RU2008744C1 - Process of chemical separation of silicon plates with deeply deposited p - n junction into individual crystals - Google Patents

Abstract

FIELD: microelectronics. SUBSTANCE: pattern of separating tracks on both sides of silicon plate is formed by process of photolithography. On one side pattern of separating tracks is produced in the form of stripe with width not less than d_max (d_max is width of track) for which speed of etching of silicon in given etching agent is permanent. On opposite side pattern of separating tracks in direction of area of closing of etching fronts is set in the form of chain of circles having diameter d, where d<d_max, removed from each other at distance l determined by inequality specified in description of invention. In this case line of separation of crystals on which holes are arranged has roundnesses in angles of crystals over radius R≥ 2h_av, where h_av is average width of track between crystals on common base. EFFECT: facilitated manufacture, improved quality.

Description

 The invention relates to semiconductor technology and can be used in the manufacture of a semiconductor structure.

 A known method of chemical separation of silicon wafers into individual crystals [1]. However, this method does not allow batch processing of crystals.

 Closest to the claimed technical solution is a method of chemical separation of the wafers into crystals, fixed on a common basis, through the use of one-sided etching of the silicon wafer along separate tracks [2]. However, one-side etching has significant drawbacks.

- d где Х₀ - глубина травления кремния через отверстие вбок от края;
h_ср = 120 -20 мкм - средняя ширина промежутка между кристаллами на общем основании;
d ≅ d_макс = 206 мкм - диаметр маскирующего диска или отверстия в золотом покрытии, определяемый по экспериментальной зависимости глубины травления от d и от длительности травления.The aim of the invention is to eliminate the disadvantages of one-side etching, that is, reducing the etching time, reducing the overhang of the gold layer over the edge of the structure, eliminating the compression of structures into an accordion, improving the uniformity and reproducibility of etching through the use of dividing tracks of a certain topology, the elements of which are determined from physical characteristics of the etching process. For this, silicon wafers containing a deep-lying pn junction are nickel-plated by chemical deposition using a known technology, then a masking layer of a photoresist is created by photolithography on the surface of the wafer. At the same time, from the side of the common base, the masking layer of the photoresist is arranged along the crystal contour in the form of a string of circular regions separated from each other by a distance l within
0,8d≅l≅2

- d where X ₀ is the depth of etching of silicon through the hole sideways from the edge;
h _av = 120 -20 microns - the average width of the gap between the crystals on a common basis;
d ≅ d _max = 206 μm - diameter of the masking disk or hole in the gold coating, determined by the experimental dependence of the etching depth on d and on the etching duration.

Gold layers are deposited on the plate-free places of the plate according to the known technology, then the mask layer of the photoresist is removed, and silicon is etched from the surface and through the gold-free areas. Silicon etching is carried out until an average gap width h _cf = 120 ± 20 μm is formed. After washing, drying, applying a protective layer to the side chamfer, the crystals are separated by tearing or cutting a thin layer by perforation of the common base.

The invention simultaneously solves the problem of eliminating the effect of constriction of crystals into an accordion. For this, holes in the corners of the crystal are made by placing them along a curve line with a radius R:
R ≥ 2h.

-d_♂ . Кроме того, для исключения стягивания кристаллов на основании в "гармошку" между кристаллами, а точнее в углах кристаллов, создают дополнительные области - изолированные островки кремния в форме четырехугольника с вогнутыми сторонами. Эти области образуются в результате травления кремния по линии контура скругленных углов кристаллов. При этом габариты четырехугольника λ должны превышать ширину промежутка h, что гарантируется условием R ≥ 2h или R_опт≥ 2h_гр.The essence of the invention is the use of round holes in a layer of gold deposited on the plate, on the side into which the acute angle of the chamfer is shifted. The diameter of the hole d is determined from the experimental dependence of the etching depth X _j on the diameter of the hole and the duration of the process t. The distance between adjacent holes is selected from 0.8d to

-d _♂ . In addition, to prevent the crystals from contracting on the base into an accordion between the crystals, and more precisely in the corners of the crystals, additional regions are created — insulated silicon islands in the shape of a quadrangle with concave sides. These regions are formed as a result of etching of silicon along the contour line of the rounded corners of the crystals. The dimensions of the quadrangle λ must exceed the width of the gap h, which is guaranteed by the condition R ≥ 2h or R _opt ≥ 2h _gr .

