and a support structure, wherein the wafer has a plurality of protrusions on its surface and the support structure comprises at least one shape memory polymer film having an inner surface and an outer surface, wherein said outer surface is attached to the protrusions of the wafer, with the proviso, that the outer surface of the shape memory polymer film forms the exterior surface of the support structure and said exterior surface exhibits a flatness index of less than 0.02 prior to the attachment of the wafer and said shape memory polymer film has an average film thickness of 10 to 1200 μηι.

Preferred embodiments of the invention are described with the figures.

Figure 1 shows a sectional view of the support structure of the present invention.

Figure 2 shows an enlarged sectional view of a part of the support structure shown in figure 1 .

Figure 3 shows an additional enlarged sectional view of a part of the support structure shown in figure 1 .

Fig. 1 shows a sectional view of a support structure as used in the method of the present invention. Said support structure comprises a shape memory polymer film 3 which has an average thickness within a range of 10 μηη to 1200 μηη. With figure 2 an enlarged sectional view of a part of the support structure 1 shown in figure 1 is depicted. The shape memory polymer film 3 has a flatness index of less than 0.02, wherein the flatness index is defined as the ratio of a roughness of the exterior surface of the support structure 1 to an average thickness of the shape memory polymer film 3.

Figure 2 shows an enlarged sectional view of a part of the support structure 1 shown in figure 1 with an indication of the roughness of the shape memory polymer film 3. The outer surface is of course not flat and shows a special asperity. The asperity of the surface of the shape memory polymer film 3 is reflected in sections of the shape memory polymer film 3 having a more or less minor variable thickness. This variable thickness is reflected in the sectional view of the support structure 1 shown in fig. 3 in the form of peaks P-i , P₂, P3 and valleys V-i , V₂, V₃, V₄ in the outer surface/exterior surface of the shape memory polymer film 3/support structure 1. The term roughness, as used in the present invention, means the roughness average, which is defined as the arithmetic average of peak P_n and valley V_n distances D_n. The term peak P_n, as used in the present invention, refers to any protruding area of the exterior surface, whereas the term valley V_n, as used in the present invention, refers to any recess between protruding areas of the exterior surface of the support structure. The term distance D_n, as used in the present invention, means the difference in height of a peak P_n and neighbored valley V_n measured in a

substantially orthogonal direction to the longitudinal extension 2 of the shape memory film 3.

The roughness can be determined at a measurement speed of 0.5 mm/s by using a Solarius non-contacting Laser Profilometer equipped with an AF2000 autofocus sensor. The obtained data can be analyzed by using solar map universal 3.1.10 image analysis software (Gaussian filter 0.8 mm), wherein a microrougbness filtering is used, with a cutoff of 2.5 pm.

The term multitude, as used above, can refer, for example, to at least 10 different measuring points at 10 different positions along a profile section of the support structure 1 .

EXAMPLES

A. Materials

In the examples, the following materials were used: Bumped wafer

Wafer type: VPA 18 Sn₃.0Ag0.₅Cu Nitride passivation;

Bumped wafer coupons 4x4 cm with 96 μηη diameter bumps;

Bump pitch: 225 μηι;

Nominal singulated die dimension: 7x7 mm;

Street width: 750 μηη.

Shape memory polymer film (SMP films)

The shape memory polymer films were obtained by UV-curing a UV-curable composition, comprising

61 wt.-% Dimethacryloxypropyl-dimethoxy-polydimethylsiloxane

28 wt.-% isobornyl acrylate,

10 wt.-% fumed silica,

1 wt.-% of photoinitiator.

For the UV-curing an Omnicure Series 2000 high pressure 200W mercury lamp was used. The sample were cured within 75 seconds using an irradiance of 5.2 W/cm².

SMP films of dimensions 4x4 cm were used in the examples, wherein the SMP films exhibited a glass transition temperature (T_g) of about 1 10°C, as determined by DMA (Dynamic mechanical analysis).

