Fluxless Wafer Bumping by Electron Attachment
BY C. CHRISTINE DONG, RICHARD E. PATRICK, EUGENE J. KARWACKI, AND GREGORY K. ARSLANIAN, Air Products, Inc.
One of the keys to successful wafer bumping is the removal of surface oxides on the deposited solder during reflow, usually done by using organic fluxes. Because of problems associated with organic fluxes, fluxless solder reflow has become a point of interest.
Using reactive gas, especially hydrogen, to replace organic fluxes is an attractive approach since the process is clean and non-toxic. However, the lack of molecular hydrogen reactivity at the normal solder reflow temperature range is a major limitation. Therefore, high temperature and pure hydrogen must be applied to activate and accelerate the reduction process. This increases operation costs and is a flammability hazard. Plasma techniques can generate active species in H2 and increase the efficiency of oxide removal. However, these often require vacuum, which increases process costs and confines the processes into discontinued batches. Therefore, a fluxless technology solution, which is efficient, cost effective, and environmentally safe, is still a goal for many researchers. The electron attachment (EA) process was developed for activating H2 gas molecules and promoting low-temperature fluxless solder reflow under ambient pressure.
Figure 1. Mass spectrometric result showing a change of HD intensity when EA is applied at different temperatures.
When low-energy electrons (< 10 eV) collide with gas molecules, some are captured by gas molecules, producing negative ions by dissociative or direct attachment - or “electron attachment.” The energy of the electrons emitted from a cathode depends on the ratio of the electric field strength, E, to total pressure, p, or, E/p. This is because electrons are accelerated under an electric field within each free path, and decelerated when colliding with gas molecules. An increase in pressure reduces the electron energy and increases the probability of electrons approaching gas molecules to attach. Therefore, ambient pressure is much more favorable than vacuum for EA.
During EA-assisted fluxless solder reflow, a gas mixture of ≤ 4 vol% H2 in N2 is introduced into a furnace, which has a moving belt running from end to end, with heating/cooling zones along the center axis. The wafers to be reflowed are set on the moving belt, which is grounded and acts as an anode. An electron-emitting cathode is mounted overhead. When a suitable DC voltage is applied to the cathode, low-energy electrons (< 10 eV) are generated from it and drift toward the anode driven by the electric field. H2 molecules can then collide with these electrons, and excited molecular hydrogen anions (H2-) can be formed, which can dissociate into neutral hydrogen atoms (H) and atomic hydrogen anions (H-*) by dissociative attachment (Equation 1). In Equation 2, additional collisions between the hydrogen atoms and electrons can lead to a direct attachment, forming excited atomic hydrogen anions (H-*). During the EA process, N2, with a concentration ≥ 96 vol%, cannot form negative ions because the electron affinity of N2 is zero. Equation 3 shows how nitrogen can act as a third body in collision stabilization to absorb the excess energy of the excited atomic hydrogen anions (H-*) formed by direct attachment. The atomic hydrogen anions (H-) generated under EA also drift along the electric field toward the anode and chemically remove the oxide layer on solder bumps (Equation 4). Water vapor as a byproduct of the oxide reduction can be easily vented out of the furnace. Free electrons injected to or generated on the wafer surface are discharged by traveling to the solder bump positions and draining through the silicon matrix to the ground. The neutral hydrogen atoms (H) formed by dissociative attachment (Equation 1) are also active species for oxide removal; however, their role is expected to be much less significant than that of atomic hydrogen anions (H-) due to a rapid recombination and a slow diffusion of H to the wafer surface.
H2 + e- → H2-* → H- + H ……………………1
H + e- → H-* ………………………………2
H-* + N2 → H- + N2 …………………………3
Solder surface de-oxidation:
2H- + SnO → Sn + H2O + 2e- ………………4
The expected values of EA assisted H2 fluxless reflow were envisioned as follows.
The initiation temperature for hydrogen to reduce solder oxides can be significantly reduced due to the dissociation of H2 molecules under EA. It is feasible to use non-flammable H2/N2 concentrations (e.g. ≤ 4 vol% H2 in N2) to achieve the expected reduction rate because there is a forced adsorption of active species (H-) on the wafer surface driven by the electrical field. Ambient pressure is much more favorable than vacuum for EA, which makes the process simple and can accommodate the open and continual production line.
Different than plasma, EA creates a single charged environment and the hydrogen anions (H-) can repel each other, making the lifetime of the active species relatively long.
