Dr Ralph Delmdahl, product marketing manager, and Dr Dirk Müller, director of marketing, Coherent
Laser-based tools overcome many of the challenges presented by the increasingly complex interconnects (redistribution layers (RDLs), copper bumps and pillars, etc.) and ever thinner wafers utilised in stacked 2.5D, 3D and fan-out packages.
Advanced packaging challenges
An example of 2.5D is a system-in-package (SiP)—namely a package that combines disparate computing and communications dies—where heterogeneous chips are interconnected using a high-density interposer or Intel’s Embedded Multi-die Interconnect Bridge (EMIB) technology.
An example of 3D is a moulded package where two or more thinned chips are stacked on top of each other, for example, several memory chips or a logic chip and a memory chip.
Fan-out, which is preferred for high bandwidth memory (HBM) applications, is where dies are packaged in moulding compound while still on a wafer and is particularly economical.
There are several variants of each of these three advanced package types. For instance, there are three types of fan-out packages, namely chip-first/face-down; chip-first/face-up; and chip-last or RDL first. However, a common requirement for all of them is dense high-resolution interconnections, often with extremely thin wafers, in order to deliver maximum speed and functionality.
This article takes a look at structuring these interconnect layers using excimer lasers, namely pulsed ultraviolet (UV) lasers (with wavelengths of 193, 248 or 308 nm) that have long been used in smart display fabrication and are the more powerful cousins of the lasers used in front-end lithography.
Dual damascene patterning of RDL trenches and microvias
In packages such as 3D and fan-out, the interconnects are typically fabricated using the dual damascene process. The circuit is formed by way of conduction lines and vias: surface trenches and through-holes that are made conductive through sputtering of a seed layer followed by electroplating. Although traditional lithography is still useful at the current resolution of 10 x 10 µm (width x pitch), it encounters challenges at 3 x 3 or 1.5 x 1.5 µm.
Lithography
There is a limited number of photoresist materials available for lithography, and often these do not have ideal physical properties, leading to problems such as compressive stress and substrate warpage. Furthermore, trenches and vias have to be created in two separate lithographic steps, meaning that as the polymer and resist are then cured, there can be micrometre-scale shape changes due to material reflow and shrinkage, including flaring of the vias and dimensional distortions.
The aforementioned factors usually mean that the pad created for each via must be made larger than the trench to guarantee registration of the via and trench patterns. However, this increases the average line size and sacrifices usable surface area.
Excimer laser process
Whereas lithography involves photographic exposure of a resist, pulses from excimer lasers can directly remove material. Furthermore, because excimer lasers have UV outputs, the high-energy photons directly break interatomic bonds in polymers and other materials, ablating the material as vapour in a relatively cold process with virtually no peripheral thermal effects.
To implement RDL patterning, the rectangular beam of the excimer laser is reshaped and passed through an aluminium photomask that is the inverse of the via or trench pattern. The patterned beam is then projected onto the RDL surface through a reduction lens.
The excimer laser process affords the following advantages over lithography:
- It cures the polymer layer before patterning rather than after as in lithography. This means that the pattern has inherently very high fidelity and no shrinkages or distortions, etc.
- It creates both the vias and trenches without disturbing the wafer or optical alignment. This ensures perfect registration and eliminates the need to use pads and oversized vias, thus minimising wasted area.
- It provides the opportunity to sputter a barrier layer, for example, titanium nitride titanium (TiNTi) or tantalum nitride tantalum (TaNTa), before sputtering the copper seed layer. The barrier layer blocks any chance of copper migration into the polymer.
- It provides superior control over all dimensions of the vias, including both the aspect ratio and the amount of taper. The entrance diameter of the vias is defined by the mask and projection optics, but any laser drilled hole has a natural taper. By controlling the fluence (pulse energy per unit area), one can directly control the taper angle (see figure 1).
Figure 1: Varying laser fluence enables manipulation of feature side-wall angles, which can be important in subsequent deposition steps.
Speed and throughput of the excimer laser process
The depth of material removed by each pulse of the excimer laser is similar for different polymers and primarily depends on the fluence. This, in turn, is a function of the pulse energy and size of the mask projection on the polymer surface. For a relatively low fluence of 100 mJ/cm2, the etch rate is about 50 nm/pulse, and for a high fluence of 1,200 mJ/cm2, it is about 1,000 nm/pulse. Furthermore, since the etch rate is independent of the number of vias in a given area, the number of vias that can be drilled per minute actually increases as the density of vias is increased.
Another advantage of the process is that via and trench patterning is performed in one step, and can be either via first/trench last, or trench first/via last. The latter option is usually the preferred one for advanced packaging as it allows better control of the via profile and its critical bottom dimension. This method also allows for different electroplating options. The vias and trenches can be overburdened with copper, followed by chemical/mechanical polishing (CMP) to reduce them down to the required height. Alternatively, bottom-up fill can be used, which eliminates the need for CMP, thereby reducing overall process costs.
Seed layer removal
If CMP is used, it can remove overburden down to the seed layer, followed by another excimer process to remove the seed layer. In fact, excimer ablation is also well-suited to seed layer removal in general, for example, for under bump metallisation (UBM) used under copper bumps and pillars that are vital interconnects in 3D packages. This is because excimer ablation effectively removes thin seed layers via a process called spallation but leaves thicker, plated metal virtually untouched.
In contrast, the wet etching process traditionally used for seed layer removal can cause undercutting of bumps and pillars, resulting in unacceptable thinning of lines at the high spatial resolution (<10 µm) needed for the latest packages. For example, copper pillars in the <5 µm range have been documented as being undercut and rendered so mechanically fragile that bonding yields are simply not sustainable.
During the spallation process, most of the excimer beam penetrates through the ultra-thin seed layer (see figure 2). The polymer absorbs UV very strongly, meaning that all of the beam energy is absorbed in the first few atomic layers of the polymer, thus completely vapourising it. The expanding vapour becomes trapped under the mechanically weak seed layer, blowing it away in a single pulse. Importantly, no masking or optical registration is needed since the thick metal conductors (pillars, etc.), absorb the pulse energy, which is then harmlessly dissipated through the metal, without reaching or affecting the underlying substrate in any way.
Since spallation is a single pulse process, it supports very fast throughput; depending on the laser power, a 300 mm patterned substrate can be stripped of residual seed layer in just ten seconds. Moreover, as most of the laser light reaches the target at near normal incidence, there are no shadowing effects and none of the problematic undercutting found with the wet etching process.
Figure 2: Schematic showing spallation where the excimer light penetrates the ultra-thin seed metal layer and is absorbed at the polymer interface; the vapourisation of the polymer surface causes explosive release of the overlaying seed metal. The conductive features are too thick to be affected by this subtle process.
Summary
As in other areas of microelectronics fabrication and related industries, advanced packaging is currently overcoming the challenges faced by established processes due to relentless miniaturisation. The excimer laser is set to provide a high-throughput solution for several advanced packaging processes, as it has previously in front-end lithography, laser annealing for high-brightness displays and laser lift-off (LLO) for flexible displays.