Electroless Cu for VLSI
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60 70 Temperature (°C) Figure 1. Deposition rate of electroless Cu as a function of (a) pH and (b) temperature of the solution with NaOH.
MRS BULLETIN/JUNE 1993
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board level, signal delay, crosstalk, electromigration, and stress-induced migration become important concerns.1 Cu holds promise as an alternative metallization material to Al alloy due to its low resistivity and ability to reliably carry high-current densities. Cu has a bulk resistivity of 1.68 /xil-cm, whereas Al has a bulk resistivity of 2.65 ixCl-cm. The only metal with a resistivity lower than Cu is Ag. Since Cu has a melting point and atomic weight both higher than Al, it is expected to have better resistance to electromigration, although properties such as grain structure and resistance to corrosion at high temperatures may also affect electromigration characteristics. Cu diffuses rapidly in Si and can form centers with deep energy levels in the Si bandgap. These deep energy levels provide a mechanism for excess minority carriers recombining with majority carriers. Consequently, Cu will induce generationrecombination leakage currents in pnjunctions and jeopardize the performance of bipolar and MOS transistors. The deeplevel enhanced recombination may decrease the minority carrier lifetime in the base of bipolar transistors, leading to a reduction in the current gain. Copper-induced leakage currents can also limit the performance of light detectors (e.g., charge-coupled devices, charge-injection devices, photodiodes) that convert a photon flux to a charge packet or electric current. High concentrations of Cu near the Si-SiO2 interface can result in a high concentration of surface states. It is evident that care must be taken to prevent Cu from diffusing into device regions. Using anisotropic dry etching to define fine features with high aspect ratios in Cu is not trivial. Since the chlorides and fluorides of Cu are not very volatile at room temperature, the substrate temperature has to be raised to fairly high temperatures (>150-250°C) during etching. This complicates the masking procedure. Also
the dry-etching chemistry may cause Cu corrosion. As an alternative to dry etching, selective deposition may be used to form patterned lines. Selective deposition by electrolytic plating has been used extensively to fabricate Cu lines on circuit boards. The boards are mounted on a cathode, and along with an anode are immersed into a plating solution that contains Cu ions. As a voltage is applied between the electrodes, the current drives the Cu ions toward the boards, forming metallic Cu. The electroless deposition process is similar to the electrolytic plating process except that no external electrodes are needed; thus, the name "electroless." The electroless Cu deposition process was first investigated for application in VLSI by Pai and Ting.2 As the cost of most semiconductor fabrication equipment rises rapidly, less expensive alternatives may become more desirable. Electroless deposition is per-
Table I: Typical
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