Cleaving is a simple yet rapid method used to prepare samples of silicon and other semiconductor materials; however, despite being a single crystal material, sapphire does not cleave well.
Existing methods include sawing and cleaving, but yields can be insufficient due to fractures that propagate in unwanted directions and loss of material during the process. Cryogenic cooling and laser scribing are described in literature as methods that can minimize unwanted fractures, chipping, delamination and loss of material. However, these methods are expensive, time consuming and can pose other undesirable problems such as poor edge quality and thermal damage caused due to temperature changes.
Since cleaving is quick and inexpensive with no loss of material, LatticeGear acquired numerous 3" sapphire wafers so as to revisit cleaving of sapphire using two newly developed methods. These methods are distinguished from handheld scribing and cleaving because they incorporate new techniques such as diamond microline indentation, backside scribing and cleaving into mechanical platforms.
The “smart" mechanics (levers, knobs, and dials) of these platforms allow a repeatable process and eliminate variation in results attributable to operator experience. Furthermore, they allow new test conditions that are otherwise not feasible with handheld processes.
Two steps are needed to cleave a sample:
Step 1. Weak Point Creation
The weak point is a “defect" formed on the sample. It will be the starting point for the cleave. It is not feasible to split a substrate into two separate pieces without first making a weak point by means of a diamond scriber or indenter.
The weak point made on the edge of the sample (Figure 1) is significant as it defines the quality and accuracy of the cleave through the crystal plane of the substrate because the cleave propagates from the weak point. If the weak point is made at an angle that is deep, or wide and not straight, it causes fractures (even micro-fractures) and both the quality and accuracy of the cleaved surface will be negatively affected.
Figure 1. Weak points made by a diamond scribers, the FlipScribe and LatticeAx.
Step 2. Cleaving
Cleaving is the second step for preparing a cleaved sample. Cleaving happens by creating stress on the weak point. Then, the cleave is initiated and propagated across the sample. As shown in Figure 2, if the sample is crystalline, the best weak point is short as it initiates a cleave following a crystal plane. The ensuing cross section will have a mirror finish, as shown in Figure 3.
If the sample is amorphous, the sample will break without a crystal plane, the cleave will spread in the direction of the weak point and it may not be straight unless a long scribe is made scribed across the entire distance of the desired line of cleavage, as shown in Figure 2. The ensuing cross section will not have a mirror finish. This “long scribe" approach can also be used on crystalline material when the cleavage line has to be counter to the crystal plane. As shown in Figure 4, a silicon sample cleaved at 45 degrees to the (100) crystal plane.
It should be noted that since the cleave is counter to the crystal plane, the edge is rough. Cleaving can be performed by splitting the sample into two pieces with fingers, pliers or pins.
Figure 2. Short scribe is used for crystalline materials and long scribe is used for amorphous materials or for cleaving counter to a crystal plane. The orange lines show the direction of the cleave.
Figure 3. Copper film on (100) silicon cleaved after making a short scribe. Cleaved edge shows mirror finish.
Figure 4. Sample cleaved at 45 degrees to (100) silicon using a long scribe.
In this study, the LatticeAx®, FlipScribe® and cleaving pliers are used for creating the weak point and cleaving.
Method 1. Use of the LatticeAx® to Cleanly Cleave a 3" Sapphire Wafer
The LatticeAx integrates weak point production and cleaving into one tool. A wedge-shaped diamond indenter is used to make the weak point.
The microline indent is very short (750 to 1000 µm long and 10 µm wide). After indenting, the sample is cleaved by using downward force in 2 points at equal distance from the indent, as illustrated in Figure 5.
Figure 5. 3pt cleaving method integrated into the LatticeAx.
Using the LatticeAx’s highly accurate and repeatable microline indent and cleave process, a 3" sapphire wafer was cleanly cleaved. The microline indent of the LatticeAx was used to make the short indent at the wafer edge. Using the LatticeAx’s 3pt cleaving method, the weak point subsequently propagates along the crystal plane.
This leads to very clean cross section faces such as those needed for photonics applications, as illustrated in Figures 6 and 7.
Figure 6. Sapphire wafer cleaved using the LatticeAx microline indent and 3pt cleave method.
Figure 7. View of sapphire wafer shows clean edges after cleaving through the crystalline structure with the LatticeAx.
It took about 5 minutes to complete this process. This process, as noted above, follows crystal planes and based on how the tools are produced, may not create cleaves that are normal to each other.
A method that uses a long scribe across the entire sample should be used to make rectangular samples with edges parallel to and normal to the flat (Figure 2).