In FIG. 1 shows a fragment of the mutual arrangement of round holes in a layer of gold and crystal elements, where h is the gap width; l is the distance between the holes; in FIG. 2 shows the dependence of the total% reject on the average width of the gap on the plate h _cf ; in FIG. Figure 3 shows the dependence of the etching depth X _j on the diameter of the hole d and on the etching duration t, where curve 1 corresponds to d = 20 μm, dashed curve to 80 μm, curve 2 to 100 μm, curve 3 to 300 μm; in FIG. 4 shows the dependence of the total% reject on the ratio λ / h; in FIG. 5 shows a fragment of the mutual arrangement of masking regions with crystal elements at the intersection, where 4 are masking regions, 5 is the boundary of the side surface of the crystal, 6 is an isolated region that acts as a spacer, λ is the spacer dimension, R is the radius of the rounding of the crystal, l is the gap width between the crystals.

PRI me R. The method was tested in the manufacture of the 2D212 device having a square crystal structure with a size of 3.3x3.3 mm ² . The crystal structure of this diode is obtained by dividing a silicon wafer with a thickness of 240 + 10 μm and a diameter of d = 42 2 μm, over the entire area of which a diffusion pn junction is created at a depth of 120 + 10 μm.

A plate with a deep-lying pn junction is nickel-plated by chemical deposition, and then using the photolithographic process, masking regions of a photoresist of the FP-25 type are created on its surface. On one side of the wafer, masking regions are performed along crystal dividing lines in the form of strips of width d> d _max . The physical meaning of d _{max is} discussed in more detail in the main application. On the opposite side of the plate, masking regions are made circular and arranged along a line aligned with the stripes on the first side of the plate (see FIG. 1). On plates coated with masking areas, electrochemical deposition of gold with a thickness of 1.5-3 microns is performed. As a result of applying gold under the masking areas, gold-free areas remain through which silicon will be etched after removal of the masking photoresist. The resulting windows in gold plating have dimensions equal to the removed photoresist layer. Silicon etching is carried out in a mixture of acids HNO ₃ + HF + H ₂ COOH (6: 1: 3). Since silicon is etched on one side through holes in a solid gold layer, after etching, the crystals are bound on one common base and remain connected by a gold layer. After etching, the crystals are washed, dried, the bevel is protected, and the appearance of the crystals is controlled by group methods. Before assembly operations, the crystals are separated by perforation of the base.

During testing, the dependences of the etching rate of silicon through the holes on the diameter of the hole and on the duration were established. It was also found that the depth of etching in depth X _j is equal to the depth of etching of silicon sideways from the edge of the hole X ₀ for hole diameters d ≅ d _max . With dimensional etching, this result is obvious, since the etching rate of silicon is limited by the diffusion input and diffusion removal of the reaction products relative to the reaction boundary. That is why the reaction rate as a whole is independent of the crystallographic direction. Accordingly, the depth of etching in depth is equal to the depth of etching to the side from the edge: X _j ≈ X ₀ .

Real silicon wafers are not perfectly plane-parallel and are characterized by an average spread in thickness over the area Δ h _av = ± 10 μm. For complete separation of crystals on such a plate, the etching process has to be "overexposed". The criterion of "overexposure" is the boundary value of the width of the through gap h _g between the crystals on a common basis (see Fig. 1).

h_гр = 120 ±10 мкм, зависимость % брака от h_ср имеет излом и % брака резко возрастает для значений h_cp < h_гр. Рост брака обусловлен ломкой перемычек между кристаллами при обработках кристаллов на общем основании и при разделении основания.Experimentally, h _cf was determined by etching a series of plates at different times and measuring the gap width at five symmetric points on the plate (in the center and along the edge on the main axes). According to the measurement data h, the arithmetic mean value h _{cf is} calculated at five points and this value is compared with the results of controlling the appearance and electrical parameters measured after protection and complete separation of the crystals by perforation of the base. Complete separation of the plates was carried out by cutting or tearing the gold layer along the perforation. In FIG. Figure 2 shows the dependence of% of the total defect in appearance and reverse current on the value of h _cf. From FIG. 2 shows that when h _av

h _gr = 120 ± 10 μm, the dependence of% reject on h _cf has a kink and% reject decreases sharply for values of h _cp <h _gr. The growth of marriage is due to the breaking of the bridges between the crystals when processing the crystals on a common basis and when the base is divided.

