Cleaving is a fast and simple technique used for preparing samples of silicon and other semiconductor materials; however, sapphire, despite being a single crystal material, does not cleave well. Existing methods include cleaving and sawing; however, yields can be unsatisfactory due to fractures that propagate in unwanted directions and loss of material during the process. Laser scribing and cryogenic cooling are mentioned in literature as methods that can help decrease unwanted fractures, chipping, delamination, and loss of material, but these methods are time consuming, costly, and can present other undesirable issues such as poor edge quality and thermal damage because of temperature changes.
Since cleaving is low cost and fast with no loss of material, several 3” sapphire wafers were attained by LatticeGear in order to revisit cleaving of sapphire using two recently developed methods. These methods are distinguished from handheld scribing and cleaving as they integrate new techniques such as diamond microline indentation, backside scribing and cleaving into mechanical platforms. The “smart” mechanics (knobs, dials, levers) of these platforms assist a repeatable process and remove variation in results attributable to operator experience. Furthermore, they enable new test conditions that are not possible with handheld processes.
Figure 1. Weak points made by a diamond scriber, the FlipScribe and LatticeAx.
Two steps are needed for cleaving a sample:
Step 1. Weak point creation.
The weak point is a “defect” developed on the sample. It will be the initiation point for the cleave. It is not possible to divide a substrate into two pieces without initially making a weak point using a diamond scriber or indenter. The weak point produced on the edge of the sample (Figure 1) is extremely vital as it defines the quality and accuracy of the cleave via the substrate’s crystal plane since the cleave propagates from the weak point. If the weak point produced at an angle, is wide, or is deep and not straight, it causes fractures (even micro-fractures) and both the quality and accuracy of the cleaved surface will be negatively affected.
Step 2. Cleaving.
Cleaving is the second step in preparing a cleaved sample. Cleaving takes place by creating stress on the weak point. The cleave is then initiated and propagates all through the sample. If the sample is crystalline, the best weak point is short (Figure 2) as it initiates a cleave following a crystal plane. The resulting cross section will have a mirror finish (Figure 3). If the sample is amorphous, the sample will break but without a crystal plane, following which the cleave will propagate in the direction of the weak point and may not be straight unless a long scribe is made across the complete distance of the desired line of cleavage (Figure 2).
The resulting cross section will not have a mirror finish. This “long scribe” approach is also employed on the crystalline material when the line of cleavage needs to be counter to the crystal plane. Figure 4 presents a silicon sample cleaved at 45 degrees to the (100) crystal plane. It should be noted that the edge is rough because the cleave is counter to the crystal plane. Cleaving can be performed by splitting the sample in two 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.
Tools used in this study for developing the weak point and cleaving are the FlipScribe®, LatticeAx®, and cleaving pliers.
Figure 4. Sample cleaved at 45 degrees to (100) silicon using a long scribe.
Method 1. Use of the LatticeAx® to Cleanly Cleave a 3” Sapphire Wafer.
The LatticeAx incorporates weak point generation and cleaving into a single tool. The weak point is made with the help of a wedge-shaped diamond indenter. The microline indent is a short indent (750-1000 μm long and 10 μm wide). After indenting, the sample is cleaved by putting a downward force in 2 points at equal distance from the indent (Figure 5).
A 3” sapphire wafer was cleanly cleaved using the LatticeAx’s highly repeatable and accurate microline indent and cleave process. The microline indent of LatticeAx was used to make the short indent at the edge of the wafer. This is followed by the weak point propagating along the crystal plane using the LatticeAx’s 3pt cleaving method. This resulted in extremely clean cross-section faces such as those needed for photonics applications (Figures 6 and 7).
The process took almost five minutes. As noted above, this process follows crystal planes and based on how the devices are made, may not produce cleaves that are normal to each other. To produce rectangular samples, with edges parallel to and normal to the flat, one would have to use a method that employs a long scribe across the whole sample as shown in Figure 2.
Figure 5. 3pt cleaving method integrated into the LatticeAx.
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.
Method 2. Use of the FlipScribe to scribe, cleave and downsize a 3” sapphire wafer.
The FlipScribe is a scribing machine capable of scribing the backside of the sample while the operator views targets on the frontside of the sample. Samples are either guided manually or guided with the help of sample holders (Figure 8) over the scriber tip. Figure 9 presents the position of the scriber contacting the backside of the sample during scribing. The scriber tilt and height can be adjusted; this was found to be important for optimizing a process to prepare samples along lithography versus crystal planes, or for amorphous material.
Note that for applications where the wafer had to be diced along the scribe line of the electronic structures, the FlipScribe is used for countering the cleave along the A-crystal planes of sapphire. Scribing will “force” the sample to break along the die scribe lines which are generally orthogonal. In Figure 10, the left image presents 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 image presents a sample cleaved using the FlipScribe.
This preparation resulted in a sample (10 mm on a side) with its sides following the lithography. This is generally needed for cross sections and when testing the performance of a die. A weak point (long scribe line in this case) developed with a hand scriber is commonly too large, deep, and destructive. If it is “too weak”, the cleave naturally propagates via the natural, strong crystal plane. The cleave will always follow “the path of least resistance”. The FlipScribe scriber height and tilt can be optimized for the material and consequently preset for a repeatable process. The holder (Figure 8) secures the sample assuring a shallow, straight, and thin scribe line that produces a “strong weak point” in order to initiate the cleave.
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.
This work demonstrates that even though sapphire is a difficult material, it can be cleaved in a successful manner.
Cleaving Die from a 3” Sapphire Wafer
The 3” wafer, 470 μm thick (shown in Figure 8) was scribed and cleaved with the help of the FlipScribe and a combination of the Cleanbreak Pliers and Small Sample Cleaving Pliers. In this situation, a short scribe was made perpendicular to the flat because in this direction, the crystal plane was parallel to the lithography. Parallel to the flat, long scribes were made to force the cleave to follow the lithography and not the sapphire crystal plane.
Scribing was carried out on the FlipScribe employing a custom designed 3” wafer holder. After scribing, the wafer was cleaved using the Cleanbreak Pliers as presented in Figure 11. Figure 12 presents the wafer after cleaving both perpendicular and parallel to the flat. To cleave the sapphire wafer into smaller samples, a small piece holder was used for gripping the sample and make clean, straight scribes. Small samples were cleaved using pliers optimized for small samples (Figure 13). Figure 14 presents the results of multiple cleaves on the sapphire wafer using this methodology. These results demonstrate that sapphire wafers can be cleaved without loss of material and fractures.
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.
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.
Figure 15. Bare sapphire wafer during cleaving with Cleanbreak Pliers.
Figure 16. Bare sapphire wafer after cleaving into quarters using the FlipScribe and Clean¬break Pliers.
Bare C-M plane sapphire wafers, 50.8 mm in diameter and 420 μm thick were purchased in order to verify if this process could be repeated. As shown in Figures 15 and 16, the wafers were cleaved into quarters using the same method described above. The wafers were scribed with the help of the FlipScribe and cleaved using Cleanbreak pliers.
With the right process and tools, it is possible to cleanly cleave sapphire with control over the cleave direction. Both the FlipScribe and the LatticeAx are valuable additions to the laboratory when sapphire wafer downsizing for testing or cross-section analysis is needed. The LatticeAx preparation creates mirror finish cleaved edges since the sample cleaves along a crystal plane. The FlipScribe backside scriber does not touch the sample frontside; it develops 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.
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