Cone bumps

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Cone bumps are cone shape microbumps.

Introduction

Reseachers at Kyushu Institute of Technology proposed the compliant bumps such as pyramid bumps and cone bumps. In 2005, they demonstrated bonding with Au cone bumps with bump size/pitch of 10 µm/20 µm [1].


In 2011, N. Watanabe and T. Asano reported Cu-Cu bonding at room temperature using Cu cone bumps and Cu landing with cross-shaped slit [2].


Formation Process

Figure 1 shows the formation process of the Cu cone bumps [2]. First, a 250-nm-thick TiW film and an 80-nm-thick Cu film (or Au film) were deposited sequentially on a Si wafer as a seed layer for electroplating [Fig. 1(a)]. Next, a resist pattern with undercut holes was fabricated by photolithography [Fig. 1(b)]. ZPN1150 (Nippon Zeon) was used as the photoresist. The undercut profile was controlled by adjusting the exposure time, the post-exposure-bake time, the post-exposure-bake temperature, and the development time. Then, Cu electroplating was carried out to fill the undercut holes in the resist film [Fig. 1(c)]. The conditions for electroplating were as follows: the plating bath was MICROFAB Cu MSA100 (Enthone), the bath voltage was 0.2–0.3 V DC, the electric current density was 1.15 A/dm2, the temperature of the plating bath was 25 °C, and the plating time was 15 min. Then, the photoresist was removed by using a solvent [Fig. 1(d)]. Finally, the seed layer was removed by etching the Cu film (or Au film) with Ar-ion etching and the TiW film with reactive-ion etching using a mixture of CF4 and O2 gases [Fig. 1(e)].

Formation of Cu cone bumps.png
Figure 1. Formation process of the Cu cone bump: (a) formation of the seed layer, (b) photolithography, (c) Cu electroplating, (d) removal of the photoresist, and (e) removal of the seed layer.


References

  1. N. Watanabe, T. Kojima, and T. Asano, “Wafer-level compliant bump for three-dimensional LSI with high-density area bump connections,” in IEEE InternationalElectron Devices Meeting, 2005. IEDM Technical Digest., 2005, pp. 671–674. Available: http://dx.doi.org/10.1109/IEDM.2005.1609440
  2. 2.0 2.1 N. Watanabe and T. Asano, “Room-Temperature Cu–Cu Bonding in Ambient Air Achieved by Using Cone Bump,” Appl. Phys. Express, vol. 4, no. 1, p. 16501, Jan. 2011. Available: http://dx.doi.org/10.1143/APEX.4.016501