Synova discusses a water based laser system that allows the user to move down to the die level without fear of damaging the chip

High power lasers can provide a powerful cutting tool for chip packaging. Using a water jet to guide the laser beam has the added advantage of clearing debris and cooling the wafer away from the cut. Synova reports on how its water-guided Laser Microjet can be extended into die marking in the wafer probe process.

Laser cutting is entering the chip packaging market as a more effective and accurate method for singulating dies compared with wafer sawing. One technique to improve the laser's accuracy even further is to guide the laser through a water jet using total internal reflection (Figure 1) in a similar way to optical fibres. But this technology can also be used to make permanent and distinct marks on defective dies after during the wafer probe testing stage of the manufacturing process.

Fig.1: Principle of coupling the laser beam into the water jet

Before the dies are singulated, their function is tested individually by a probing system and any bad die is spotted in a digital test protocol (wafer mapping). With this data, bad dies can be physically marked so that they are not mistaken for good dies later downstream in the production line. The common way to mark bad dies is to make a dot with ink.

The water jet guided laser alternative is faster and safer - any kind of mark shape and size (down to 28µm) is possible and the marks are permanent. The laser can create groove marks through the active surface of the chip. Hence, one can combine marking and dicing on the same machine, leaving clear and permanent marks without any risk of damaging the good dies.

The water jet further cools any particles and debris that may come from the process. In combination with a water protection film, the wafer is marked without leaving any depositions or particles on the good dies - this is very difficult to achieve with conventional lasers.

Dicing and inking

The most standard set-up today is to combine probing with inking systems through shared software. Because tests take more time than inking, several probing machines are often coupled with a single inking machine. Using the Laser Microjet for chip marking relies on a different concept: the chip marking is integrated into the dicing system, simplifying the process flow.

The water jet-guided laser is used mainly for dicing thin wafers in silicon and compound semiconductors such as gallium arsenide (GaAs) because of its high speed (50 to 200mm/s) and the cleanness of the cuts. However, the tool has other interesting uses: scribing (for layer removal, as well as scribe-and-break), hole drilling and edge grinding.

Edge grinding consists of removing the wafer edge containing micro-cracks - by removing the cracks the wafer strength is increased. Die marking brings further use for the Laser Microjet as the marking step can now be carried out with the same machine.

The water jet guided laser consists of a hair-thin low-pressure water jet with a high power laser beam inside. The laser gives the energy to ablate the material, while the water jet guides the focused laser beam to the work piece. Further, the use of water allows an efficient cooling of the edges. Compared with conventional lasers the result is much cleaner - wafers are free from debris and particles due to the protection of the wafer by a water layer.

Defective die marking with the Laser Microjet is in fact a short grooving with reduced quality requirements. Indeed, the purpose is to destroy the die so there is no need for a high grooving quality. What is crucial though is that neighbouring good dies not be damaged by the process.

A clean process

The main problem in using conventional lasers in this kind of applications is the production of lots of debris that can contaminate the surrounding good dies - micrometric and nanometric particles are spread over the surface and are very difficult to remove afterwards because of strong interface forces. Because the Laser Microjet uses a low-pressure water jet combined with a high-power laser beam, air-born particle generation from gaseous ablation products is strongly reduced. Furthermore, the wafer surface is protected with a thin water film during the process - the few particles generated during the laser ablation are maintained in suspension and cannot attach to the wafer surface. Unlike classical laser grooving, there is also no risk of hot particles attaching by fusion to the surface of the good dies, as the water rapidly cools the particles. None of the damage generated by the conventional laser to neighbouring dies can occur with the Laser Microjet [1].

Fast and permanent marks

The Laser Microjet has several advantages over inking machines: high speed, low maintenance and no errors (ink dots sometimes go also on good dies). Another important advantage is the mark itself. Ink dots are not always clearly visible, particularly on uneven structured surfaces and show limitations on size, location and durability. They can sometimes be removed during the sawing process. The Laser Microjet produces clearly visible and permanent marks on the die surface. All kind of shapes and all kind of sizes (down to 28µm) can be programmed, depending on chip size and requirements. For example, marks can be spots, rings or crosses (Figure 2).

Fig.2: Different kinds of marks produced by the Laser Microjet (surface as cut) on a 50µm thick silicon wafer

The marking rate is achieved with a distance of 2mm between the marks. The fastest is the 200µm round spot and the rate is 14 marks per second with a 2mm distance between the marks and up to 20 marks per second for shorter distances. These speeds are much higher than those of inking machines, which are usually around 3 marks per second.

Furthermore, laser marking guarantees that the defective chip is definitely destroyed and therefore unusable - the malfunction will be detected by any basic electrical test as often performed on the final product. Regarding confidentiality questions, this property is also an advantage. The Laser Microjet marking guarantees that the die cannot be used for any purpose after grooving.

Laser marking

The particular feature of free-shape grooving is interesting for another application too: standard chip or wafer marking. While the standard marking process (conventional laser) must work on the backside of the wafer, because of possible heat damage, the Laser Microjet can also carve any kind of mark on the front side. The first marking in Figure 3 (top) was made with a frequency doubled Nd:YAG laser on the steel lid of a sensor. The pulse repetition rate of the laser was 40kHz and the average power 3W. With a nozzle diameter of 50µm and a water jet pressure of 250bar, the resulting mark speed was 2mm/s. The second image (bottom) is a bigger mark made on a silicon wafer with a nozzle diameter of 100µm in one pass with a 50W average power laser and a pulse repetition rate of 25kHz.

Fig.3: Front-side marking: metal lid of a sensor, 40x magnification, without water film control (top) and silicon wafer, 15x magnification (bottom)

Another interesting advantage of the Laser Microjet over conventional lasers is that it is not disrupted by wafer warp. Indeed, as the wafers become thinner and larger, warping can appear with gravity pulling down the wafer centre. Because the laser beam is guided by the water jet, changes in distance between the nozzle and the work piece are not an issue - this is not the case for conventional lasers, where warping can cause serious problems.

Authors:

Delphine Perrottet, Frank Wagner, Roy Housh, Bernold Richerzhagen, Synova.

References

[1] B Richerzhagen, T Nilsson, C Boillat, R Housh, Shrinking Diameters, Industrial Laser Solutions, April 2004