Carrier film

A polyester film with an average thickness of 125 μηη and dimensions of 4x4 cm was used as a carrier film.

Different support structures were used in the examples. The support structures comprised in all cases the aforementioned shape memory polymer (SMP) film, having an inner surface and an outer surface, wherein the outer surface of the shape memory polymer film formed the exterior surface of the support structure. The roughness, average film thickness, and the flatness index of each SMP film is given in Table 1 . In some examples the support structure additionally comprised a carrier film, wherein the carrier film was attached to the inner surface of the SMP film. Support structures, comprising a shape memory polymer film on a carrier were obtained by curing-in-place techniques, wherein oxygen was excluded during the curing process.

B. Test methods

In the examples, the following test methods were used: Glass transition temperature (T_fl)

The glass transition temperature (T_g) of the shape memory polymer films were determined by DMA (Dynamic mechanical analysis) using a Rheometrics Solids Analyzer (RSA-3) from TA Instruments-Waters LLC, New Castle. Samples for DMA were prepared by laser cutting specimens to 30 mmx5mmx1 mm from bulk material. The samples were equilibrated at -90°C for 2 min then raised to 200°C at a rate of 10°C/min. The glass transition temperature was defined as the peak of the tan δ curve from the DMA testing.

Roughness

The roughness was measured along a line of 0.08 cm on the exterior surface of the support structure. The roughness given in Table 1 is the arithmetic average of 15 different

measurements at 15 different positions on the exterior surface of the support structure. The roughness was determined at a measurement speed of 0.5 mm/s by using a Solarius non- contacting Laser Profilometer equipped with an AF2000 autofocus sensor. The obtained data were analyzed by using solar map universal 3.1 .10 image analysis software (Gaussian filter 0.8 mm), wherein a microroughness filtering was used, with a cutoff of 2.5 μηι.

Average film thickness T of the SMP film

The average film thickness T of the shape memory polymer film was determined along the profile section as shown in Fig. 3. The average film thickness T given in Table 1 is the arithmetic average of 10 film thickness values T_n obtained at 10 different positions along the profile section of the shape memory polymer film. The average film thickness T along the profile was determined by scanning electron microscopy (SEM) using a scanning electron microscope JEOL JSM-6060 SEM.

Flatness index (Fl)

The flatness index is defined as the ratio of roughness of the exterior surface of the support structure to the average film thickness of the shape memory polymer film. C. Wafer handling

Forming of a support structure- wafer assembly

A support structure-wafer assembly was formed by contacting the bumped side of the wafer with the exterior surface of the support structure at a pressure of 6 MPa at a temperature of 130°C for 15 minutes. Afterwards, the assembly was cooled within 2 minutes to a temperature of 22°C at a pressure of 6 MPa.

Removal of the support structure

The SMP film was activated by exposing the support structure-wafer assembly to a temperature of 130°C for 2 minutes.

The support structure-wafer assembly was than cooled to 23°C and the support structure was removed from the wafer surface by applying a peel force. The peel force reported in Table 1 is the 90° peel force needed for peeling the adhered surfaces (exterior surface of the support structure and bumped wafer surface) apart. Said 90° peel force was determined at 23°C according to ASTM D6862-04 test method using a TXT plus tensile tester (available from Stable Micro Systems, Surrey UK) using 5 Kg load cell and a crosshead speed of 25 mm/min.

Table 1

^[a| A removal is regarded as being complete if less than 5 wt.-%, based on the total weight of the support structure, remain on the wafer surface after the support structure has been removed from the wafer surface.

The examples of Table 1 demonstrate that support structures having a flatness index (Fl) of less than 0.02 can be removed easily and substantially completely from the wafer surface by applying a low peel force, whereas support structures having a flatness index (Fl) of than 0.02 (comparative example) require a much higher peel force and can not be removed completely from the wafer surface.

