In-Process Bump Inspection

Manufacturers can lose millions of dollars to bump connectivity and adhesion defects. Yield losses at this stage are particularly costly, since the wafers have completed the fabrication process and production costs are fully vested. To help boost yields or protect against potential losses, bump processes incorporate various inspection techniques to automatically collect data after each process step. The inspection data typically includes an image of the wafer and a wafer map that indicates which die passed or failed inspection.

Advanced macro inspection, two- and three-dimensional bump inspection, and backside inspection can collect various types of in-process bump and critical dimension (CD) line data.

Advanced macro inspection compares the acquired image to a reference image and detects differences. It is useful at most steps in the bump process because it can detect surface debris or defects when they occur in process — before they create adhesion and connectivity issues. Repeat particles or defects also serve as a good indicator of underlying process or equipment issues.

2-D bump inspection is an analysis algorithm usually applied to data collected during an advanced macro inspection (thus eliminating the need for an additional inspection pass). It can determine bumps' diameter and detect out-of-position bumps, surface particles and bridge bumps — bumps connected by metal. Additionally, the inspection can measure straight and curved CD lines to verify whether they are within tolerance.

3-D bump inspection is a confocal technique that can detect bump height, co-planarity, and morphology and roughness. Employed after plating, 3-D bump inspection can collect the data required to predict connectivity issues with the package and underlying process or equipment issues.

Backside inspection analyzes a reflected light image to detect chemical residue and other irregularities on the backside of wafers. Backside residue can cause the corresponding die to stick to the film frame and crack when the die sorter tries to pick it up. Additionally, backside inspection can detect particles that may cause "hot spots" in the lithography process.

Generally, bump process steps include polyimide patterning, base-layer metal deposition, photoresist deposition and patterning, electroplating, photoresist strip and base-layer metal etch, and reflow (Fig. 1 ). The following sections briefly describe each step and illustrate inspection opportunities.

1. An overview of the solder bumping process.

Polyimide patterning — Also known as lithography and after-develop, polyimide patterning deposits a translucent polyimide layer onto the wafer, forming a thermal barrier against subsequent bump processes. The metal pad and tungsten via form a direct connection to the underlying circuits. Advanced macro inspection is useful at this step in detecting surface particles and scumming caused by incomplete photoresist removal on the metal pads during develop. Surface particles on the polyimide layer prevent proper metal deposition. In some instances, particles create bubbles in the metal that can later cause packaging issues. Additionally, if the bubbles burst, an open forms in the metal down to the polyimide layer, resulting in a killer defect. 2-D bump inspection can also be used to measure straight or curved CD lines — verifying whether they are within specification. Lines outside of specified tolerances cause connectivity issues. It is worthwhile to note that wafers determined defective at this step can still be reworked.

2. Surface particles and scumming can cause adhesion and connectivity issues.

3. Missing metal viewed with advanced macro inspection.

Patterning — Photoresist is spun onto the base-layer metal, then exposed and developed to create vias to the base-layer metal, and finally hard-baked to stabilize the photoresist, which is semi-transparent. Operations can utilize advanced macro inspection and/or 2-D bump inspection after develop to inspect vias for excess resist or particles, as shown in Figure 5 (the photoresist is difficult to inspect because of surface reflectivity). Scumming or particles in or around the via can cause adhesion issues with the plating layer.

When debris prevents the proper adhesion of the plating layer, a void can develop in a bump, resulting in insufficient material deposition in the centroid. Later, packaging might deposit in the void and onto underlying metal layers, killing the die.

4. Particle buildup in the chamber can lead to surface debris.

Electroplating — Inspecting the height of bumps at electroplating is important, since packaging issues often result from bumps that are outside acceptable tolerances. Bumps that are too tall push up the package, preventing other bumps from establishing a connection. Similarly, bumps that are too short cannot form a connection. Bumps cannot be reworked after strip, so inspecting at electroplating offers a final opportunity for operations to catch bumping issues before strip. Although rework is not possible when bumps are too tall, plating can be added to bumps that are too short. 2-D bump inspection is the preferred method for inspecting bumps at this step because of the photoresist's reflectivity. The algorithm uses the bump's width to calculate its height.