Figures 2a, b, c. SEM micrographs of the initial mushroom shape of the electroplated bumps (2a), reflowed bumps without (2b), and with applying EA (2c).
The process is energy effective since N2 as a dilution gas, and being more than 96 vol%, is inert to EA.
The major challenge to establishing the EA process was to have a cathode, which could efficiently and uniformly generate electrons in a large scale and have a long lifetime of operation and low manufacturing cost. There was no commercially available cathode found to be acceptable. Therefore, a novel cathode had to be developed especially for this application. The cathode was set 1 cm above the wafer surface. When a single-polarity pulsed DC voltage with a frequency of 10 KHz and amplitude of 2 to 3 KV was applied to the cathode, a stable and uniform electron emission was obtained in a gas mixture of H2 and N2. The emission current could be varied from 0.1 to 0.5 mA/pin by using the current control capability of a power source. The cathode has a simple structure and thus a low manufacturing cost. It also proved to have a long operation lifetime, due to the special nature of EA, where the ions formed are negatively charged and a blasting effect of the cathode by positive ions is largely minimized.
To demonstrate the occurrence of the dissociative attachment, it would be ideal to directly detect atomic hydrogen (H). However, because of the low mass and the rapid recombination, it was not possible to detect atomic hydrogen using a mass spectrometer. Therefore, an alternative approach to monitor the hydrogen dissociation was developed. This involved detecting HD formation in a gas environment containing H2 and deuterium (D2). An increase in HD intensity when EA was applied would be indicative of the dissociative attachments taking place for both H2 and D2 molecules (Equations 5 to 7).
H2 dissociative attachment:
H2 + e- → H2-* → H- + H ……………………5
D2 dissociative attachment:
D2 + e- → D2-* → D- + D ……………………6
H + D → HD …………………………………7
Figure 1 shows mass spectrometric results analyzed from the gas outlet of the furnace at different process temperatures. Each isothermal analysis of the effluent started when furnace temperature was reached and equilibrated at a set point under a N2 flow. Five minutes after data acquisition started, the gas inlet was switched from N2 to a gas mixture of H2/D2 at a volume ratio of 1:1. EA was then applied during the time period of 15 to 25 minutes after data acquisition started. For each isothermal experiment the HD intensity increases when EA is applied, and falls back to its original level after EA is stopped. The initiation temperature for having HD formation under EA is around 100°C, and with increasing process temperature the peak HD intensity increases. This is reasonable since the dissociative attachment is known to be an endothermic reaction. Therefore, it proves that H2 can be dissociated under EA at a temperature much lower than that of thermal dissociation.
The isothermal experiment was also repeated by introducing N2 in the gas mixture of H2/D2. It was found that the peak HD intensity formed under EA decreased with increasing N2 concentration, which was the same as anticipated. There was also no NH3 peak detected when EA was applied. Ionization energy of H2 is quite close to that of N2. The observation further confirms that the demonstrated H2 and D2 dissociations are induced by dissociative attachments rather than gas ionizations.
To demonstrate a promoted fluxless solder reflow under EA, flip chips with electroplated solder bumps of 90%lead/10%tin in composition and 40 μm in diameter were heated in the furnace from room temperature to 310°C (m.p. of the solder is 305°C) in a gas environment containing 4 vol% H2 in N2, and then cooled immediately after reaching the peak temperature. For the run with EA being applied, it was applied during heating from 250 to 310°C.
Without applying EA, the surface of the solder bumps is all wrinkled, and the bump height is small due to the oxide skin covering the molten solder during reflow. With applying EA the reflowed solder bumps are very smooth and spherical in shape, indicating an oxide-free solder surface (Figures 2a-c).
A novel hydrogen fluxless technology based on EA for wafer bumping has been demonstrated. The process uses EA to produce atomic hydrogen anions. Fluxless solder reflow can be achieved at ambient pressure and normal reflow temperature by using a non-flammable hydrogen concentration (≤ 4 vol%).
C. CHRISTINE DONG, lead research scientist; RICHARD E. PATRICK, senior principle research technician; EUGENE J. KARWACKI, marketing manager, new product development; AND GREGORY K. ARSLANIAN, global segment manager, microelectronics applications, may be contacted at Air Products and Chemicals, Inc. Electronics Division, 7201 Hamilton Blvd. Allentown, PA 18195-1501; 610/481-4911; E-mail: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org.