Cleaving is a fast and simple method used for preparing samples of semiconductor materials, including silicon. In contrast, sapphire does not cleave well, despite being a single crystal. While sawing and cleaving are currently used for downsizing sapphire, the yields are unsatisfactory because of loss of material during the process and fractures in unwanted directions. Cryogenic cooling and laser scribing are methods that are cited in literature and can reduce delamination, unwanted fractures, loss of material, and chipping. However, these methods are time consuming, costly, and can present other undesired problems like edge quality and thermal damage caused by temperature changes.
As cleaving is fast and cost-effective without any loss of material, LatticeGear acquired several 3” sapphire wafers to revisit cleaving of sapphire using two recently developed techniques. Unlike handheld scribing and cleaving, these techniques combine diamond indenting, scribing, and cleaving into a mechanical platform. In addition, the “smart” mechanics (levers, dials, knobs) allow a repeatable process and remove differences in results based on the operator. It even enables for new test conditions that are impossible with handheld processes.
Sample cleaving involves two steps.
Step 1. Weak Point Creation
A defect created on the sample is called a weak point, and this will be the initiation point for the cleave. A substrate cannot be split into two pieces without initially making a weak point using a diamond indenter or scriber. The weak point is made on the sample edge (Figure 1) and is very significant because it defines the quality and accuracy of the cleave and also the cleave propagates from the weak point. If the weak point, made at an angle, is wide, or causes fractures, it can adversely affect the quality and accuracy of the cleaved surface.
Figure 1. Weak points made by scribers, the FlipScribe and LatticeAx.
Step 2. Cleaving
Cleaving is the second step in preparing a cleaved sample. When stress is created on the weak point, it results in cleaving. The cleave is then initiated and propagated across the sample. For a crystalline sample, short is the best weak point (Figure 2) as it initiates a cleave following a crystal plane. The resulting cross section will exhibit a mirror finish (Figure 3). For an amorphous sample, the sample will break but the cleave will propagate in the direction of the weak point without a crystal plane and may not be straight unless a long scribe is made scribed across the entire preferred line of cleavage (Figure 2). The resulting cross section will not exhibit a mirror finish. When the line of cleavage needs to be counter to the crystal plane, this “long scribe” approach is also employed on crystalline material. A silicon sample cleaved at 45 degrees to the (100) crystal plane is shown in Figure 4. It is to 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 pins, pliers, fingers, or indenting and cleaving tool such as the LatticeAx.
Figure 2. Short scribe used for crystalline materials and long scribe for amorphous or a cleave 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 a mirror finish.
Figure 4. Sample cleaved at 45 degrees to (100) silicon using a long scribe.
The FlipScribe® and the LatticeAx® are the two tools used in this study for creating the weak point and cleaving.
Method 1. Use of the LatticeAx® to Cleanly Cleave a 3” Sapphire Wafer
The LatticeAx integrates cleaving and weak point generation into a single tool. A wedge-shaped diamond indenter is used to make the weak point. The microline indent is a short indent measuring 10 mm wide and 750-1000 mm long. After indenting, a downward force is applied in two points at equal distance from the indent to cleave the sample. (See Figure 5). The following video shows how it is done.
The LatticeAx’s standard, highly repeatable and accurate microline indent and cleave process was used to cleanly cleave a 3″ sapphire wafer. Then, using the LatticeAx’s microline indent, the short indent was made at the edge of the wafer and this weak point subsequently propagated along the crystal plane using the LatticeAx’s 3pt cleaving technique. This resulted in very clean cross-section faces such as those preferred for photonics applications (Figures 6 and 7).
The process took around five minutes. As mentioned above, this process follows crystal planes and based on the way the devices are made, may not create cleaves that are normal to each other. In order to make rectangular samples, with edges parallel to and normal to the flat, one would have to use a method that uses a long scribe across the whole sample, as illustrated in Figure 2.
The LatticeAx method is used to downsize from wafer to tiny pieces, particularly when very clean, mirror finish edges are needed.
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 showing clean cleaved edges of the sapphire wafer cleaved using the LatticeAx.
