Method for separating integrated circuits formed on a substrate.

0001This invention relates to large scale semiconductor arrays and, more particularly, to aligning a plurality of integrated circuit semiconductor chips into a linear or two-dimensional array.

0002There is a growing need for an inexpensive and simple method for abutting or aligning semiconductor integrated circuit (IC) chips and interconnecting them to produce page-width linear arrays or two-dimensional arrays extending up to a full page size.There is a demand for large area arrays for various applications such as displays, read or write image bars, and thermal ink jet printers. Conventionally, a plurality of IC chips are formed from a wafer die and are separated from each other by a dicing technique. The chips are then tested and the "good" chips are bonded together to form a larger array. The dicing technique creates fractures along the etched crystal plane, leaving surfaces and sometimes damaging passivation layers on the circuit surface near the fracture. A further problem with known dicing techniques is that chips to be aligned are not the same height, and a slight lateral misalignment results since the aligned surfaces are at an angle to each other. Chip misalignment can result from wafer-to-wafer variations.

0003The present invention is directed towards forming large-area arrays by butting together a plurality of integrated circuit chips using special techniques. Accordingly the present invention provides a method of separating from each other plural integrated circuits formed contemporaneously on the same substrate (chip).

0004The following publications are considered as material: US-A- 4,612,554 discloses an ink jet printhead apparatus comprising two identical parts, and a method for producing the same. A plurality of V-shaped grooves are anisotropically etched on each part of the apparatus between a linear array of heating elements. The grooves allow the structures to be mated in an automatically self-aligned manner.

0005US-A- 4,604,161 teaches a method for fabricating an image sensor array. An active surface of an array chip is etched to form a V-shaped groove which marks an array end. A groove is then cut into the opposite inactive surface with a centerline parallel to, but offset from, the centerline of the V-shaped groove. Forcible fracturing of the chip along the {111} crystalline plane between the grooves produces a straight array end for butting against the end of a like array.

0006JP-A- 60-157236 discloses a method of dicing a semiconductor substrate. A first full-cut or half-cut is made into one side of the semiconductor substrate with a first blade. Then, another cut is made into the other side of the semiconductor substrate, opposite the first cut, with a blade wider than the first blade.

0007US-A- 4,589,952 discloses a method of making trenches having substantially vertical sidewalls in a silicon substrate, wherein a thick photoresist layer, a silicon nitride layer and a thin photoresist layer are sequentially formed on the surface of the substrate and reactive ion etched to form the trenches.

0008US-A- 4,542,397 discloses the formation of large scale arrays from integrated chips on a wafer, which is characterized as having a parallelogrammatic like geometry.

0009The present invention will now be described by way of example with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a side view of an <100> axial wafer of semiconductive crystalline material;</li><li>Figure 2 is a top view of a silicon wafer prior to separation of the individual chips therefrom;</li><li>Figure 3 is an enlarged side view, in cross-section, of a portion of the wafer shown in Figure 2, showing details of a first processing and etching step;</li><li>Figure 4 shows the wafer cross-section of Figure 3, following a second processing step;</li><li>Figure 5 shows the wafer cross-section of Figure 3, following the chip separation;</li><li>Figures 6 and 7 shows buttable chip edges of geometry different from that of the Figure 5 embodiment;</li><li>Figure 8 is an enlarged side view in cross-section of a portion of the wafer shown in Figure 2, showing details of a second chip processing and separation technique;</li><li>Figures 9-11 show enlarged cross-sectional views of a portion of the wafer of Figure 2, showing a third chip processing and separation technique;</li><li>Figure 12 shows a side view of the wafer following the processing and separation steps shown in Figures 9-11;</li><li>Figure 13 shows separated chips from Figure 12 butted together;</li><li>Figure 14 shows another embodiment wherein chips from the wafer of Figure 2 are formed by a dice cut meeting a reactive ion trench;</li><li>Figure 15 shows the butting together of chips formed by the process of Figure 14, and</li><li>Figures 16 and 17 show a side view of a dual wafer, ink jet print head formed by the etching and processing technique of the present invention.</li></ul>

0010As shown in Figures 1 and 2, a wafer 10 of semiconductor material has two opposed surfaces 12 and 14. For purposes of description of the invention, wafer 10 is silicon with a <100> axial orientation. However, the technique disclosed below is applicable in principle for most crystalline materials and is not restricted to silicon.