- d ♂ (1)
Выбор величины d и Х₀ для расчета l по выражению (1) аналогичен выбору d в основном изобретении и осуществляется при помощи экспериментальной зависимости Х_j от d и от длительности травления (см. фиг. 3).Separation of crystals by perforation imposes a restriction on the choice of the distance l between adjacent masking regions (or holes). According to the geometric construction in FIG. 1, the limitation of the value of l from above is associated with d, with the etching depth X ₀ and h _{gr the} ratio:
l≅2

- d ♂ (1)
The choice of the values of d and X ₀ for calculating l from expression (1) is similar to the choice of d in the main invention and is carried out using the experimental dependence of X _j on d and on the etching duration (see Fig. 3).

h₀/2 = 125 мкм. Смещение острого выступа на боковой поверхности от места выхода р-n-перехода - на 25 мкм, являющееся достаточным для получения воспроизводимых электропараметров на кристаллах. На основе этих исходных данных находим глубину травления с каждой стороны пластины: со стороны широких промежутков d > d_макс, глубина травления должна быть Х₀₁ ≃ h₀/2 + 25 = 125 + 25 = 150 мкм. Глубина травления со стороны отверстий равна соответственно: X_j2 ≃ 250 - 150 = 100 мкм за то же время. По графической зависимости на фиг. 3 можно определить длительность травления на глубину ≈ 150 мкм через широкие промежутки (для d > d_гр): по кривой 3: получаем t = 30±5 мин. Далее для этой же длительности определяем диаметр отверстия (или маски) по параметру кривой, проходящей через точку пересечения координаты глубины травления Х_j2 = 100 мкм и координаты длительности t = 30 мин, параметр пунктирной линии составляет d = 80 мкм + 10 мкм. Это является целью расчета по зависимости фиг. 3. Теперь по формуле (1) рассчитываем длину промежутка между отверстиями в золотом основании:
l= 2

- 80 = 80 мкм, т. е. l = 80 + 10 мкм.
С другой стороны, величина l не может быть малой, поскольку снижается прочность основания по перфорации и она начинает самопроизвольно рваться при механических нагрузках в процессе групповых обработок (промывка, сушка, нанесение защитного покрытия и т. д. ).An example of the choice of l: let the average plate thickness h _0av = 240 ± 10 μm. The maximum thickness h _0max = 250 microns. Depth pn junction X

h _0/2 = 125 μm. The offset of the sharp protrusion on the lateral surface from the exit point of the pn junction is 25 μm, which is sufficient to obtain reproducible electrical parameters on crystals. Based on these data we find initial etch depth of each side plate: from the wide spacing d> d _max, etching depth must be ₀₁ X ≃ h _{0/2 + 25 = 125 +} 25 = 150 microns. The etching depth from the side of the holes is respectively: X _j2 ≃ 250 - 150 = 100 μm at the same time. According to the graphical dependence in FIG. 3, it is possible to determine the etching duration to a depth of ≈ 150 μm through wide gaps (for d> d _gr ): from curve 3: we obtain t = 30 ± 5 min. Next, for the same duration, we determine the diameter of the hole (or mask) by the parameter of the curve passing through the point of intersection of the coordinate of the etching depth X _j2 = 100 μm and the coordinate of duration t = 30 min, the dashed line parameter is d = 80 μm + 10 μm. This is the purpose of the calculation according to FIG. 3. Now, using the formula (1), we calculate the length of the gap between the holes in the gold base:
l = 2

- 80 = 80 microns, i.e. l = 80 + 10 microns.
On the other hand, the value of l cannot be small, since the perforation strength of the base decreases and it begins to break spontaneously under mechanical loads during group treatments (washing, drying, applying a protective coating, etc.).

 A special experiment determined the criterion of the tensile strength of the base for perforation in the form of the ratio between l and d. During the study, photo masks were used with different ratios of l and d, but satisfying condition (1). With their help, crystals were made and the total% of marriage was determined by the presence of discontinuities in perforation and by the flow of protective material on the back side of the plate. An elastic compound KLT-30 was used as a protective material. The results of the study are summarized in the graphical dependence of% of the total marriage on the ratio l / d (see Fig. 4). From FIG. 4 it can be seen that when l / d <0.8 (2)% of the defect begins to increase sharply, which corresponds to a loss in perforation strength.

- d (3)
После подстановки исходных данных d = 80 мкм, Х_j2 = 100 мкм, h_гр = 120 мкм имеем: 64 мкм ≅ l ≅ 80 мкм. При групповых обработках основание с кристаллами может стягиваться в "гармошку", что ограничивает возможности групповых обработок. Для предотвращения этого эффекта на перекрестках промежутков выполняют вспомогательные области в виде островков из кремния, имеющих форму четырехугольников с вогнутыми сторонами (см. фиг. 5). Эти области делают изолированными от кристалла, так чтобы при разделении кристаллов по перфорации они выпадали или оставались с одним из кристаллов, не влияя на его работу в приборе. Указанный островок является остатком от скругления углов кристалла (см. фиг. 5). Габариты островка-распорки λ , ширина промежутка между кристаллами h и радиус закругления углов кристалла R связаны друг с другом, согласно геометрическому построению на фиг. 5, соотношением: R²-Rλ+(λ/2)²= Rh+(h/2)²(4)
Опытная проверка показала, что для выполнения функции распорки габариты островка λ должны превышать ширину промежутка h:
λ≥h (5)
Это условие обосновывается резким возрастанием % суммарного брака - по внешнему виду плюс по обратному току кристалла, обусловленным стягиванием кристаллов в "гармошку" при λ≅ h.So, there is a restriction on the length of the gap both above and below, which can be expressed by the relation:
0,8d≅l≅2

