PROTECTIVE FILM FOR BACK-GRINDING WAFER

Abstract

A protective film for back-grinding wafers primarily includes a base layer and a multi-film structure. The multi-film structure at least includes two TPU films, wherein the one (HPTU film) having a higher Vicat softening temperatures and/or heat deflection temperature than the other (LPTU film) contacts the base layer. The protective film optionally includes an adhesive film on one side of the LTPU film. The multi-film structure may include one or more TPU films between the HPTU and the LPTU films.

Claims

1. A protective film for back-grinding wafers, the protective film comprising: a base layer; and a multi-film structure comprising two thermoplastic polyurethane (TPU) films, wherein one (HTPU film) having a higher Vicat softening temperature than the other (LTPU film) contacts with the base layer.

2. The protective film for back-grinding wafers of claim 1, wherein the base layer is made from polyolefin (PO) or polyethylene terephthalate (PET).

3. The protective film for back-grinding wafers of claim 1, wherein the LTPU film has a Shore A hardness ranging from 60 to 75.

4. The protective film for back-grinding wafers of claim 1, wherein the HTPU film has a heat deflection temperature ranging from 100° C. to 140° C., and the LTPU film has a heat deflection temperature ranging from 60° C. to 90° C.

5. The protective film for back-grinding wafers of claim 1, wherein the HTPU film has a Vicat softening temperature ranging from 100° C. to 150° C., and the LTPU film has a Vicat softening temperature ranging from 70° C. to 100° C.

6. The protective film for back-grinding wafers of claim 1, further comprising a TPU film having a Vicat softening temperature and a heat deflection temperature not higher than those of the HTPU film and not lower than those of the LTPU film.

7. The protective film for back-grinding wafers of claim 1, further comprising an adhesive layer formed on the LTPU film.

8. The protective film for back-grinding wafers of claim 7, wherein the adhesive layer is a layer of UV curable pressure sensitive adhesive.

9. The protective film for back-grinding wafers of claim 8, wherein the UV curable pressure sensitive adhesive comprises poly(methyl methacrylate) (PMMA) and epoxy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows the protective film of this invention including four layers/films.

[0023] FIG. 2 shows the protective film of this invention including five layers/films.

[0024] FIG. 3 shows the protective film of this invention including three layers/films.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] To illustrate the protective film for back-grinding wafers of this invention, the preferred embodiments are described but not to limit this invention.

[0026] In this description, Vicat softening temperatures are measured according to ASTM D1525, heat deflection temperatures are measured according to ASTM 648, Shore A hardness is measured according to ASTM D2240-05 and adhesive strength is measured according to JIS Z0237 8.

[0027] The protective film for back-grinding wafers of this invention is characterized by the multi-film structure including at least two thermoplastic polyurethane (TPU) films with different heat deflection temperatures and Vicat softening temperatures. The TPU film having a heat deflection temperature and a Vicat softening temperature higher than those of the other is referred to as the HTPU film and the other is referred to as the LTPU film. The HTPU film has low adhesive strength or even non-adhesive. Between the TPU films of the multi-film structure, there is perhaps no obvious interface because of similar properties thereof. In the multi-film structure, the TPU film having a higher Vicat softening temperature (and/or a higher heat deflection temperature) is closer to the base layer. When being attached to a wafer at an operation temperature, the TPU film having a lower Vicat softening temperature (and/or heat deflection temperature) covers the wafer and is deformed by the shape of bumps and circuit lines thereon so as to protect the wafer during the back-grinding process. The TPU film having a higher Vicat softening temperature (and/or heat deflection temperature) retains sufficient strength to support the protective film. These TPU films are made from very similar materials and therefore perform good adjustability with the shape of the wafer.

[0028] The TPU films can be supplied from commercial products or produced from reactions. The commercial products can be in the form of particles or films which may be further processed with different equipment. The reactions usually include reactants such as isocyanate, polyol and additives.

[0029] The base layer can be made from any materials, for example, polyolefine (PO), poly ester (PE), polyethylene terephthalate (PET) and ethylene vinyl acetate (EVA), among which PET is used in the following examples.

[0030] The adhesive can be made from any materials for the back-grinding process of wafers, for example, the UV curable pressure sensitive adhesive (PSA) and the heat curable PSA. In the following examples, the UV curable PMMA adhesive with epoxy is used and referred to as E-g-PMMA. For E-g-PMMA, “g” indicates a ratio of epoxy to PMMA and ranges from 5 to 20, for example, 5, 10 and 15. E-g-PMMA may further include other additives such as curing agents, UV initiators, and inhibitors. A preferred formula of the E-g-PMMA PSA includes 2,4-diisocyanato-1-methyl-benzene (TDI, 1.5 wt %) serving as a curing agent, benzophenone (1.5 wt %) serving as a UV initiator and acetate (0.1 wt %) serving as an inhibitor.

[0031] In this invention, the LTPU film may replace the above adhesive to directly cover the wafer by adding additives therein or a means.

EXAMPLE 1

[0032] Commercial TPU particles are used. The HTPU has a Vicat softening temperature of 100° C. and a heat deflection temperature of 100° C. The LTPU has a Vicat softening temperature of 70° C., a heat deflection temperature of 60° C. and a Shore A hardness of 70.