Method 2. Use of the FlipScribe® to Scribe, Cleave and Downsize a 3” Sapphire Wafer
The FlipScribe is a scribing machine that scribes the sample’s backside while the operator observes targets on the sample’s frontside. Samples are either manually guided or guided with the help of sample holders, as represented in Figure 8, over the scriber tip. Shown in Figure 9 is the position of the scriber contacting the backside of the sample during scribing. It is possible to adjust the scriber height and tilt and this was found to be the key to optimizing a process to prepare samples along lithography versus crystal planes.
Figure 8. The FlipScribe is a scribing machine that makes the scribe on the backside of the sample.
Figure 9. Diagram showing the sample on the FlipScribe worksurface and position of the scriber.
It is to be noted that for applications where the wafer has to be diced along the scribe line of the electronic structures, the cleave is countered along the sapphire's a-crystal planes using the FlipScribe scribing machine. Scribing will “force” the sample to break along the die scribe lines that are usually orthogonal. The left image in Figure 10 represents a sample cleaved after manually scribing with a pen style diamond scriber, and here it must be noted that the sample cleaves along a crystal plane which is not parallel to the lithography. The right image represents a sample cleaved using the FlipScribe scribing machine.
This preparation resulted in a sample (10 mm on a side). This is typically preferred when testing the performance of a die and for cross-sections. When a weak point is (long scribe line in this case) made with hand scriber, it is normally too deep, large, and destructive. If it is “too weak” the cleave typically propagates through the robust, natural crystal plane. “The path of least resistance” is always followed by the cleave. It is possible to optimize the tilt and height of the FlipScribe scriber for the material and subsequently preset for a repeatable process. The holder secures (Figure 8) the sample, ensuring a thin, shallow, and straight scribe line creating a “strong weak point” to start the cleave. It shows that although sapphire is a difficult material, it can still be cleaved.
Figure 10. Comparison of sapphire scribed and cleaved with handheld scribers with a sample scribe using the FlipScribe.
Figure 11. Sapphire wafer ready for cleaving with Cleanbreak pliers.
Cleaving Die from a 3” Sapphire Wafer
Figure 8 shows the 3 inch and 470 µm thick wafer, which was scribed and cleaved using the FlipScribe machine and a combination of the Small Sample Cleaving Pliers and Cleanbreak Pliers. Here, a short scribe was made perpendicular to the flat, because in this direction the crystal plane was parallel to the lithography. Long scribes were made parallel to the flat so that the cleave is forced to follow the lithography and not the sapphire crystal plane.
A custom designed 3” wafer holder was used to carry out scribing on the FlipScribe. Once scribing was done, the wafer was cleaved using the Cleanbreak Pliers as depicted in Figure 11. Shown in Figure 12 is the wafer after cleaving both perpendicular and parallel to the flat. For cleaving sapphire wafer into smaller samples, a small piece holder was used to grip the sample and make straight and clean scribes. Using pliers optimized for small samples, small die sized samples were cleaved (Figure 13).
These results demonstrate that sapphire wafers can be cleaved without loss of material and fractures.
Figure 12. Sapphire wafer after cleaving perpendicular and parallel to the flat.
Figure 13. Cleaving a small sample with the small sample cleaving pliers (note the large rules are 5 mm apart).
Figure 14. Sapphire wafer after cleaving into small samples.
This process could be repeated to verify it. Bare C-M plane sapphire wafers, with a thickness of 420 mm and a diameter of 50.8 mm, were purchased. The same method explained above was used to cleave the wafers into quarters. The wafers were then scribed using the FlipScribe and cleaved using Cleanbreak pliers.
Figure 15. Bare sapphire wafer during cleaving with the Cleanbreak pliers.
Figure 16. Bare sapphire wafer after cleaving into quarters using the FlipScribe and Cleanbreak pliers.
Sapphire can be cleaved cleanly and repeatedly using the right process and tools. Both the FlipScribe and the LatticeAx are important additions to the laboratory when sapphire wafer downsizing for testing or cross-section analysis is needed. Since the sample cleaves along a crystal plane, the LatticeAx preparation yields mirror finish cleaved edges. The FlipScribe backside scriber does not disturb the frontside of the sample and creates 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.