- d (3)
After substituting the initial data, d = 80 μm, X _j2 = 100 μm, h _gr = 120 μm, we have: 64 μm ≅ l ≅ 80 μm. In group treatments, the base with crystals can be pulled together in an accordion, which limits the possibilities of group treatments. To prevent this effect, auxiliary regions in the form of silicon islands having the shape of quadrangles with concave sides (see Fig. 5) are made at the intersections of the gaps. These areas are isolated from the crystal, so that when the crystals are separated by perforation, they fall out or remain with one of the crystals, without affecting its operation in the device. The specified island is the remainder from the rounding of the corners of the crystal (see Fig. 5). The dimensions of the spacer island λ, the gap between the crystals h and the radius of curvature of the corners of the crystal R are connected to each other, according to the geometric construction in FIG. 5, by the ratio: R ² -Rλ + (λ / 2) ² = Rh + (h / 2) ² (4)
An experimental verification showed that to fulfill the spacer function, the dimensions of the island λ must exceed the gap width h:
λ≥h (5)
This condition is justified by a sharp increase in% of the total defect - in appearance plus in the reverse current of the crystal, due to the contraction of the crystals into an accordion at λ≅ h.

Relation (4) allows condition (5) to be expressed in terms of the radius of curvature R:
R≥ 2h (6)
Since the optimal gap width h _opt = h _gr = 120 ± 10 μm, the optimal radius R _opt ≥ 2h _gr = 2 x (120 ± 10) = 240 ± 20 μm.

 In the manufacture of the photomask for the crystal of the 2D212 instrument, the radius of the corner of the crystal angle was chosen to be 300 μm. The diameter of the hole for perforation is selected to 80 μm, and the distance between the holes is 70 μm.

Using this photomask, more than 40,000 pieces were made. experienced instruments. The etching process was carried out, as in the main invention, in a mixture of hydrofluoric, nitric and acetic acids. In this case, the axis of the cassette with the plates rotated with an inclination to the horizon of 73 ^about with a frequency of 75 rpm. The etching duration was 30 + 5 min. The total yield of suitable experimental crystals exceeded the yield by current technology by 15 ± 5%. The proposal has been prepared for implementation at the crystal manufacturing site of the 2D212 device at P.O. "Photon" in Tashkent. (56) 1. Japan Patent N 99 (5) A 04, N 43-9081, 1968.

 2. Japan Patent N 50-25906, cl. H 01 L 21/306, 1975.

Claims (1)

METHOD FOR CHEMICAL SEPARATION OF SILICON PLATES WITH A DEPTH-FREE P - N-TRANSITION TO SEPARATE CRYSTALS, including nickel plating of the plate surface, forming a pattern of dividing tracks by photolithography, applying nickel-plated areas of the gold plate to the photoresist, removing photoresist and etching the tracks, that, in order to reduce the duration, improve reproducibility and uniformity of etching, reduce the overhang of the gold layer over the edge of the crystal and exclude cheniya compression crystals on a base prior to formation of the pattern dividing tracks define the width d _max track above which the silicon etch rate in the etchant used is constant, establish the dependence of silicon etching rate of the hole diameter d <d _max, through which is carried etching on the basis of the depth p - the n-junction and the magnitude of the required mixing of the region where the etching fronts are adjacent from the plane of the p-n-junction, the required ratio d _{ma x} and d is calculated, and then a section casting paths on both sides of the silicon wafer by photolithography, moreover, on one side of the wafer, a pattern of dividing paths is made in the form of a strip with a width of at least d _max , and on the opposite side of the wafer in the direction of mixing of the area of contact between the etching fronts, a pattern of dividing paths is made in the form of a chain of circles with a diameter of d separated from one another by a distance l defined by the inequality
0,8d≅l <2

- d
where X ₀ - the value of lateral etching, silicon equal to the etching depth;
h _av = (120 ± 20) microns - the average width of the path between the crystals on a common basis,
the crystal dividing line along which the holes are located has roundings in the corners of the structures along a radius R ≥ 2h _sr .

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