Method 2. Use of the FlipScribe to Scribe, Cleave and Downsize a 3" Sapphire Wafer
The FlipScribe is a scribing tool that scribes the backside of the sample while the operator observes targets on the frontside of the sample. As shown in Figure 8, samples can be guided either manually or with the help of sample holders over the scriber tip. Figure 9 reveals the position of the scriber contacting the backside of the sample during scribing.
The tilt and height of the scriber can be adjusted, and this was found to be the key to optimize a process to prepare samples along crystal planes versus lithography, or for amorphous material.
Figure 8. The FlipScribe is a scribing machine that makes the scribe on the backside of the sapphire.
Figure 9. Diagram showing the sample on the FlipScribe worksurface and position of the scriber.
It should be noted that for applications that need the wafer to be diced along the scribe line of the electronic structures, the FlipScribe is employed to counter the cleave along the A-crystal planes of sapphire.
Scribing will "force" the sample to split along the die scribe lines which are typically orthogonal. In Figure 10, the left side image reveals a sample cleaved after manually scribing with a pen style diamond scriber. Note that the sample cleaves along a crystal plane which is not parallel to the lithography. The right side image reveals a sample cleaved using the FlipScribe.
This process resulted in a sample (10 mm on a side) with its sides following the lithography. Typically, this is preferred for cross sections and while testing the performance of a die. A weak point (long scribe line in this case) created with a hand scriber is generally large, too deep and destructive. If it is "too weak", the cleave naturally propagates through the strong, natural crystal plane. The cleave will constantly follow “the path of least resistance".
The tilt and height of the FlipScribe scriber can be optimized for the material and then preset for a repeatable process. As shown in Figure 8, the holder secures the sample assuring a thin, shallow and straight scribe line that makes a “strong weak point" to begin the cleave.
Figure 10. Comparison of sapphire scribed and cleaved with handheld scribers with a sample scribe using the FlipScribe. Left: Sapphire after manual scribe and cleave. Right: Sapphire scribed using the FlipScribe, then cleaved with LatticeGear’s Small Sample Pliers.
This work reveals that although sapphire is a difficult material, it can be effectively cleaved.
Cleaving Die from a 3" Sapphire Wafer
The 3" wafer, 470 µm thick, shown in Figure 8, was scribed and cleaved with the FlipScribe and a combination of the Small Sample Cleaving Pliers and Cleanbreak Pliers. In this study, a short scribe was made perpendicular to the flat as in this direction, the crystal plane was parallel to the lithography. In parallel to the flat, long scribes were made to force the cleave to follow the lithography and not the sapphire crystal plane.
Using a custom designed 3" wafer holder, scribing was carried out on the FlipScribe. After scribing, the Cleanbreak Pliers (Figure 11) was used to cleave the wafer. Figure 12 illustrates the wafer after cleaving both parallel and perpendicular to the flat.
A small piece holder was used to grip the sample and make clean, straight scribes in order to cleave the sapphire wafer into smaller samples, which were cleaved using pliers improved for small samples (Figure 13). The results of multiple cleaves on the sapphire wafer using this methodology as shown in Figure 14. These results reveal that it is possible to cleave sapphire wafers without loss of material and fractures.
Figure 11. Sapphire wafer ready for cleaving with Cleanbreak pliers.
Figure 12. Sapphire wafer after cleaving perpendicular and parallel to the flat.
Figure 13. Cleaving a small sample with the small sample cleaving pliers.
Figure 14. Sapphire wafer after cleaving into small samples.
Bare C-M plane sapphire wafers, 50.8 mm in diameter and 420 µm thick were purchased to verify this process could be repeated. As shown in Figures 15 and 16, the wafers were cleaved into quarters using the same process explained above.
The FlipScribe was used to scribe the wafers while Cleanbreak pliers were used to cleave the wafers.
Figure 15. Bare sapphire wafer during cleaving with Cleanbreak Pliers.
Figure 16. Bare sapphire wafer after cleaving into quarters using the FlipScribe and Cleanbreak Pliers.
Sapphire can be cleanly cleaved with control over the cleave direction using the right process and tools. The LatticeAx and the FlipScribe are important additions to the laboratory when there is a requirement for sapphire wafer downsizing for testing or cross section analysis.
The LatticeAx preparation creates mirror finish cleaved edges as the sample cleaves along a crystal plane. The FlipScribe backside scriber does not contact the sample frontside; it generates a clean break defined by the scribe line which can also be aligned with a surface target.
This information has been sourced, reviewed and adapted from materials provided by LatticeGear.
For more information on this source, please visit LatticeGear.