Point Contact Transistor Grind Contact Face Flat

# Grind the Contact Face Flat: A Standard Set by a Distance That Does Not Exist Yet

Step 2 left behind an oversize block with a damaged shell on every sawn face and no flat reference surface anywhere on it. Step 3 grinds one of those faces toward its finished dimension — the face that will eventually carry both point contacts, ground now even though neither point contact will touch it for many steps to come. The base face that will carry the metal electrode is deliberately left alone here; it gets its own grind, immediately before metallization, much later in this sequence. Working on the contact face first, with no urgency behind it, is itself a choice: this face can sit idle and verified for as long as the rest of the sequence needs, while the base face is handled right before it matters, to minimize how long a freshly prepared surface sits exposed before metal lands on it.

This Face's Standard Is Set by a Spacing Not Yet Chosen the distance this grind has to respect belongs to an operation many steps in the future contact face, ground in this step point contacts land here, many steps later point spacing, thousandths of an inch — not yet decided this grind's roughness target scales to a distance the process has not committed to yet which is why the standard has to be set from the device's eventual geometry, not from the face's current state the base face, by contrast, is left untouched — its own grind comes much later, right before metallization

This face's roughness standard is a tolerance against a distance, not against any fixed general-purpose grinding convention. The emitter and collector points that will eventually land here sit only a few thousandths of an inch apart, and the entire point-contact mechanism depends on an injected carrier crossing exactly that gap through the germanium immediately beneath both points. A surface irregularity that would be irrelevant to a broad, averaging contact can sit directly inside or beneath that narrow gap, and because the device's active region is defined by the gap rather than by any larger reference dimension, the standard this face must meet scales with the gap, not with a number carried over from some other process. A grind quality adequate for contacts spaced millimeters apart becomes marginal once two points must sit only thousandths of an inch apart on the same surface, sharing that surface's local defects.

Grinding's Own Damage Must Clear Before the Etch, Not Survive It every grind leaves a subsurface damage layer; its depth is this step's responsibility, its removal is the next step's subsurface damage layer: dislocations, microcracks from the abrasive undamaged bulk — the carrier path Step 1 qualified abrasive grit, removing material from above damage depth Coarser grit fast removal, deep damage layer Finer final pass slower removal, shallow damage the etch can fully clear a damage layer too deep for the following etch's allowance survives into the active region as permanent lattice disorder

Every grind leaves behind damage this step did not ask for and the next step has to clean up. Material removal by an abrasive wheel does not stop neatly at the finished surface; it drives dislocations and microcracks some distance beneath it, and that depth scales with the grit size used — coarser abrasive removes stock faster but buries damage deeper, finer abrasive removes stock slower but confines the damage closer to the surface. This has nothing to do with the device's own geometry, and everything to do with what the following etch step's allowance has to be sized against. Choosing too coarse a final pass trades grinding time for an etch step that can no longer remove all the disturbed lattice before a point contact ever touches it — the same failure mode the machining allowance in Step 2 was meant to prevent, recreated one operation later by the grind itself.

Leaving the base face untouched here is not an oversight; it is the point. Preparing only one face at a time means the other face can be ground and etched much closer to the moment it is actually needed, which matters most for whichever face metal will be deposited onto, since a freshly etched surface promotes far more dependable adhesion than one that has sat exposed to ambient conditions since early in the sequence. This face has no such urgency — nothing happens to it again until conditions much later in the build — so grinding it now, verifying it, and letting it wait is the right order for this face specifically, even though the opposite order will turn out to be right for the other one.

StepProcess operationInputOutputSpecificationConstraint
3.1Set the finished roughness standard for the contact face from the planned point spacingPlanned point spacing for the eventual emitter and collector placementContact face grind specificationRoughness small relative to the intended point spacing, not against a fixed general toleranceA roughness standard borrowed from a wider-spaced device would under-specify this device's contact face
3.2Choose grit progression and final pass sized to the etch allowance that will followContact face grind specification from 3.1Grinding plan for this faceFinal grit sized so its expected subsurface damage depth is within the following etch step's planned allowanceA final grit too coarse for the point spacing recreates the exact risk Step 2's machining allowance addressed
3.3Grind the contact face to its specificationAs-cut body from Step 2, grinding plan from 3.2Contact face ground flatMeasured roughness within the specification from 3.1A contact face ground off-specification places point contacts on a surface whose local defects can sit inside the critical gap
3.4Verify the contact face against its own standard before releasing it to the etchGround contact face from 3.3Verified ground contact faceMeasured roughness within the specification from 3.1, confirmed before etchingA verified grind that is actually off-specification wastes the following etch step's effort correcting the wrong thing
3.5Leave the base face in its as-cut state, undisturbed, for grinding later in the sequenceAs-cut body from Step 2Base face, deliberately unground at this stageNo grinding performed on the base face in this stepGrinding the base face prematurely exposes it to ambient conditions for far longer than its eventual metallization needs
3.6Record the contact face's grind specification and verification result against the body's provenanceVerified contact face from 3.4Documented, contact-face-ground bodySpecification and verification recorded against the record from Step 2.6An undocumented grind leaves a later discrepancy in roughness or point behavior untraceable to this specific operation

Step 3 does not finish this body's two faces; it finishes the one that benefits from being prepared early and left alone. The contact face answers to a distance the process has not even chosen yet, which is reason enough to settle it now and verify it well before anything else depends on it. The base face's own grind is deferred on purpose, to a point in the sequence where finishing it immediately before metallization matters more than finishing it early ever would. Treating both faces the same here would be the easier instruction to write, and it would get the timing wrong for at least one of them.

Take point contact transistor grind contact face flat further

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