5. Excess debris in vias is shown here.

Photoresist strip and base-layer metal etch — During strip, wafers are run through a solvent spray to strip away the photoresist bump processes that use 2-D bump inspection to calculate the diameter of bumps (which can be used to calculate their height), detect bridge bumps and measure CD lines. Bumps that are outside the specified levels will not adhere well to the package, indicating potential issues in electroplating. 3-D bump inspection can calculate bumps' height, co-planarity and morphology. Far more effective than a profilometer, 3-D bump inspection measures the roughness of a bump's surface, predicting the quality of its connection with the package — and measuring the effectiveness of electroplating. Bumps that are too rough cannot form enough contacts with the package, increasing the potential for an open or non-connection. Bumps' co-planarity is another important predictor of package adhesion. Bumps that are shorter than others cannot form a connection with the package. Surface particle detection using advanced macro inspection is also useful at this step. Residual resist or particles left on the wafer are good indicators of process issues during strip.

After strip, the wafer is run through a wet chemical etch to remove the metal layer where it is not protected by the bump, exposing the polyimide layer. 2-D bump inspection can detect residual metal or acid materials, as shown in Figure 6 . Repeat debris, or lines outside of specifications, can indicate process or equipment issues. Operations can also measure CD lines using 2-D bump inspection. 3-D bump inspection can measure the height of bumps or detect debris. Performing a 3-D inspection at this step provides useful information about process and equipment effectiveness.

6. Debris on bump edge (left) and polyimide (right) after stripping the photoresist and etching the base-layer metal.

Backside inspection — Backside inspection can also provide useful information at this step. Sometimes, chemical residue left behind after etch migrates onto the wafer's backside (Fig. 7 ). Any die with chemical residue on the backside will later adhere to the film frame — blue tape. When the die sorter tries to pick up the die, it could crack. Additionally, backside inspection can detect particles that may cause "hot spots" in the lithography process. Without automated backside inspection, operations must manually inspect the backside of wafers. If a defect is found, the lot is paused and the wafer is sent back to the fab so the chemical residue can be stripped off — contributing to the cost of the wafer. If an automated backside inspection system detects debris, the system marks the die, so the pick-and-place equipment will ignore it, leaving it on the film frame.

7. The window on the left shows part of a wafer map. The red dots represent particles on the wafer’s backside. The window on the right contains a brightfield image of some chemical residue on the wafer’s backside.

Reflow — Reflow is the last step before the package is placed on the device. Heat is applied to the wafer to round off the bumps, creating a more uniform shape and enabling greater connectivity with the package. This is the most well-known inspection step. Operations use 3-D bump inspection to check the co-planarity of reflowed bumps on a die. Bumps that are outside the co-planarity threshold cause connectivity issues with the package. Bumps that are too tall push up the package, preventing the shorter bumps from making a connection.

The Table summarizes the application of different inspection techniques at various steps in the bump process.

Before plating, advanced macro inspection can be used at polyimide patterning, base-layer metal and patterning to detect debris that can impact metal deposition or create adhesion defects. Equipment issues can also be detected by monitoring surface particles. 2-D bump inspection can measure CD lines to identify those outside tolerances before they cause connectivity issues. Faulty wafers identified before plating can also be reworked.

After plating, 2-D bump inspection can be used at electroplating and strip to inspect CD lines and measure bumps' diameters. Bumps that are too tall or short will cause packaging issues. At strip, advanced macro and 2-D and 3-D bump inspections can be employed. 2-D bump inspection can measure bumps' height and detect bridge bumps. 3-D bump inspection can measure bumps' height, co-planarity and morphology to catch defects that can cause connectivity issues with the package. Advanced macro inspection can detect surface particles to measure the effectiveness of the strip process.

2-D bump inspection can be used as a process monitor at base-layer metal to detect residual metal or acid, or to ensure CD lines are within tolerances. 3-D bump inspection can also serve as a process monitor, detecting excess debris or measuring bumps' height to ensure solid connectivity with the package. Backside inspection can also be used to find chemical residue that can cause die to crack when they are picked off the film frame or particles that can cause hot-spot anomalies during lithography.

3-D bump inspection can check the co-planarity of bumps at reflow, detecting bumps that can cause connectivity issues with the package and measuring the effectiveness of the process.

Advanced macro inspection, 2-D and 3-D bump inspection, and backside inspection provide valuable data for process engineers. The particular mix of inspection technologies described here will vary from process to process. Inspections play a vital role in process development and process control. In process development, they provide the feedback engineers need to tune the process and achieve production yields. In process control, they detect process excursions and aid in the diagnosis of root cause. Early detection and correction is especially important for bump processes, because completed wafers make very expensive scrap. By accelerating the process development cycle and shortening the time required to detect and recover from yield excursions, bump inspection can contribute dramatically to process profitability.