0011In Figure 2, the surface of wafer 10 has been marked off with imaginary lines 16 and 18 creating a grid pattern within which IC chips 20 are formed. Chips 20 may be identical to, or different from, each other depending upon the particular application. For purposes of description, chips 20 comprise a plurality of identical circuits which can be individually assembled to form a full page thermal ink jet printing device. Each chip 20 contains an array of electronic sensing or printing elements and necessary associated circuitry to enable interconnection off the chip. For specific inkjet applications, each chip 20 is formed with transistor drive logic and resistive heater elements integrated therein. Following the appropriate processing steps, the chips 20 are separated and then bonded together to form, for example, a resistive plate which together has a channel plate, comprise a full page thermal ink jet printhead.

0012Prior to processing of the wafer 10 to form the desired circuits, processing steps are performed along the separation lines 16 and 18 so as to enable the final etching steps which will separate the chips. Figure 3 shows a sectional view of the circled area from Figure 2, the circled area extending on both sides of one of the separation lines 16. It is understood that the following description pertains to chip separation processing along all of the lines 16, 18. As shown in Figure 3, a p+ layer 30 is formed in wafer 10, approximately 10 µm below the surface. Layer 30, approximately 2-3 µm thick, will be used as an orientation-dependent etch stop when the wafer is etched from the back as will be seen. Layer 30 is formed beneath a lightly doped p- layer 32 approximately 10 µm thick. The first process step is to etch vertical trenches 34 in the wafer surface by a reactive ion etching process. This technique is distinguished by a high degree of directional (anisotropic) etching so that almost vertical sidewalls can be formed. Trench 34 is typically 2-3 µm wide and about 10 µm deep extending through layers 32 and 30. Trench 34 is oxidized and then filled with a low pressure CVD polysilicon forming field oxide layers 36 and polysilicon plug 38.

0013A normal wafer fabrication sequence is now implemented to form logic and sensor or printing elements of the completed circuit. Once the logic, and resistor fabrication sequence is complete, the chip is passivated with PSG followed by coating of top and bottom surfaces with a plasma-enhanced Si₃N₄ coating 40,42 respectively (Figure 4). The PSG is patterned to expose bonding pads before deposition of the plasma nitride. The plasma nitride on the back of the wafer is patterned and aligned using an IR aligner. The plasma nitride coating protects the completed circuitry from attack by orientation dependent etch, (dotted lines) which is shown as terminating at p+ layer 30. The front side of the wafer can alternatively be protected by being clamped against silicone or ethyelene-propylene elastomeric material with 'Teflon' or stainless steel. The etching is made through an ODE etch mask whose alignment with trench 34 is not critical so long as some latitude is designed into the mask. The ODE etch is interrupted so that a short wet oxide etch (buffered oxide etchant) can be used to etch through the oxide layer 44 at the bottom of the trench (Figure 3). The wafer is then replaced in the ODE solution to etch away polysilicon block 38. Figure 5 shows the wafer at this point with two adjoining chips 20 being separated as a result of retrench 34 being met by the ODE etch. At this point the Si₃N₄ layers 40, 42 are stripped away with a plasma etch and metallization applied to the back of the wafer for ohmic contact. As shown in Figure 5 , this separation results in planar butting surface 50, 52 for each of the chips 20. The chips are free from sharp edges and, assuming both chips are tested and found to be operational, they can be butted together. Since the butting surfaces are planar, some lateral (vertical) mis-alignment can be accepted to ensure proper chip-to-chip alignment. This processing and etch separation is performed along all of the lines 16, 18 (Figure 2) separating all the chips 20. The chips can then be tested and are aligned and bonded to other chips to form the desired large area array.

0014A second method can be used to form the chips described with reference to Figures 3-5. As a first example, trench 34, as shown in these figures, is formed with planar butting edges 50, 52. Alternatively, and as shown in Figures 6 and 7, the edges can be formed with an irregular, wedge-shaped contour. The countoured edges are formed by using an appropriate photomask to expose the resist. Each edge 50a, 52a and 50b, 52b is separately matched with the associated butting edge so that the two butted edges will fit together in tongue and groove fashion. This modification allows precise alignment in both x and y directions and ensures side-to-side as well as end-to-end alignment.