Claims

1 . A method for handling a wafer, comprising the steps of: a) providing a wafer and a support structure, wherein the wafer has a plurality of protrusions on its surface,

and wherein the support structure comprises at least one shape memory polymer film having an inner surface and an outer surface, wherein the outer surface of the shape memory polymer film forms the exterior surface of the support structure and said exterior surface exhibits a flatness index (Fl) of less than 0.02 and said shape memory polymer film has an average film thickness of 10 to 1200 μηη; b) bringing the protrusions into contact with the exterior surface of the support structure at a pressure of 0.1 to 100 MPa, preferably at a pressure of 1 to 100 MPa and at a temperature above the glass transition temperature (T_g) of the at least one shape memory polymer to form an assembly; c) cooling the formed assembly to a temperature of more than 20°C below the glass transition temperature (T_g) of the at least one shape memory polymer at a pressure of 0.1 to 100 MPa, preferably at a pressure of 1 to 100 MPa.

2. The method of claim 1 , wherein the exterior surface of the support structure exhibits a flatness index (Fl) of less than 0.01.

3. The method of claim 1 and/or 2, wherein step b) comprises contacting the exterior surface of the support structure and the wafer at a temperature above the glass transition temperature (T_g) of the shape memory polymer for 30 seconds to 100 minutes at a pressure of 4 to 50 MPa.

4. The method of any one of claims 1 to 3, wherein step c) comprises cooling the formed assembly to a temperature of more than 20°C below the glass transition temperature (T_g) of the at least one shape memory polymer at a pressure of 4 to 50 MPa.

5. The method of any one of claims 1 to 4, further comprising the additional step d) of exposing the wafer to a grinding operation to thin said wafer.

6. The method of any one of claims 1 to 5 further comprising the additional step of exposing the support structure to a temperature above the glass transition temperature (T_g) of the shape memory polymer for a period of time sufficient to remove said support structure form the wafer.

7. The method of any one of claims 1 to 6, wherein the shape memory polymer is a reaction product formed by curing a curable composition, comprising

i) at least one crosslinkable component which forms an elastomer when cured; and ii) distributed within said crosslinkable component are polymeric particles which remain discrete in the cured elastomer and have a melt temperature below the degradation temperature of the cured elastomer.

8. The method of claim 7, wherein the crosslinkable component includes a compound having the formula:

MA R⁵ MA

I I I

(c¾o) ._c— sio— (sio)_n— si— (oa K_c

(CH₃)_C R⁵ (CH_.,)_C wherein MA is a methacryloxypropyl group, n is from 1 to 1200 and c is 0 or 1 ; and R⁵ is a substituted or unsubstituted Ci_-2o hydrocarbon or Ci_-2o hydrocarbonoxy radical.

9. The method of any one of claims 1 to 8, wherein the protrusions are solder bumps.

10. The method of any one of claims 1 to 9, wherein the support structure comprises at least one carrier film and the carrier film is attached to the inner surface of the shape memory polymer film.

1 1 . The method of claim 10, wherein the carrier film comprises at least one polymer selected from polyethylenes, polypropylenes, polycarbonates, polyesters,

polyethyleneterephthalates, polyvinylchlorides, copolymers of ethylene and vinyl acetate and/or combinations thereof.

12. The method of claim 10 and/or 1 1 , wherein the carrier film comprises at least two different layers.

13. The method of any one of claims 10 to 12, wherein the carrier film has an average film thickness in the range of 50 to 200 μηη.

14. An assembly, comprising a wafer and a support structure, wherein the assembly is obtained by a method of any one of claims 1 to 13.

15. An assembly, comprising a wafer and a support structure, wherein the wafer has a plurality of protrusions on its surface and the support structure comprises at least one shape memory polymer film having an inner surface and an outer surface, wherein said outer surface is attached to the protrusions of the wafer, with the proviso, that the outer surface of the shape memory polymer film forms the exterior surface of the support structure and said exterior surface exhibits a flatness index of less than 0.02 prior to the attachment of the wafer and said shape memory polymer film has an average film thickness of 10 to 1200 μπι.