[0033] The HTPU particles and the LTPU particles from their respective feeders are extruded through the co-extrusion dies to be laminated and combined as a multi-film structure. The extruders for the HTPU and the LTPU are respectively set at 120° C. and 100° C. Thicknesses of the HTPU film and the LTPU film are respectively 300 μm and 50 μm. The extruded multi-film structure is cooled and rolled.

[0034] The multi-film structure is then attached to the PET base layer with the HTPU side. The PET base layer has a thickness of 80 μm.

[0035] Consequently, the adhesive (E-g-PMMA) is spread on the LTPU film to obtain the protective film for back-grinding wafers. As shown in FIG. 1, the protective film includes the base layer 10, the multi-film structure 20 and the adhesive layer 30. The multi-film structure 20 includes the HTPU film 21 contacting with the base layer 10 on one side and the LTPU film 22 formed on the other side of the HTPU film 21. The Vicat softening temperature and the heat deflection temperature of the HTPU film 21 are relatively higher than those of the LTPU film 22. The adhesive layer 30 is spread on the other side of the LTPU film 22.

EXAMPLE 2

[0036] The procedures of Example 1 are repeated except that the HTPU has a Vicat softening temperature of 110° C. and a heat deflection temperature of 100° C. and the LTPU has a Vicat softening temperature of 80° C., a heat deflection temperature of 70° C. and a Shore A hardness of 70.

[0037] The extruders for the HTPU and the LTPU are respectively set at 140° C. and 130° C.

EXAMPLE 3

[0038] The procedures of Example 1 are repeated except that the HTPU has a Vicat softening temperature of 120° C. and a heat deflection temperature of 120° C. and the LTPU has a Vicat softening temperature of 90° C., a heat deflection temperature of 80° C. and a Shore A hardness of 70.

[0039] The extruders for the HTPU and the LTPU are respectively set at 180° C. and 160° C.

EXAMPLE 4

[0040] Commercial TPU films are used. The HTPU has a Vicat softening temperature of 130° C., a heat deflection temperature of 130° C. and a thickness of 300 μm. The LTPU has a Vicat softening temperature of 100° C., a heat deflection temperature of 90° C., a thickness of 50 μm and a Shore A hardness of 70.

[0041] The HTPU film and the LTPU film are heated so as to be combined to form a multi-film structure which is then cooled and rolled.

[0042] The multi-film structure is then attached to the PET base layer with the HTPU side. The PET base layer has a thickness of 80 μm.

[0043] Consequently, the adhesive E-g-PMMA is spread on the LTPU film to obtain the protective film for back-grinding wafers.

EXAMPLE 5

[0044] The procedures of Example 4 are repeated except that the HTPU has a Vicat softening temperature of 140° C. and a heat deflection temperature of 140° C. and the LTPU has a Vicat softening temperature of 80° C., a heat deflection temperature of 70° C. and a Shore A hardness of 70.

EXAMPLE 6

[0045] The procedures of Example 4 are repeated except that the HTPU has a Vicat softening temperature of 150° C. and a heat deflection temperature of 150° C. and the LTPU has a Vicat softening temperature of 90° C., a heat deflection temperature of 80° C. and a Shore A hardness of 70.

Test for the Protective Films

[0046] Adhesive strength of the protective films of Examples 1-6 is measured. Before being radiated with UV light, the protective films have adhesive strength ranging from 150 gf/25 mm to 220 gf/25 mm. After being radiated with UV light, the protective films have adhesive strength ranging from 15 gf/25 mm to 80 gf/25 mm. The protective films are then attached to the wafers having the same surface conditions at 80° C. As a result, the wafers are well protected during the back-grinding process and the protective films can be easily peeled without residues. In other words, the protective films perform excellent adjustability with the bumps and circuit lines on the wafer surfaces and firm combination between the base layer, the HTPU film, the LTPU film and the adhesive layer. The protective films of Examples 2-5 are better and those of Examples 2 and 3 are the best.

[0047] Particularly, a TPU film having a Vicat softening temperature (and/or a heat deflection temperature) not higher than those of the HTPU film and not lower than those of the LTPU film can be formed between the HTPU film and the LTPU film.

[0048] The multi-film structure is not restricted by the HTPU film and the LTPU film as in the above Examples. FIG. 2 shows an “MTPU” film 23 having a “middle” Vicat softening temperature ranging between the HTPU film 21 and the LTPU film 22 and formed between them.

[0049] The multi-film structure is also not restricted by only one HTPU film or only one LTPU film as in the above Examples. Optionally, there can be two or more HTPU film or LTPU film according to the required thickness.

[0050] In this invention, the above adhesive layer can be replaced with the LTPU film. As shown in FIG. 3, the protective film for back-grinding wafers includes the base layer 10 and the multi-film structure 20 including the HTPU film 21 and the LTPU film 22. The HTPU film 21 contacts with the base layer 10 and the LTPU film 22 is formed on the other side of the HTPU film 21. By adding additives therein or a means, the LTPU film 22 can well cover the wafer without a separate adhesive layer.

[0051] The effects for protecting the wafers may be influenced by various factors, for example, compositions, thickness and hardness, of the TPU films, the adhesive layer and the base layer, surface conditions of the wafers, the operating temperatures, etc. The above Examples are used to verify that the multi-film structure of TPU is feasible for protecting wafers during the back-grinding process.