0015According to a another modification, it may be desirable for certain applications to use a p+ silicon substrate rather than the p- substrate 14 shown in Figure 5. For example, p-/p+ wafers are useful starting material for advanced CMOS chips, including high density memory. In order to replace the p- layer 14 of Figure 5 with a p+ layer the resistivity of the p+ layer must be high enough so that it is etched in the ODE etchant at the same rate as the p- wafer. In practice, a resistivity of 0.01ohm cm is appropriate.

0016According to a third modification, for certain applications where the active circuits can be offset towards the center of the wafer (e.g. for thermal ink jet resistors or ionographic printheads), it may be possible to dispense with the p- layer 32 (Figure 3) and restrict the p+ layer 30 to areas immediately adjacent the trench butt edge surfaces. As shown in Figure 8, p+ layer 60 is formed by diffusion through a mask which exposes only the areas adjacent the edges of trenches 62. Thus, and referring to Figure 3 for comparison, p- layer 32 is not required and the p+ layer 60 is restricted to the areas shown adjacent the trench edges.

0017According to another aspect of the invention, and referring to Figures 9-11, prior to processing of wafer 14, a two µm SiO₂ mask 70 is deposited on the top surface of the wafer. A trench 72, (Figure 2) 3 µm wide and 10 µm deep, generally following lines 16 and 18, is formed (etched) by a reactive ion etching process. The SiO₂ mask 70 is then stripped and wafer 14 is reoxidized. If trench 72 has been made with 3 µm or less width, a 2 µm thermal oxidation will close over the top of the trench allowing subsequent processing on the wafer surface without the interference of an etch trench (Figure 10). At the end of the wafer process, a via etch removes the SiO₂ near the trench; a resist layer 74 is reapplied and imaged and a final SiO₂ etch removes the SiO₂ from the trench (Figure 11). The purpose of the via etch is to provide a bare silicon substrate for the resist to protect the integrated chip circuitry and avoid severe undercutting of the chip passivating SiO₂ during the trench SiO₂ etch.

0018Referring now to Figure 12, a side view of the processed wafer shows the vertical trenches 72. Grooves 74, having centerlines offset from the centerlines of the trenches, are formed by known dicing techniques. Wafer 14 is then subjected to a fracturing force at each of the trenches 72 causing separation as indicated by the dotted lines. The separated chips 20, assuming they tested satisfactorily, are butted together as shown in Figure 13. Butting surfaces 84a, 84b have flat vertical edges instead of the sharp edges characterized in the prior art. Thus variations in two butting chips 20 do not result in any lateral displacement.

0019A still further embodiment is shown in Figures 14 and 15. In Figure 14, the grooves 40 are formed to intersect trenches 72. With this method, precise butting surfaces 84c, 84d, are produced without the possible damage attendant upon the fracture technique of the Figures 12 and 13 embodiment.

0020The chip processing and separation methods described above are generic to any application requiring the precise, damage-free butting of single die pieces. In the specific application of a thermal jet printer, a two-wafer sandwich is required to be separated into individual buttable chips. This can be accomplished by the methods described, provided that the top wafer (the channel plate) is designed so as not to prevent the separation of the sandwich after the trench-die cut frees the bottom (resistor) wafer. Figures 16, 17 show how this is accomplished. Figure 16 shows an enlarged partial cross section having an upper wafer 90 and a lower wafer 92. Upper wafer 90 has formed thereon a plurality of channel plates 94, while sandwich 92 has integrally formed therein a plurality of resistor heater plates 96. The channel plates 94 are fabricated so that, in the separation areas, the channel plates are not connected to each other, the connections being at the front and back of the channel die, in areas that can be easily cut away by a dicing saw after the heater and channel plates are firmly adhesively bonded. When this is done, it only remains to form dice cuts 98 (Figure 17) along the back of the heater wafer down to the reactive ion etching trenches 100 to free the sandwich die from the wafer.

0021Once the chips have been separated by any of the techniques described above, they may be precisely butted together, or aligned by any of several known techniques. For example, the chips 20 may be aligned by means of a robot vacuum, operating under a closed loop optical pattern recognition system.